[#307] add package trace, integrate it with logging, and adopt it throughout zrepl

package trace:

- introduce the concept of tasks and spans, tracked as linked list within ctx
    - see package-level docs for an overview of the concepts
    - **main feature 1**: unique stack of task and span IDs
        - makes it easy to follow a series of log entries in concurrent code
    - **main feature 2**: ability to produce a chrome://tracing-compatible trace file
        - either via an env variable or a `zrepl pprof` subcommand
        - this is not a CPU profile, we already have go pprof for that
        - but it is very useful to visually inspect where the
          replication / snapshotter / pruner spends its time
          ( fixes #307 )

usage in package daemon/logging:

- goal: every log entry should have a trace field with the ID stack from package trace

- make `logging.GetLogger(ctx, Subsys)` the authoritative `logger.Logger` factory function
    - the context carries a linked list of injected fields which
      `logging.GetLogger` adds to the logger it returns
    - `logging.GetLogger` also uses package `trace` to get the
      task-and-span-stack and injects it into the returned logger's fields

cli/cli.go

@@ -1,11 +1,13 @@
 package cli
 
 import (
+	"context"
 	"fmt"
 	"os"
 
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 )
@@ -75,7 +77,7 @@ type Subcommand struct {
 	Short            string
 	Example          string
 	NoRequireConfig  bool
-	Run              func(subcommand *Subcommand, args []string) error
+	Run              func(ctx context.Context, subcommand *Subcommand, args []string) error
 	SetupFlags       func(f *pflag.FlagSet)
 	SetupSubcommands func() []*Subcommand
 
@@ -96,7 +98,11 @@ func (s *Subcommand) Config() *config.Config {
 
 func (s *Subcommand) run(cmd *cobra.Command, args []string) {
 	s.tryParseConfig()
-	err := s.Run(s, args)
+	ctx := context.Background()
+	endTask := trace.WithTaskFromStackUpdateCtx(&ctx)
+	defer endTask()
+	err := s.Run(ctx, s, args)
+	endTask()
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "%s\n", err)
 		os.Exit(1)

client/configcheck.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"context"
 	"encoding/json"
 	"fmt"
 	"os"
@@ -29,7 +30,7 @@ var ConfigcheckCmd = &cli.Subcommand{
 		f.StringVar(&configcheckArgs.format, "format", "", "dump parsed config object [pretty|yaml|json]")
 		f.StringVar(&configcheckArgs.what, "what", "all", "what to print [all|config|jobs|logging]")
 	},
-	Run: func(subcommand *cli.Subcommand, args []string) error {
+	Run: func(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 		formatMap := map[string]func(interface{}){
 			"": func(i interface{}) {},
 			"pretty": func(i interface{}) {

client/migrate.go

@@ -48,14 +48,13 @@ var migratePlaceholder0_1Args struct {
 	dryRun bool
 }
 
-func doMigratePlaceholder0_1(sc *cli.Subcommand, args []string) error {
+func doMigratePlaceholder0_1(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	if len(args) != 0 {
 		return fmt.Errorf("migration does not take arguments, got %v", args)
 	}
 
 	cfg := sc.Config()
 
-	ctx := context.Background()
 	allFSS, err := zfs.ZFSListMapping(ctx, zfs.NoFilter())
 	if err != nil {
 		return errors.Wrap(err, "cannot list filesystems")
@@ -124,7 +123,7 @@ var fail = color.New(color.FgRed)
 
 var migrateReplicationCursorSkipSentinel = fmt.Errorf("skipping this filesystem")
 
-func doMigrateReplicationCursor(sc *cli.Subcommand, args []string) error {
+func doMigrateReplicationCursor(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	if len(args) != 0 {
 		return fmt.Errorf("migration does not take arguments, got %v", args)
 	}
@@ -137,8 +136,6 @@ func doMigrateReplicationCursor(sc *cli.Subcommand, args []string) error {
 		return fmt.Errorf("exiting migration after error")
 	}
 
-	ctx := context.Background()
-
 	v1cursorJobs := make([]job.Job, 0, len(cfg.Jobs))
 	for i, j := range cfg.Jobs {
 		if jobs[i].Name() != j.Name() {

client/pprof.go

@@ -1,67 +1,10 @@
 package client
 
-import (
-	"errors"
-	"log"
-	"os"
-
-	"github.com/zrepl/zrepl/cli"
-	"github.com/zrepl/zrepl/config"
-	"github.com/zrepl/zrepl/daemon"
-)
-
-var pprofArgs struct {
-	daemon.PprofServerControlMsg
-}
+import "github.com/zrepl/zrepl/cli"
 
 var PprofCmd = &cli.Subcommand{
-	Use:   "pprof off | [on TCP_LISTEN_ADDRESS]",
-	Short: "start a http server exposing go-tool-compatible profiling endpoints at TCP_LISTEN_ADDRESS",
-	Run: func(subcommand *cli.Subcommand, args []string) error {
-		if len(args) < 1 {
-			goto enargs
-		}
-		switch args[0] {
-		case "on":
-			pprofArgs.Run = true
-			if len(args) != 2 {
-				return errors.New("must specify TCP_LISTEN_ADDRESS as second positional argument")
-			}
-			pprofArgs.HttpListenAddress = args[1]
-		case "off":
-			if len(args) != 1 {
-				goto enargs
-			}
-			pprofArgs.Run = false
-		}
-
-		RunPProf(subcommand.Config())
-		return nil
-	enargs:
-		return errors.New("invalid number of positional arguments")
-
+	Use: "pprof",
+	SetupSubcommands: func() []*cli.Subcommand {
+		return []*cli.Subcommand{PprofListenCmd, pprofActivityTraceCmd}
 	},
 }
-
-func RunPProf(conf *config.Config) {
-	log := log.New(os.Stderr, "", 0)
-
-	die := func() {
-		log.Printf("exiting after error")
-		os.Exit(1)
-	}
-
-	log.Printf("connecting to zrepl daemon")
-
-	httpc, err := controlHttpClient(conf.Global.Control.SockPath)
-	if err != nil {
-		log.Printf("error creating http client: %s", err)
-		die()
-	}
-	err = jsonRequestResponse(httpc, daemon.ControlJobEndpointPProf, pprofArgs.PprofServerControlMsg, struct{}{})
-	if err != nil {
-		log.Printf("error sending control message: %s", err)
-		die()
-	}
-	log.Printf("finished")
-}

client/pprof_activity_trace.go

@@ -0,0 +1,44 @@
+package client
+
+import (
+	"context"
+	"io"
+	"log"
+	"os"
+
+	"golang.org/x/net/websocket"
+
+	"github.com/zrepl/zrepl/cli"
+)
+
+var pprofActivityTraceCmd = &cli.Subcommand{
+	Use:   "activity-trace ZREPL_PPROF_HOST:ZREPL_PPROF_PORT",
+	Short: "attach to zrepl daemon with activated pprof listener and dump an activity-trace to stdout",
+	Run:   runPProfActivityTrace,
+}
+
+func runPProfActivityTrace(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
+	log := log.New(os.Stderr, "", 0)
+
+	die := func() {
+		log.Printf("exiting after error")
+		os.Exit(1)
+	}
+
+	if len(args) != 1 {
+		log.Printf("exactly one positional argument is required")
+		die()
+	}
+
+	url := "ws://" + args[0] + "/debug/zrepl/activity-trace" // FIXME dont' repeat that
+
+	log.Printf("attaching to activity trace stream %s", url)
+	ws, err := websocket.Dial(url, "", url)
+	if err != nil {
+		log.Printf("error: %s", err)
+		die()
+	}
+
+	_, err = io.Copy(os.Stdout, ws)
+	return err
+}

client/pprof_listen.go

@@ -0,0 +1,68 @@
+package client
+
+import (
+	"context"
+	"errors"
+	"log"
+	"os"
+
+	"github.com/zrepl/zrepl/cli"
+	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/daemon"
+)
+
+var pprofListenCmd struct {
+	daemon.PprofServerControlMsg
+}
+
+var PprofListenCmd = &cli.Subcommand{
+	Use:   "listen off | [on TCP_LISTEN_ADDRESS]",
+	Short: "start a http server exposing go-tool-compatible profiling endpoints at TCP_LISTEN_ADDRESS",
+	Run: func(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
+		if len(args) < 1 {
+			goto enargs
+		}
+		switch args[0] {
+		case "on":
+			pprofListenCmd.Run = true
+			if len(args) != 2 {
+				return errors.New("must specify TCP_LISTEN_ADDRESS as second positional argument")
+			}
+			pprofListenCmd.HttpListenAddress = args[1]
+		case "off":
+			if len(args) != 1 {
+				goto enargs
+			}
+			pprofListenCmd.Run = false
+		}
+
+		RunPProf(subcommand.Config())
+		return nil
+	enargs:
+		return errors.New("invalid number of positional arguments")
+
+	},
+}
+
+func RunPProf(conf *config.Config) {
+	log := log.New(os.Stderr, "", 0)
+
+	die := func() {
+		log.Printf("exiting after error")
+		os.Exit(1)
+	}
+
+	log.Printf("connecting to zrepl daemon")
+
+	httpc, err := controlHttpClient(conf.Global.Control.SockPath)
+	if err != nil {
+		log.Printf("error creating http client: %s", err)
+		die()
+	}
+	err = jsonRequestResponse(httpc, daemon.ControlJobEndpointPProf, pprofListenCmd.PprofServerControlMsg, struct{}{})
+	if err != nil {
+		log.Printf("error sending control message: %s", err)
+		die()
+	}
+	log.Printf("finished")
+}

client/signal.go

@@ -1,6 +1,8 @@
 package client
 
 import (
+	"context"
+
 	"github.com/pkg/errors"
 
 	"github.com/zrepl/zrepl/cli"
@@ -11,7 +13,7 @@ import (
 var SignalCmd = &cli.Subcommand{
 	Use:   "signal [wakeup|reset] JOB",
 	Short: "wake up a job from wait state or abort its current invocation",
-	Run: func(subcommand *cli.Subcommand, args []string) error {
+	Run: func(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 		return runSignalCmd(subcommand.Config(), args)
 	},
 }

client/status.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"context"
 	"fmt"
 	"io"
 	"math"
@@ -180,7 +181,7 @@ var StatusCmd = &cli.Subcommand{
 	Run: runStatus,
 }
 
-func runStatus(s *cli.Subcommand, args []string) error {
+func runStatus(ctx context.Context, s *cli.Subcommand, args []string) error {
 	httpc, err := controlHttpClient(s.Config().Global.Control.SockPath)
 	if err != nil {
 		return err

client/stdinserver.go

@@ -17,7 +17,7 @@ import (
 var StdinserverCmd = &cli.Subcommand{
 	Use:   "stdinserver CLIENT_IDENTITY",
 	Short: "stdinserver transport mode (started from authorized_keys file as forced command)",
-	Run: func(subcommand *cli.Subcommand, args []string) error {
+	Run: func(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 		return runStdinserver(subcommand.Config(), args)
 	},
 }

client/testcmd.go

@@ -39,7 +39,7 @@ var testFilter = &cli.Subcommand{
 	Run: runTestFilterCmd,
 }
 
-func runTestFilterCmd(subcommand *cli.Subcommand, args []string) error {
+func runTestFilterCmd(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 
 	if testFilterArgs.job == "" {
 		return fmt.Errorf("must specify --job flag")
@@ -49,7 +49,6 @@ func runTestFilterCmd(subcommand *cli.Subcommand, args []string) error {
 	}
 
 	conf := subcommand.Config()
-	ctx := context.Background()
 
 	var confFilter config.FilesystemsFilter
 	job, err := conf.Job(testFilterArgs.job)
@@ -137,10 +136,9 @@ var testPlaceholder = &cli.Subcommand{
 	Run: runTestPlaceholder,
 }
 
-func runTestPlaceholder(subcommand *cli.Subcommand, args []string) error {
+func runTestPlaceholder(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 
 	var checkDPs []*zfs.DatasetPath
-	ctx := context.Background()
 
 	// all actions first
 	if testPlaceholderArgs.all {
@@ -197,11 +195,11 @@ var testDecodeResumeToken = &cli.Subcommand{
 	Run: runTestDecodeResumeTokenCmd,
 }
 
-func runTestDecodeResumeTokenCmd(subcommand *cli.Subcommand, args []string) error {
+func runTestDecodeResumeTokenCmd(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 	if testDecodeResumeTokenArgs.token == "" {
 		return fmt.Errorf("token argument must be specified")
 	}
-	token, err := zfs.ParseResumeToken(context.Background(), testDecodeResumeTokenArgs.token)
+	token, err := zfs.ParseResumeToken(ctx, testDecodeResumeTokenArgs.token)
 	if err != nil {
 		return err
 	}

client/version.go

@@ -1,6 +1,7 @@
 package client
 
 import (
+	"context"
 	"fmt"
 	"os"
 
@@ -25,7 +26,7 @@ var VersionCmd = &cli.Subcommand{
 	SetupFlags: func(f *pflag.FlagSet) {
 		f.StringVar(&versionArgs.Show, "show", "", "version info to show (client|daemon)")
 	},
-	Run: func(subcommand *cli.Subcommand, args []string) error {
+	Run: func(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
 		versionArgs.Config = subcommand.Config()
 		versionArgs.ConfigErr = subcommand.ConfigParsingError()
 		return runVersionCmd()

client/zfsabstractions_create_step_hold.go

@@ -28,7 +28,7 @@ var zabsCmdCreateStepHold = &cli.Subcommand{
 	},
 }
 
-func doZabsCreateStep(sc *cli.Subcommand, args []string) error {
+func doZabsCreateStep(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	if len(args) > 0 {
 		return errors.New("subcommand takes no arguments")
 	}
@@ -44,8 +44,6 @@ func doZabsCreateStep(sc *cli.Subcommand, args []string) error {
 		return errors.Errorf("jobid must be set")
 	}
 
-	ctx := context.Background()
-
 	v, err := zfs.ZFSGetFilesystemVersion(ctx, f.target)
 	if err != nil {
 		return errors.Wrapf(err, "get info about target %q", f.target)

client/zfsabstractions_list.go

@@ -32,9 +32,8 @@ var zabsCmdList = &cli.Subcommand{
 	},
 }
 
-func doZabsList(sc *cli.Subcommand, args []string) error {
+func doZabsList(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	var err error
-	ctx := context.Background()
 
 	if len(args) > 0 {
 		return errors.New("this subcommand takes no positional arguments")

client/zfsabstractions_release.go

@@ -43,9 +43,8 @@ var zabsCmdReleaseStale = &cli.Subcommand{
 	SetupFlags:      registerZabsReleaseFlags,
 }
 
-func doZabsReleaseAll(sc *cli.Subcommand, args []string) error {
+func doZabsReleaseAll(ctx context.Context, sc *cli.Subcommand, args []string) error {
 	var err error
-	ctx := context.Background()
 
 	if len(args) > 0 {
 		return errors.New("this subcommand takes no positional arguments")
@@ -68,10 +67,9 @@ func doZabsReleaseAll(sc *cli.Subcommand, args []string) error {
 	return doZabsRelease_Common(ctx, abstractions)
 }
 
-func doZabsReleaseStale(sc *cli.Subcommand, args []string) error {
+func doZabsReleaseStale(ctx context.Context, sc *cli.Subcommand, args []string) error {
 
 	var err error
-	ctx := context.Background()
 
 	if len(args) > 0 {
 		return errors.New("this subcommand takes no positional arguments")

daemon/daemon.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/job"
@@ -23,9 +24,8 @@ import (
 	"github.com/zrepl/zrepl/zfs/zfscmd"
 )
 
-func Run(conf *config.Config) error {
-
-	ctx, cancel := context.WithCancel(context.Background())
+func Run(ctx context.Context, conf *config.Config) error {
+	ctx, cancel := context.WithCancel(ctx)
 
 	defer cancel()
 	sigChan := make(chan os.Signal, 1)
@@ -39,6 +39,7 @@ func Run(conf *config.Config) error {
 	if err != nil {
 		return errors.Wrap(err, "cannot build logging from config")
 	}
+	outlets.Add(newPrometheusLogOutlet(), logger.Debug)
 
 	confJobs, err := job.JobsFromConfig(conf)
 	if err != nil {
@@ -48,14 +49,23 @@ func Run(conf *config.Config) error {
 	log := logger.NewLogger(outlets, 1*time.Second)
 	log.Info(version.NewZreplVersionInformation().String())
 
+	ctx = logging.WithLoggers(ctx, logging.SubsystemLoggersWithUniversalLogger(log))
+	trace.RegisterCallback(trace.Callback{
+		OnBegin: func(ctx context.Context) { logging.GetLogger(ctx, logging.SubsysTraceData).Debug("begin span") },
+		OnEnd: func(ctx context.Context, spanInfo trace.SpanInfo) {
+			logging.
+				GetLogger(ctx, logging.SubsysTraceData).
+				WithField("duration_s", spanInfo.EndedAt().Sub(spanInfo.StartedAt()).Seconds()).
+				Debug("finished span " + spanInfo.TaskAndSpanStack(trace.SpanStackKindAnnotation))
+		},
+	})
+
 	for _, job := range confJobs {
 		if IsInternalJobName(job.Name()) {
 			panic(fmt.Sprintf("internal job name used for config job '%s'", job.Name())) //FIXME
 		}
 	}
 
-	ctx = job.WithLogger(ctx, log)
-
 	jobs := newJobs()
 
 	// start control socket
@@ -84,6 +94,7 @@ func Run(conf *config.Config) error {
 
 	// register global (=non job-local) metrics
 	zfscmd.RegisterMetrics(prometheus.DefaultRegisterer)
+	trace.RegisterMetrics(prometheus.DefaultRegisterer)
 
 	log.Info("starting daemon")
 
@@ -98,6 +109,8 @@ func Run(conf *config.Config) error {
 	case <-ctx.Done():
 		log.WithError(ctx.Err()).Info("context finished")
 	}
+	log.Info("waiting for jobs to finish")
+	<-jobs.wait()
 	log.Info("daemon exiting")
 	return nil
 }
@@ -120,14 +133,11 @@ func newJobs() *jobs {
 	}
 }
 
-const (
-	logJobField string = "job"
-)
-
 func (s *jobs) wait() <-chan struct{} {
 	ch := make(chan struct{})
 	go func() {
 		s.wg.Wait()
+		close(ch)
 	}()
 	return ch
 }
@@ -202,9 +212,8 @@ func (s *jobs) start(ctx context.Context, j job.Job, internal bool) {
 	s.m.Lock()
 	defer s.m.Unlock()
 
-	jobLog := job.GetLogger(ctx).
-		WithField(logJobField, j.Name()).
-		WithOutlet(newPrometheusLogOutlet(j.Name()), logger.Debug)
+	ctx = logging.WithInjectedField(ctx, logging.JobField, j.Name())
+
 	jobName := j.Name()
 	if !internal && IsInternalJobName(jobName) {
 		panic(fmt.Sprintf("internal job name used for non-internal job %s", jobName))
@@ -219,7 +228,6 @@ func (s *jobs) start(ctx context.Context, j job.Job, internal bool) {
 	j.RegisterMetrics(prometheus.DefaultRegisterer)
 
 	s.jobs[jobName] = j
-	ctx = job.WithLogger(ctx, jobLog)
 	ctx = zfscmd.WithJobID(ctx, j.Name())
 	ctx, wakeup := wakeup.Context(ctx)
 	ctx, resetFunc := reset.Context(ctx)
@@ -229,8 +237,8 @@ func (s *jobs) start(ctx context.Context, j job.Job, internal bool) {
 	s.wg.Add(1)
 	go func() {
 		defer s.wg.Done()
-		jobLog.Info("starting job")
-		defer jobLog.Info("job exited")
+		job.GetLogger(ctx).Info("starting job")
+		defer job.GetLogger(ctx).Info("job exited")
 		j.Run(ctx)
 	}()
 }

daemon/hooks/hook_logging.go

@@ -6,29 +6,17 @@ import (
 	"context"
 	"sync"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/util/envconst"
 )
 
-type contextKey int
-
-const (
-	contextKeyLog contextKey = 0
-)
-
 type Logger = logger.Logger
 
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLog, log)
-}
-
 func GetLogger(ctx context.Context) Logger { return getLogger(ctx) }
 
 func getLogger(ctx context.Context) Logger {
-	if log, ok := ctx.Value(contextKeyLog).(Logger); ok {
-		return log
-	}
-	return logger.NewNullLogger()
+	return logging.GetLogger(ctx, logging.SubsysHooks)
 }
 
 const MAX_HOOK_LOG_SIZE_DEFAULT int = 1 << 20

daemon/hooks/hooks_test.go

@@ -10,9 +10,11 @@ import (
 	"text/template"
 
 	"github.com/stretchr/testify/require"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/zfs"
 )
@@ -69,6 +71,9 @@ func curry(f comparisonAssertionFunc, expected interface{}, right bool) (ret val
 }
 
 func TestHooks(t *testing.T) {
+	ctx, end := trace.WithTaskFromStack(context.Background())
+	defer end()
+
 	testFSName := "testpool/testdataset"
 	testSnapshotName := "testsnap"
 
@@ -418,9 +423,8 @@ jobs:
 
 			cbReached = false
 
-			ctx := context.Background()
 			if testing.Verbose() && !tt.SuppressOutput {
-				ctx = hooks.WithLogger(ctx, log)
+				ctx = logging.WithLoggers(ctx, logging.SubsystemLoggersWithUniversalLogger(log))
 			}
 			plan.Run(ctx, false)
 			report := plan.Report()

daemon/job/active.go

@@ -8,12 +8,13 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/common/log"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/filters"
 	"github.com/zrepl/zrepl/daemon/job/reset"
 	"github.com/zrepl/zrepl/daemon/job/wakeup"
-	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/daemon/pruner"
 	"github.com/zrepl/zrepl/daemon/snapper"
 	"github.com/zrepl/zrepl/endpoint"
@@ -79,7 +80,7 @@ func (a *ActiveSide) updateTasks(u func(*activeSideTasks)) activeSideTasks {
 }
 
 type activeMode interface {
-	ConnectEndpoints(rpcLoggers rpc.Loggers, connecter transport.Connecter)
+	ConnectEndpoints(ctx context.Context, connecter transport.Connecter)
 	DisconnectEndpoints()
 	SenderReceiver() (logic.Sender, logic.Receiver)
 	Type() Type
@@ -98,14 +99,14 @@ type modePush struct {
 	snapper       *snapper.PeriodicOrManual
 }
 
-func (m *modePush) ConnectEndpoints(loggers rpc.Loggers, connecter transport.Connecter) {
+func (m *modePush) ConnectEndpoints(ctx context.Context, connecter transport.Connecter) {
 	m.setupMtx.Lock()
 	defer m.setupMtx.Unlock()
 	if m.receiver != nil || m.sender != nil {
 		panic("inconsistent use of ConnectEndpoints and DisconnectEndpoints")
 	}
 	m.sender = endpoint.NewSender(*m.senderConfig)
-	m.receiver = rpc.NewClient(connecter, loggers)
+	m.receiver = rpc.NewClient(connecter, rpc.GetLoggersOrPanic(ctx))
 }
 
 func (m *modePush) DisconnectEndpoints() {
@@ -176,14 +177,14 @@ type modePull struct {
 	interval       config.PositiveDurationOrManual
 }
 
-func (m *modePull) ConnectEndpoints(loggers rpc.Loggers, connecter transport.Connecter) {
+func (m *modePull) ConnectEndpoints(ctx context.Context, connecter transport.Connecter) {
 	m.setupMtx.Lock()
 	defer m.setupMtx.Unlock()
 	if m.receiver != nil || m.sender != nil {
 		panic("inconsistent use of ConnectEndpoints and DisconnectEndpoints")
 	}
 	m.receiver = endpoint.NewReceiver(m.receiverConfig)
-	m.sender = rpc.NewClient(connecter, loggers)
+	m.sender = rpc.NewClient(connecter, rpc.GetLoggersOrPanic(ctx))
 }
 
 func (m *modePull) DisconnectEndpoints() {
@@ -376,15 +377,18 @@ func (j *ActiveSide) SenderConfig() *endpoint.SenderConfig {
 }
 
 func (j *ActiveSide) Run(ctx context.Context) {
+	ctx, endTask := trace.WithTaskAndSpan(ctx, "active-side-job", j.Name())
+	defer endTask()
 	log := GetLogger(ctx)
-	ctx = logging.WithSubsystemLoggers(ctx, log)
 
 	defer log.Info("job exiting")
 
 	periodicDone := make(chan struct{})
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
-	go j.mode.RunPeriodic(ctx, periodicDone)
+	periodicCtx, endTask := trace.WithTask(ctx, "periodic")
+	defer endTask()
+	go j.mode.RunPeriodic(periodicCtx, periodicDone)
 
 	invocationCount := 0
 outer:
@@ -400,17 +404,15 @@ outer:
 		case <-periodicDone:
 		}
 		invocationCount++
-		invLog := log.WithField("invocation", invocationCount)
-		j.do(WithLogger(ctx, invLog))
+		invocationCtx, endSpan := trace.WithSpan(ctx, fmt.Sprintf("invocation-%d", invocationCount))
+		j.do(invocationCtx)
+		endSpan()
 	}
 }
 
 func (j *ActiveSide) do(ctx context.Context) {
 
-	log := GetLogger(ctx)
-	ctx = logging.WithSubsystemLoggers(ctx, log)
-	loggers := rpc.GetLoggersOrPanic(ctx) // filled by WithSubsystemLoggers
-	j.mode.ConnectEndpoints(loggers, j.connecter)
+	j.mode.ConnectEndpoints(ctx, j.connecter)
 	defer j.mode.DisconnectEndpoints()
 
 	// allow cancellation of an invocation (this function)
@@ -433,20 +435,22 @@ func (j *ActiveSide) do(ctx context.Context) {
 			return
 		default:
 		}
+		ctx, endSpan := trace.WithSpan(ctx, "replication")
 		ctx, repCancel := context.WithCancel(ctx)
 		var repWait driver.WaitFunc
 		j.updateTasks(func(tasks *activeSideTasks) {
 			// reset it
 			*tasks = activeSideTasks{}
-			tasks.replicationCancel = repCancel
+			tasks.replicationCancel = func() { repCancel(); endSpan() }
 			tasks.replicationReport, repWait = replication.Do(
 				ctx, logic.NewPlanner(j.promRepStateSecs, j.promBytesReplicated, sender, receiver, j.mode.PlannerPolicy()),
 			)
 			tasks.state = ActiveSideReplicating
 		})
-		log.Info("start replication")
+		GetLogger(ctx).Info("start replication")
 		repWait(true) // wait blocking
 		repCancel()   // always cancel to free up context resources
+		endSpan()
 	}
 
 	{
@@ -455,16 +459,18 @@ func (j *ActiveSide) do(ctx context.Context) {
 			return
 		default:
 		}
+		ctx, endSpan := trace.WithSpan(ctx, "prune_sender")
 		ctx, senderCancel := context.WithCancel(ctx)
 		tasks := j.updateTasks(func(tasks *activeSideTasks) {
 			tasks.prunerSender = j.prunerFactory.BuildSenderPruner(ctx, sender, sender)
-			tasks.prunerSenderCancel = senderCancel
+			tasks.prunerSenderCancel = func() { senderCancel(); endSpan() }
 			tasks.state = ActiveSidePruneSender
 		})
-		log.Info("start pruning sender")
+		GetLogger(ctx).Info("start pruning sender")
 		tasks.prunerSender.Prune()
-		log.Info("finished pruning sender")
+		GetLogger(ctx).Info("finished pruning sender")
 		senderCancel()
+		endSpan()
 	}
 	{
 		select {
@@ -472,16 +478,18 @@ func (j *ActiveSide) do(ctx context.Context) {
 			return
 		default:
 		}
+		ctx, endSpan := trace.WithSpan(ctx, "prune_recever")
 		ctx, receiverCancel := context.WithCancel(ctx)
 		tasks := j.updateTasks(func(tasks *activeSideTasks) {
 			tasks.prunerReceiver = j.prunerFactory.BuildReceiverPruner(ctx, receiver, sender)
-			tasks.prunerReceiverCancel = receiverCancel
+			tasks.prunerReceiverCancel = func() { receiverCancel(); endSpan() }
 			tasks.state = ActiveSidePruneReceiver
 		})
-		log.Info("start pruning receiver")
+		GetLogger(ctx).Info("start pruning receiver")
 		tasks.prunerReceiver.Prune()
-		log.Info("finished pruning receiver")
+		GetLogger(ctx).Info("finished pruning receiver")
 		receiverCancel()
+		endSpan()
 	}
 
 	j.updateTasks(func(tasks *activeSideTasks) {

daemon/job/job.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/endpoint"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/zfs"
@@ -14,21 +15,8 @@ import (
 
 type Logger = logger.Logger
 
-type contextKey int
-
-const (
-	contextKeyLog contextKey = iota
-)
-
 func GetLogger(ctx context.Context) Logger {
-	if l, ok := ctx.Value(contextKeyLog).(Logger); ok {
-		return l
-	}
-	return logger.NewNullLogger()
-}
-
-func WithLogger(ctx context.Context, l Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLog, l)
+	return logging.GetLogger(ctx, logging.SubsysJob)
 }
 
 type Job interface {

daemon/job/passive.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/filters"
@@ -164,12 +165,14 @@ func (j *PassiveSide) SenderConfig() *endpoint.SenderConfig {
 func (*PassiveSide) RegisterMetrics(registerer prometheus.Registerer) {}
 
 func (j *PassiveSide) Run(ctx context.Context) {
-
+	ctx, endTask := trace.WithTaskAndSpan(ctx, "passive-side-job", j.Name())
+	defer endTask()
 	log := GetLogger(ctx)
 	defer log.Info("job exiting")
-	ctx = logging.WithSubsystemLoggers(ctx, log)
 	{
-		ctx, cancel := context.WithCancel(ctx) // shadowing
+		ctx, endTask := trace.WithTask(ctx, "periodic") // shadowing
+		defer endTask()
+		ctx, cancel := context.WithCancel(ctx)
 		defer cancel()
 		go j.mode.RunPeriodic(ctx)
 	}
@@ -179,8 +182,14 @@ func (j *PassiveSide) Run(ctx context.Context) {
 		panic(fmt.Sprintf("implementation error: j.mode.Handler() returned nil: %#v", j))
 	}
 
-	ctxInterceptor := func(handlerCtx context.Context) context.Context {
-		return logging.WithSubsystemLoggers(handlerCtx, log)
+	ctxInterceptor := func(handlerCtx context.Context, info rpc.HandlerContextInterceptorData, handler func(ctx context.Context)) {
+		// the handlerCtx is clean => need to inherit logging and tracing config from job context
+		handlerCtx = logging.WithInherit(handlerCtx, ctx)
+		handlerCtx = trace.WithInherit(handlerCtx, ctx)
+
+		handlerCtx, endTask := trace.WithTaskAndSpan(handlerCtx, "handler", fmt.Sprintf("job=%q client=%q method=%q", j.Name(), info.ClientIdentity(), info.FullMethod()))
+		defer endTask()
+		handler(handlerCtx)
 	}
 
 	rpcLoggers := rpc.GetLoggersOrPanic(ctx) // WithSubsystemLoggers above

daemon/job/snapjob.go

@@ -2,15 +2,16 @@ package job
 
 import (
 	"context"
+	"fmt"
 	"sort"
 
 	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/filters"
 	"github.com/zrepl/zrepl/daemon/job/wakeup"
-	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/daemon/pruner"
 	"github.com/zrepl/zrepl/daemon/snapper"
 	"github.com/zrepl/zrepl/endpoint"
@@ -89,15 +90,18 @@ func (j *SnapJob) OwnedDatasetSubtreeRoot() (rfs *zfs.DatasetPath, ok bool) {
 func (j *SnapJob) SenderConfig() *endpoint.SenderConfig { return nil }
 
 func (j *SnapJob) Run(ctx context.Context) {
+	ctx, endTask := trace.WithTaskAndSpan(ctx, "snap-job", j.Name())
+	defer endTask()
 	log := GetLogger(ctx)
-	ctx = logging.WithSubsystemLoggers(ctx, log)
 
 	defer log.Info("job exiting")
 
 	periodicDone := make(chan struct{})
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
-	go j.snapper.Run(ctx, periodicDone)
+	periodicCtx, endTask := trace.WithTask(ctx, "snapshotting")
+	defer endTask()
+	go j.snapper.Run(periodicCtx, periodicDone)
 
 	invocationCount := 0
 outer:
@@ -112,8 +116,10 @@ outer:
 		case <-periodicDone:
 		}
 		invocationCount++
-		invLog := log.WithField("invocation", invocationCount)
-		j.doPrune(WithLogger(ctx, invLog))
+
+		invocationCtx, endSpan := trace.WithSpan(ctx, fmt.Sprintf("invocation-%d", invocationCount))
+		j.doPrune(invocationCtx)
+		endSpan()
 	}
 }
 
@@ -161,8 +167,9 @@ func (h alwaysUpToDateReplicationCursorHistory) ListFilesystems(ctx context.Cont
 }
 
 func (j *SnapJob) doPrune(ctx context.Context) {
+	ctx, endSpan := trace.WithSpan(ctx, "snap-job-do-prune")
+	defer endSpan()
 	log := GetLogger(ctx)
-	ctx = logging.WithSubsystemLoggers(ctx, log)
 	sender := endpoint.NewSender(endpoint.SenderConfig{
 		JobID: j.name,
 		FSF:   j.fsfilter,

daemon/logging/build_logging.go

@@ -9,20 +9,10 @@ import (
 
 	"github.com/mattn/go-isatty"
 	"github.com/pkg/errors"
-
 	"github.com/zrepl/zrepl/config"
-	"github.com/zrepl/zrepl/daemon/hooks"
-	"github.com/zrepl/zrepl/daemon/pruner"
-	"github.com/zrepl/zrepl/daemon/snapper"
-	"github.com/zrepl/zrepl/endpoint"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"github.com/zrepl/zrepl/logger"
-	"github.com/zrepl/zrepl/replication/driver"
-	"github.com/zrepl/zrepl/replication/logic"
-	"github.com/zrepl/zrepl/rpc"
-	"github.com/zrepl/zrepl/rpc/transportmux"
 	"github.com/zrepl/zrepl/tlsconf"
-	"github.com/zrepl/zrepl/transport"
-	"github.com/zrepl/zrepl/zfs/zfscmd"
 )
 
 func OutletsFromConfig(in config.LoggingOutletEnumList) (*logger.Outlets, error) {
@@ -70,6 +60,8 @@ func OutletsFromConfig(in config.LoggingOutletEnumList) (*logger.Outlets, error)
 type Subsystem string
 
 const (
+	SubsysMeta         Subsystem = "meta"
+	SubsysJob          Subsystem = "job"
 	SubsysReplication  Subsystem = "repl"
 	SubsysEndpoint     Subsystem = "endpoint"
 	SubsysPruning      Subsystem = "pruning"
@@ -81,30 +73,103 @@ const (
 	SubsysRPCControl   Subsystem = "rpc.ctrl"
 	SubsysRPCData      Subsystem = "rpc.data"
 	SubsysZFSCmd       Subsystem = "zfs.cmd"
+	SubsysTraceData    Subsystem = "trace.data"
+	SubsysPlatformtest Subsystem = "platformtest"
 )
 
-func WithSubsystemLoggers(ctx context.Context, log logger.Logger) context.Context {
-	ctx = logic.WithLogger(ctx, log.WithField(SubsysField, SubsysReplication))
-	ctx = driver.WithLogger(ctx, log.WithField(SubsysField, SubsysReplication))
-	ctx = endpoint.WithLogger(ctx, log.WithField(SubsysField, SubsysEndpoint))
-	ctx = pruner.WithLogger(ctx, log.WithField(SubsysField, SubsysPruning))
-	ctx = snapper.WithLogger(ctx, log.WithField(SubsysField, SubsysSnapshot))
-	ctx = hooks.WithLogger(ctx, log.WithField(SubsysField, SubsysHooks))
-	ctx = transport.WithLogger(ctx, log.WithField(SubsysField, SubsysTransport))
-	ctx = transportmux.WithLogger(ctx, log.WithField(SubsysField, SubsysTransportMux))
-	ctx = zfscmd.WithLogger(ctx, log.WithField(SubsysField, SubsysZFSCmd))
-	ctx = rpc.WithLoggers(ctx,
-		rpc.Loggers{
-			General: log.WithField(SubsysField, SubsysRPC),
-			Control: log.WithField(SubsysField, SubsysRPCControl),
-			Data:    log.WithField(SubsysField, SubsysRPCData),
-		},
-	)
-	return ctx
+var AllSubsystems = []Subsystem{
+	SubsysMeta,
+	SubsysJob,
+	SubsysReplication,
+	SubsysEndpoint,
+	SubsysPruning,
+	SubsysSnapshot,
+	SubsysHooks,
+	SubsysTransport,
+	SubsysTransportMux,
+	SubsysRPC,
+	SubsysRPCControl,
+	SubsysRPCData,
+	SubsysZFSCmd,
+	SubsysTraceData,
+	SubsysPlatformtest,
 }
 
-func LogSubsystem(log logger.Logger, subsys Subsystem) logger.Logger {
-	return log.ReplaceField(SubsysField, subsys)
+type injectedField struct {
+	field  string
+	value  interface{}
+	parent *injectedField
+}
+
+func WithInjectedField(ctx context.Context, field string, value interface{}) context.Context {
+	var parent *injectedField
+	parentI := ctx.Value(contextKeyInjectedField)
+	if parentI != nil {
+		parent = parentI.(*injectedField)
+	}
+	// TODO sanity-check `field` now
+	this := &injectedField{field, value, parent}
+	return context.WithValue(ctx, contextKeyInjectedField, this)
+}
+
+func iterInjectedFields(ctx context.Context, cb func(field string, value interface{})) {
+	injI := ctx.Value(contextKeyInjectedField)
+	if injI == nil {
+		return
+	}
+	inj := injI.(*injectedField)
+	for ; inj != nil; inj = inj.parent {
+		cb(inj.field, inj.value)
+	}
+}
+
+type SubsystemLoggers map[Subsystem]logger.Logger
+
+func SubsystemLoggersWithUniversalLogger(l logger.Logger) SubsystemLoggers {
+	loggers := make(SubsystemLoggers)
+	for _, s := range AllSubsystems {
+		loggers[s] = l
+	}
+	return loggers
+}
+
+func WithLoggers(ctx context.Context, loggers SubsystemLoggers) context.Context {
+	return context.WithValue(ctx, contextKeyLoggers, loggers)
+}
+
+func GetLoggers(ctx context.Context) SubsystemLoggers {
+	loggers, ok := ctx.Value(contextKeyLoggers).(SubsystemLoggers)
+	if !ok {
+		return nil
+	}
+	return loggers
+}
+
+func GetLogger(ctx context.Context, subsys Subsystem) logger.Logger {
+	return getLoggerImpl(ctx, subsys, true)
+}
+
+func getLoggerImpl(ctx context.Context, subsys Subsystem, panicIfEnded bool) logger.Logger {
+	loggers, ok := ctx.Value(contextKeyLoggers).(SubsystemLoggers)
+	if !ok || loggers == nil {
+		return logger.NewNullLogger()
+	}
+	l, ok := loggers[subsys]
+	if !ok {
+		return logger.NewNullLogger()
+	}
+
+	l = l.WithField(SubsysField, subsys)
+
+	l = l.WithField(SpanField, trace.GetSpanStackOrDefault(ctx, *trace.StackKindId, "NOSPAN"))
+
+	fields := make(logger.Fields)
+	iterInjectedFields(ctx, func(field string, value interface{}) {
+		fields[field] = value
+	})
+	l = l.WithFields(fields)
+
+	return l
 }
 
 func parseLogFormat(i interface{}) (f EntryFormatter, err error) {

daemon/logging/logging_context.go

@@ -0,0 +1,24 @@
+package logging
+
+import "context"
+
+type contextKey int
+
+const (
+	contextKeyLoggers contextKey = 1 + iota
+	contextKeyInjectedField
+)
+
+var contextKeys = []contextKey{
+	contextKeyLoggers,
+	contextKeyInjectedField,
+}
+
+func WithInherit(ctx, inheritFrom context.Context) context.Context {
+	for _, k := range contextKeys {
+		if v := inheritFrom.Value(k); v != nil {
+			ctx = context.WithValue(ctx, k, v) // no shadow
+		}
+	}
+	return ctx
+}

daemon/logging/logging_formatters.go

@@ -22,6 +22,7 @@ const (
 const (
 	JobField    string = "job"
 	SubsysField string = "subsystem"
+	SpanField   string = "span"
 )
 
 type MetadataFlags int64
@@ -85,7 +86,7 @@ func (f *HumanFormatter) Format(e *logger.Entry) (out []byte, err error) {
 		fmt.Fprintf(&line, "[%s]", col.Sprint(e.Level.Short()))
 	}
 
-	prefixFields := []string{JobField, SubsysField}
+	prefixFields := []string{JobField, SubsysField, SpanField}
 	prefixed := make(map[string]bool, len(prefixFields)+2)
 	for _, field := range prefixFields {
 		val, ok := e.Fields[field]
@@ -174,8 +175,8 @@ func (f *LogfmtFormatter) Format(e *logger.Entry) ([]byte, error) {
 	}
 
 	// at least try and put job and task in front
-	prefixed := make(map[string]bool, 2)
-	prefix := []string{JobField, SubsysField}
+	prefixed := make(map[string]bool, 3)
+	prefix := []string{JobField, SubsysField, SpanField}
 	for _, pf := range prefix {
 		v, ok := e.Fields[pf]
 		if !ok {

daemon/logging/trace/trace.go

@@ -0,0 +1,386 @@
+// package trace provides activity tracing via ctx through Tasks and Spans
+//
+// Basic Concepts
+//
+// Tracing can be used to identify where a piece of code spends its time.
+//
+// The Go standard library provides package runtime/trace which is useful to identify CPU bottlenecks or
+// to understand what happens inside the Go runtime.
+// However, it is not ideal for application level tracing, in particular if those traces should be understandable
+// to tech-savvy users (albeit not developers).
+//
+// This package provides the concept of Tasks and Spans to express what activity is happening within an application:
+//
+//  - Neither task nor span is really tangible but instead contained within the context.Context tree
+//  - Tasks represent concurrent activity (i.e. goroutines).
+//  - Spans represent a semantic stack trace within a task.
+//
+// As a consequence, whenever a context is propagated across goroutine boundary, you need to create a child task:
+//
+//   go func(ctx context.Context) {
+//     ctx, endTask = WithTask(ctx, "what-happens-inside-the-child-task")
+//     defer endTask()
+//     // ...
+//   }(ctx)
+//
+// Within the task, you can open up a hierarchy of spans.
+// In contrast to tasks, which have can multiple concurrently running child tasks,
+// spans must nest and not cross the goroutine boundary.
+//
+//  ctx, endSpan = WithSpan(ctx, "copy-dir")
+//  defer endSpan()
+//  for _, f := range dir.Files() {
+//    func() {
+//      ctx, endSpan := WithSpan(ctx, fmt.Sprintf("copy-file %q", f))
+//      defer endspan()
+//      b, _ := ioutil.ReadFile(f)
+//      _ = ioutil.WriteFile(f + ".copy", b, 0600)
+//    }()
+//  }
+//
+// In combination:
+//  ctx, endTask = WithTask(ctx, "copy-dirs")
+//  defer endTask()
+//  for i := range dirs {
+//    go func(dir string) {
+//      ctx, endTask := WithTask(ctx, "copy-dir")
+//      defer endTask()
+//      for _, f := range filesIn(dir) {
+//        func() {
+//          ctx, endSpan := WithSpan(ctx, fmt.Sprintf("copy-file %q", f))
+//          defer endspan()
+//          b, _ := ioutil.ReadFile(f)
+//          _ = ioutil.WriteFile(f + ".copy", b, 0600)
+//        }()
+//      }
+//    }()
+//  }
+//
+// Note that a span ends at the time you call endSpan - not before and not after that.
+// If you violate the stack-like nesting of spans by forgetting an endSpan() invocation,
+// the out-of-order endSpan() will panic.
+//
+// A similar rule applies to the endTask closure returned by WithTask:
+// If a task has live child tasks at the time you call endTask(), the call will panic.
+//
+// Recovering from endSpan() or endTask() panics will corrupt the trace stack and lead to corrupt tracefile output.
+//
+//
+// Best Practices For Naming Tasks And Spans
+//
+// Tasks should always have string constants as names, and must not contain the `#` character. WHy?
+// First, the visualization by chrome://tracing draws a horizontal bar for each task in the trace.
+// Also, the package appends `#NUM` for each concurrently running instance of a task name.
+// Note that the `#NUM` suffix will be reused if a task has ended, in order to avoid an
+// infinite number of horizontal bars in the visualization.
+//
+//
+// Chrome-compatible Tracefile Support
+//
+// The activity trace generated by usage of WithTask and WithSpan can be rendered to a JSON output file
+// that can be loaded into chrome://tracing .
+// Apart from function GetSpanStackOrDefault, this is the main benefit of this package.
+//
+// First, there is a convenience environment variable 'ZREPL_ACTIVITY_TRACE' that can be set to an output path.
+// From process start onward, a trace is written to that path.
+//
+// More consumers can attach to the activity trace through the ChrometraceClientWebsocketHandler websocket handler.
+//
+// If a write error is encountered with any consumer (including the env-var based one), the consumer is closed and
+// will not receive further trace output.
+package trace
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/zrepl/zrepl/util/chainlock"
+)
+
+var metrics struct {
+	activeTasks prometheus.Gauge
+}
+var taskNamer *uniqueConcurrentTaskNamer = newUniqueTaskNamer()
+
+func init() {
+	metrics.activeTasks = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "zrepl",
+		Subsystem: "trace",
+		Name:      "active_tasks",
+		Help:      "number of active (tracing-level) tasks in the daemon",
+	})
+}
+
+func RegisterMetrics(r prometheus.Registerer) {
+	r.MustRegister(metrics.activeTasks)
+}
+
+type traceNode struct {
+	id         string
+	annotation string
+	parentTask *traceNode
+
+	mtx chainlock.L
+
+	activeChildTasks int32 // only for task nodes, insignificant for span nodes
+	parentSpan       *traceNode
+	activeChildSpan  *traceNode // nil if task or span doesn't have an active child span
+
+	startedAt time.Time
+	endedAt   time.Time
+}
+
+func (s *traceNode) StartedAt() time.Time { return s.startedAt }
+func (s *traceNode) EndedAt() time.Time   { return s.endedAt }
+
+// Returned from WithTask or WithSpan.
+// Must be called once the task or span ends.
+// See package-level docs for nesting rules.
+// Wrong call order / forgetting to call it will result in panics.
+type DoneFunc func()
+
+var ErrTaskStillHasActiveChildTasks = fmt.Errorf("end task: task still has active child tasks")
+
+// Start a new root task or create a child task of an existing task.
+//
+// This is required when starting a new goroutine and
+// passing an existing task context to it.
+//
+// taskName should be a constantand must not contain '#'
+//
+// The implementation ensures that,
+// if multiple tasks with the same name exist simultaneously,
+// a unique suffix is appended to uniquely identify the task opened with this function.
+func WithTask(ctx context.Context, taskName string) (context.Context, DoneFunc) {
+
+	var parentTask *traceNode
+	nodeI := ctx.Value(contextKeyTraceNode)
+	if nodeI != nil {
+		node := nodeI.(*traceNode)
+		if node.parentSpan != nil {
+			parentTask = node.parentTask
+		} else {
+			parentTask = node
+		}
+	}
+	// find the first ancestor that hasn't ended yet (nil if need be)
+	if parentTask != nil {
+		parentTask.mtx.Lock()
+	}
+	for parentTask != nil && !parentTask.endedAt.IsZero() {
+		thisParent := parentTask
+		parentTask = parentTask.parentTask
+		// lock parent first such that it isn't modified by other callers to thisParent
+		if parentTask != nil {
+			parentTask.mtx.Lock()
+		}
+		thisParent.mtx.Unlock()
+	}
+	// invariant: either parentTask != nil and we hold the lock on parentTask, or parentTask is nil
+
+	taskName, taskNameDone := taskNamer.UniqueConcurrentTaskName(taskName)
+
+	this := &traceNode{
+		id:               genID(),
+		annotation:       taskName,
+		parentTask:       parentTask,
+		activeChildTasks: 0,
+		parentSpan:       nil,
+		activeChildSpan:  nil,
+
+		startedAt: time.Now(),
+		endedAt:   time.Time{},
+	}
+
+	if parentTask != nil {
+		this.parentTask.activeChildTasks++
+		parentTask.mtx.Unlock()
+	}
+
+	ctx = context.WithValue(ctx, contextKeyTraceNode, this)
+
+	chrometraceBeginTask(this)
+
+	metrics.activeTasks.Inc()
+
+	endTaskFunc := func() {
+
+		// only hold locks while manipulating the tree
+		// (trace writer might block too long and unlike spans, tasks are updated concurrently)
+		alreadyEnded := func() (alreadyEnded bool) {
+			if this.parentTask != nil {
+				defer this.parentTask.mtx.Lock().Unlock()
+			}
+			defer this.mtx.Lock().Unlock()
+
+			if this.activeChildTasks != 0 {
+				panic(errors.Wrapf(ErrTaskStillHasActiveChildTasks, "end task: %v active child tasks", this.activeChildTasks))
+			}
+
+			// support idempotent task ends
+			if !this.endedAt.IsZero() {
+				return true
+			}
+			this.endedAt = time.Now()
+
+			if this.parentTask != nil {
+				this.parentTask.activeChildTasks--
+				if this.parentTask.activeChildTasks < 0 {
+					panic("impl error: parent task with negative activeChildTasks count")
+				}
+			}
+			return false
+		}()
+		if alreadyEnded {
+			return
+		}
+
+		chrometraceEndTask(this)
+
+		metrics.activeTasks.Dec()
+
+		taskNameDone()
+	}
+
+	return ctx, endTaskFunc
+}
+
+var ErrAlreadyActiveChildSpan = fmt.Errorf("create child span: span already has an active child span")
+var ErrSpanStillHasActiveChildSpan = fmt.Errorf("end span: span still has active child spans")
+
+// Start a new span.
+// Important: ctx must have an active task (see WithTask)
+func WithSpan(ctx context.Context, annotation string) (context.Context, DoneFunc) {
+	var parentSpan, parentTask *traceNode
+	nodeI := ctx.Value(contextKeyTraceNode)
+	if nodeI != nil {
+		parentSpan = nodeI.(*traceNode)
+		if parentSpan.parentSpan == nil {
+			parentTask = parentSpan
+		} else {
+			parentTask = parentSpan.parentTask
+		}
+	} else {
+		panic("must be called from within a task")
+	}
+
+	this := &traceNode{
+		id:              genID(),
+		annotation:      annotation,
+		parentTask:      parentTask,
+		parentSpan:      parentSpan,
+		activeChildSpan: nil,
+
+		startedAt: time.Now(),
+		endedAt:   time.Time{},
+	}
+
+	parentSpan.mtx.HoldWhile(func() {
+		if parentSpan.activeChildSpan != nil {
+			panic(ErrAlreadyActiveChildSpan)
+		}
+		parentSpan.activeChildSpan = this
+	})
+
+	ctx = context.WithValue(ctx, contextKeyTraceNode, this)
+	chrometraceBeginSpan(this)
+	callbackEndSpan := callbackBeginSpan(ctx)
+
+	endTaskFunc := func() {
+
+		defer parentSpan.mtx.Lock().Unlock()
+		if parentSpan.activeChildSpan != this && this.endedAt.IsZero() {
+			panic("impl error: activeChildSpan should not change while != nil because there can only be one")
+		}
+
+		defer this.mtx.Lock().Unlock()
+		if this.activeChildSpan != nil {
+			panic(ErrSpanStillHasActiveChildSpan)
+		}
+
+		if !this.endedAt.IsZero() {
+			return // support idempotent span ends
+		}
+
+		parentSpan.activeChildSpan = nil
+		this.endedAt = time.Now()
+
+		chrometraceEndSpan(this)
+		callbackEndSpan(this)
+	}
+
+	return ctx, endTaskFunc
+}
+
+type StackKind struct {
+	symbolizeTask func(t *traceNode) string
+	symbolizeSpan func(s *traceNode) string
+}
+
+var (
+	StackKindId = &StackKind{
+		symbolizeTask: func(t *traceNode) string { return t.id },
+		symbolizeSpan: func(s *traceNode) string { return s.id },
+	}
+	SpanStackKindCombined = &StackKind{
+		symbolizeTask: func(t *traceNode) string { return fmt.Sprintf("(%s %q)", t.id, t.annotation) },
+		symbolizeSpan: func(s *traceNode) string { return fmt.Sprintf("(%s %q)", s.id, s.annotation) },
+	}
+	SpanStackKindAnnotation = &StackKind{
+		symbolizeTask: func(t *traceNode) string { return t.annotation },
+		symbolizeSpan: func(s *traceNode) string { return s.annotation },
+	}
+)
+
+func (n *traceNode) task() *traceNode {
+	task := n.parentTask
+	if n.parentSpan == nil {
+		task = n
+	}
+	return task
+}
+
+func (n *traceNode) TaskName() string {
+	task := n.task()
+	return task.annotation
+}
+
+func (this *traceNode) TaskAndSpanStack(kind *StackKind) (spanIdStack string) {
+	task := this.task()
+
+	var spansInTask []*traceNode
+	for s := this; s != nil; s = s.parentSpan {
+		spansInTask = append(spansInTask, s)
+	}
+
+	var tasks []*traceNode
+	for t := task; t != nil; t = t.parentTask {
+		tasks = append(tasks, t)
+	}
+
+	var taskIdsRev []string
+	for i := len(tasks) - 1; i >= 0; i-- {
+		taskIdsRev = append(taskIdsRev, kind.symbolizeTask(tasks[i]))
+	}
+
+	var spanIdsRev []string
+	for i := len(spansInTask) - 1; i >= 0; i-- {
+		spanIdsRev = append(spanIdsRev, kind.symbolizeSpan(spansInTask[i]))
+	}
+
+	taskStack := strings.Join(taskIdsRev, "$")
+	return fmt.Sprintf("%s$%s", taskStack, strings.Join(spanIdsRev, "."))
+}
+
+func GetSpanStackOrDefault(ctx context.Context, kind StackKind, def string) string {
+	if nI := ctx.Value(contextKeyTraceNode); nI != nil {
+		n := nI.(*traceNode)
+		return n.TaskAndSpanStack(StackKindId)
+	} else {
+		return def
+	}
+}

daemon/logging/trace/trace_callback.go

@@ -0,0 +1,54 @@
+package trace
+
+import (
+	"context"
+	"time"
+
+	"github.com/zrepl/zrepl/util/chainlock"
+)
+
+type SpanInfo interface {
+	StartedAt() time.Time
+	EndedAt() time.Time
+	TaskAndSpanStack(kind *StackKind) string
+}
+
+type Callback struct {
+	OnBegin func(ctx context.Context)
+	OnEnd   func(ctx context.Context, spanInfo SpanInfo)
+}
+
+var callbacks struct {
+	mtx chainlock.L
+	cs  []Callback
+}
+
+func RegisterCallback(c Callback) {
+	callbacks.mtx.HoldWhile(func() {
+		callbacks.cs = append(callbacks.cs, c)
+	})
+}
+
+func callbackBeginSpan(ctx context.Context) func(SpanInfo) {
+	// capture the current state of callbacks into a local variable
+	// this is safe because the slice is append-only and immutable
+
+	// (it is important that a callback registered _after_ callbackBeginSpin is called does not get called on OnEnd)
+
+	var cbs []Callback
+	callbacks.mtx.HoldWhile(func() {
+		cbs = callbacks.cs
+	})
+	for _, cb := range cbs {
+		if cb.OnBegin != nil {
+			cb.OnBegin(ctx)
+		}
+	}
+	return func(spanInfo SpanInfo) {
+		for _, cb := range cbs {
+			if cb.OnEnd != nil {
+				cb.OnEnd(ctx, spanInfo)
+			}
+		}
+	}
+}

daemon/logging/trace/trace_chrometrace.go

@@ -0,0 +1,230 @@
+package trace
+
+// The functions in this file are concerned with the generation
+// of trace files based on the information from WithTask and WithSpan.
+//
+// The emitted trace files are open-ended array of JSON objects
+// that follow the Chrome trace file format:
+//   https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/preview
+//
+// The emitted JSON can be loaded into Chrome's chrome://tracing view.
+//
+// The trace file can be written to a file whose path is specified in an env file,
+// and be written to web sockets established on ChrometraceHttpHandler
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"sync"
+	"sync/atomic"
+
+	"golang.org/x/net/websocket"
+
+	"github.com/zrepl/zrepl/util/envconst"
+)
+
+var chrometracePID string
+
+func init() {
+	var err error
+	chrometracePID, err = os.Hostname()
+	if err != nil {
+		panic(err)
+	}
+	chrometracePID = fmt.Sprintf("%q", chrometracePID)
+}
+
+type chrometraceEvent struct {
+	Cat                       string   `json:"cat,omitempty"`
+	Name                      string   `json:"name"`
+	Stack                     []string `json:"stack,omitempty"`
+	Phase                     string   `json:"ph"`
+	TimestampUnixMicroseconds int64    `json:"ts"`
+	DurationMicroseconds      int64    `json:"dur,omitempty"`
+	Pid                       string   `json:"pid"`
+	Tid                       string   `json:"tid"`
+	Id                        string   `json:"id,omitempty"`
+}
+
+func chrometraceBeginSpan(s *traceNode) {
+	taskName := s.TaskName()
+	chrometraceWrite(chrometraceEvent{
+		Name:                      s.annotation,
+		Phase:                     "B",
+		TimestampUnixMicroseconds: s.startedAt.UnixNano() / 1000,
+		Pid:                       chrometracePID,
+		Tid:                       taskName,
+	})
+}
+
+func chrometraceEndSpan(s *traceNode) {
+	taskName := s.TaskName()
+	chrometraceWrite(chrometraceEvent{
+		Name:                      s.annotation,
+		Phase:                     "E",
+		TimestampUnixMicroseconds: s.endedAt.UnixNano() / 1000,
+		Pid:                       chrometracePID,
+		Tid:                       taskName,
+	})
+}
+
+var chrometraceFlowId uint64
+
+func chrometraceBeginTask(s *traceNode) {
+	chrometraceBeginSpan(s)
+
+	if s.parentTask == nil {
+		return
+	}
+	// beginning of a task that has a parent
+	// => use flow events to link parent and child
+
+	flowId := atomic.AddUint64(&chrometraceFlowId, 1)
+	flowIdStr := fmt.Sprintf("%x", flowId)
+
+	parentTask := s.parentTask.TaskName()
+	chrometraceWrite(chrometraceEvent{
+		Cat:                       "task", // seems to be necessary, otherwise the GUI shows some `indexOf` JS error
+		Name:                      "child-task",
+		Phase:                     "s",
+		TimestampUnixMicroseconds: s.startedAt.UnixNano() / 1000, // yes, the child's timestamp (=> from-point of the flow line is at right x-position of parent's bar)
+		Pid:                       chrometracePID,
+		Tid:                       parentTask,
+		Id:                        flowIdStr,
+	})
+
+	childTask := s.TaskName()
+	if parentTask == childTask {
+		panic(parentTask)
+	}
+	chrometraceWrite(chrometraceEvent{
+		Cat:                       "task", // seems to be necessary, otherwise the GUI shows some `indexOf` JS error
+		Name:                      "child-task",
+		Phase:                     "f",
+		TimestampUnixMicroseconds: s.startedAt.UnixNano() / 1000,
+		Pid:                       chrometracePID,
+		Tid:                       childTask,
+		Id:                        flowIdStr,
+	})
+}
+
+func chrometraceEndTask(s *traceNode) {
+	chrometraceEndSpan(s)
+}
+
+type chrometraceConsumerRegistration struct {
+	w io.Writer
+	// errored must have capacity 1, the writer thread will send to it non-blocking, then close it
+	errored chan error
+}
+
+var chrometraceConsumers struct {
+	register  chan chrometraceConsumerRegistration
+	consumers map[chrometraceConsumerRegistration]bool
+	write     chan []byte
+}
+
+func init() {
+	chrometraceConsumers.register = make(chan chrometraceConsumerRegistration)
+	chrometraceConsumers.consumers = make(map[chrometraceConsumerRegistration]bool)
+	chrometraceConsumers.write = make(chan []byte)
+	go func() {
+		kickConsumer := func(c chrometraceConsumerRegistration, err error) {
+			debug("chrometrace kicking consumer %#v after error %v", c, err)
+			select {
+			case c.errored <- err:
+			default:
+			}
+			close(c.errored)
+			delete(chrometraceConsumers.consumers, c)
+		}
+		for {
+			select {
+			case reg := <-chrometraceConsumers.register:
+				debug("registered chrometrace consumer %#v", reg)
+				chrometraceConsumers.consumers[reg] = true
+				n, err := reg.w.Write([]byte("[\n"))
+				if err != nil {
+					kickConsumer(reg, err)
+				} else if n != 2 {
+					kickConsumer(reg, fmt.Errorf("short write: %v", n))
+				}
+				// successfully registered
+
+			case buf := <-chrometraceConsumers.write:
+				debug("chrometrace write request: %s", string(buf))
+				var r bytes.Reader
+				for c := range chrometraceConsumers.consumers {
+					r.Reset(buf)
+					n, err := io.Copy(c.w, &r)
+					debug("chrometrace wrote n=%v bytes to consumer %#v", n, c)
+					if err != nil {
+						kickConsumer(c, err)
+					}
+				}
+			}
+		}
+	}()
+}
+
+func chrometraceWrite(i interface{}) {
+	var buf bytes.Buffer
+	err := json.NewEncoder(&buf).Encode(i)
+	if err != nil {
+		panic(err)
+	}
+	buf.WriteString(",")
+	chrometraceConsumers.write <- buf.Bytes()
+}
+
+func ChrometraceClientWebsocketHandler(conn *websocket.Conn) {
+	defer conn.Close()
+
+	var wg sync.WaitGroup
+	defer wg.Wait()
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		r := bufio.NewReader(conn)
+		_, _, _ = r.ReadLine() // ignore errors
+		conn.Close()
+	}()
+
+	errored := make(chan error, 1)
+	chrometraceConsumers.register <- chrometraceConsumerRegistration{
+		w:       conn,
+		errored: errored,
+	}
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		<-errored
+		conn.Close()
+	}()
+}
+
+var chrometraceFileConsumerPath = envconst.String("ZREPL_ACTIVITY_TRACE", "")
+
+func init() {
+	if chrometraceFileConsumerPath != "" {
+		var err error
+		f, err := os.Create(chrometraceFileConsumerPath)
+		if err != nil {
+			panic(err)
+		}
+		errored := make(chan error, 1)
+		chrometraceConsumers.register <- chrometraceConsumerRegistration{
+			w:       f,
+			errored: errored,
+		}
+		go func() {
+			<-errored
+			f.Close()
+		}()
+	}
+}

daemon/logging/trace/trace_context.go

@@ -0,0 +1,27 @@
+package trace
+
+import "context"
+
+type contextKey int
+
+const (
+	contextKeyTraceNode contextKey = 1 + iota
+)
+
+var contextKeys = []contextKey{
+	contextKeyTraceNode,
+}
+
+// WithInherit inherits the task hierarchy from inheritFrom into ctx.
+// The returned context is a child of ctx, but its task and span are those of inheritFrom.
+//
+// Note that in most use cases, callers most likely want to call WithTask since it will most likely
+// be in some sort of connection handler context.
+func WithInherit(ctx, inheritFrom context.Context) context.Context {
+	for _, k := range contextKeys {
+		if v := inheritFrom.Value(k); v != nil {
+			ctx = context.WithValue(ctx, k, v) // no shadow
+		}
+	}
+	return ctx
+}

daemon/logging/trace/trace_convenience.go

@@ -0,0 +1,74 @@
+package trace
+
+import (
+	"context"
+	"fmt"
+	"runtime"
+	"strings"
+	"sync"
+)
+
+// use like this:
+//
+//	defer WithSpanFromStackUpdateCtx(&existingCtx)()
+//
+//
+func WithSpanFromStackUpdateCtx(ctx *context.Context) DoneFunc {
+	childSpanCtx, end := WithSpan(*ctx, getMyCallerOrPanic())
+	*ctx = childSpanCtx
+	return end
+}
+
+// derive task name from call stack (caller's name)
+func WithTaskFromStack(ctx context.Context) (context.Context, DoneFunc) {
+	return WithTask(ctx, getMyCallerOrPanic())
+}
+
+// derive task name from call stack (caller's name) and update *ctx
+// to point to be the child task ctx
+func WithTaskFromStackUpdateCtx(ctx *context.Context) DoneFunc {
+	child, end := WithTask(*ctx, getMyCallerOrPanic())
+	*ctx = child
+	return end
+}
+
+// create a task and a span within it in one call
+func WithTaskAndSpan(ctx context.Context, task string, span string) (context.Context, DoneFunc) {
+	ctx, endTask := WithTask(ctx, task)
+	ctx, endSpan := WithSpan(ctx, fmt.Sprintf("%s %s", task, span))
+	return ctx, func() {
+		endSpan()
+		endTask()
+	}
+}
+
+// create a span during which several child tasks are spawned using the `add` function
+func WithTaskGroup(ctx context.Context, taskGroup string) (_ context.Context, add func(f func(context.Context)), waitEnd DoneFunc) {
+	var wg sync.WaitGroup
+	ctx, endSpan := WithSpan(ctx, taskGroup)
+	add = func(f func(context.Context)) {
+		wg.Add(1)
+		defer wg.Done()
+		ctx, endTask := WithTask(ctx, taskGroup)
+		defer endTask()
+		f(ctx)
+	}
+	waitEnd = func() {
+		wg.Wait()
+		endSpan()
+	}
+	return ctx, add, waitEnd
+}
+
+func getMyCallerOrPanic() string {
+	pc, _, _, ok := runtime.Caller(2)
+	if !ok {
+		panic("cannot get caller")
+	}
+	details := runtime.FuncForPC(pc)
+	if ok && details != nil {
+		const prefix = "github.com/zrepl/zrepl"
+		return strings.TrimPrefix(strings.TrimPrefix(details.Name(), prefix), "/")
+	}
+	return ""
+}

daemon/logging/trace/trace_convenience_test.go

@@ -0,0 +1,16 @@
+package trace
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetCallerOrPanic(t *testing.T) {
+	withStackFromCtxMock := func() string {
+		return getMyCallerOrPanic()
+	}
+	ret := withStackFromCtxMock()
+	// zrepl prefix is stripped
+	assert.Equal(t, "daemon/logging/trace.TestGetCallerOrPanic", ret)
+}

daemon/logging/trace/trace_debug.go

@@ -0,0 +1,15 @@
+package trace
+
+import (
+	"fmt"
+	"os"
+)
+
+const debugEnabled = false
+
+func debug(format string, args ...interface{}) {
+	if !debugEnabled {
+		return
+	}
+	fmt.Fprintf(os.Stderr, format+"\n", args...)
+}

daemon/logging/trace/trace_genID.go

@@ -0,0 +1,47 @@
+package trace
+
+import (
+	"encoding/base64"
+	"math/rand"
+	"os"
+	"strings"
+	"time"
+
+	"github.com/zrepl/zrepl/util/envconst"
+)
+
+var genIdPRNG = rand.New(rand.NewSource(1))
+
+func init() {
+	genIdPRNG.Seed(time.Now().UnixNano())
+	genIdPRNG.Seed(int64(os.Getpid()))
+}
+
+var genIdNumBytes = envconst.Int("ZREPL_TRACE_ID_NUM_BYTES", 3)
+
+func init() {
+	if genIdNumBytes < 1 {
+		panic("trace node id byte length must be at least 1")
+	}
+}
+
+func genID() string {
+	var out strings.Builder
+	enc := base64.NewEncoder(base64.RawStdEncoding, &out)
+	buf := make([]byte, genIdNumBytes)
+	for i := 0; i < len(buf); {
+		n, err := genIdPRNG.Read(buf[i:])
+		if err != nil {
+			panic(err)
+		}
+		i += n
+	}
+	n, err := enc.Write(buf[:])
+	if err != nil || n != len(buf) {
+		panic(err)
+	}
+	if err := enc.Close(); err != nil {
+		panic(err)
+	}
+	return out.String()
+}

daemon/logging/trace/trace_test.go

@@ -0,0 +1,205 @@
+package trace
+
+import (
+	"context"
+	"fmt"
+	"testing"
+
+	"github.com/gitchander/permutation"
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRegularSpanUsage(t *testing.T) {
+	root, endRoot := WithTask(context.Background(), "root")
+	defer endRoot()
+
+	s1, endS1 := WithSpan(root, "parent")
+	s2, endS2 := WithSpan(s1, "child")
+	_, endS3 := WithSpan(s2, "grand-child")
+	require.NotPanics(t, func() { endS3() })
+	require.NotPanics(t, func() { endS2() })
+
+	// reuse
+	_, endS4 := WithSpan(s1, "child-2")
+	require.NotPanics(t, func() { endS4() })
+
+	// close parent
+	require.NotPanics(t, func() { endS1() })
+}
+
+func TestMultipleActiveChildSpansNotAllowed(t *testing.T) {
+	root, endRoot := WithTask(context.Background(), "root")
+	defer endRoot()
+
+	s1, _ := WithSpan(root, "s1")
+	_, endS2 := WithSpan(s1, "s1-child1")
+
+	require.PanicsWithValue(t, ErrAlreadyActiveChildSpan, func() {
+		_, _ = WithSpan(s1, "s1-child2")
+	})
+
+	endS2()
+
+	require.NotPanics(t, func() {
+		_, _ = WithSpan(s1, "s1-child2")
+	})
+}
+
+func TestForkingChildSpansNotAllowed(t *testing.T) {
+	root, endRoot := WithTask(context.Background(), "root")
+	defer endRoot()
+
+	s1, _ := WithSpan(root, "s1")
+	sc, endSC := WithSpan(s1, "s1-child")
+	_, _ = WithSpan(sc, "s1-child-child")
+
+	require.PanicsWithValue(t, ErrSpanStillHasActiveChildSpan, func() {
+		endSC()
+	})
+}
+
+func TestRegularTaskUsage(t *testing.T) {
+	// assert concurrent activities on different tasks can end in any order
+	closeOrder := []int{0, 1, 2}
+	closeOrders := permutation.New(permutation.IntSlice(closeOrder))
+	for closeOrders.Next() {
+		t.Run(fmt.Sprintf("%v", closeOrder), func(t *testing.T) {
+			root, endRoot := WithTask(context.Background(), "root")
+			defer endRoot()
+
+			c1, endC1 := WithTask(root, "c1")
+			defer endC1()
+			c2, endC2 := WithTask(root, "c2")
+			defer endC2()
+
+			// begin 3 concurrent activities
+			_, endAR := WithSpan(root, "aR")
+			_, endAC1 := WithSpan(c1, "aC1")
+			_, endAC2 := WithSpan(c2, "aC2")
+
+			endFuncs := []DoneFunc{endAR, endAC1, endAC2}
+			for _, i := range closeOrder {
+				require.NotPanics(t, func() {
+					endFuncs[i]()
+				}, "%v", i)
+			}
+		})
+	}
+}
+
+func TestTaskEndWithActiveChildTaskNotAllowed(t *testing.T) {
+	root, _ := WithTask(context.Background(), "root")
+	c, endC := WithTask(root, "child")
+	_, _ = WithTask(c, "grand-child")
+	func() {
+		defer func() {
+			r := recover()
+			require.NotNil(t, r)
+			err, ok := r.(error)
+			require.True(t, ok)
+			require.Equal(t, ErrTaskStillHasActiveChildTasks, errors.Cause(err))
+		}()
+		endC()
+	}()
+
+}
+
+func TestIdempotentEndTask(t *testing.T) {
+	_, end := WithTask(context.Background(), "root")
+	end()
+	require.NotPanics(t, func() { end() })
+}
+
+func TestSpansPanicIfNoParentTask(t *testing.T) {
+	require.Panics(t, func() { WithSpan(context.Background(), "taskless-span") })
+}
+
+func TestIdempotentEndSpan(t *testing.T) {
+	root, _ := WithTask(context.Background(), "root")
+	_, end := WithSpan(root, "span")
+	end()
+	require.NotPanics(t, func() { end() })
+}
+
+func logAndGetTraceNode(t *testing.T, descr string, ctx context.Context) *traceNode {
+	n, ok := ctx.Value(contextKeyTraceNode).(*traceNode)
+	require.True(t, ok)
+	t.Logf("% 20s %p %#v", descr, n, n)
+	return n
+}
+
+func TestWhiteboxHierachy(t *testing.T) {
+	root, e1 := WithTask(context.Background(), "root")
+	rootN := logAndGetTraceNode(t, "root", root)
+	assert.Nil(t, rootN.parentTask)
+	assert.Nil(t, rootN.parentSpan)
+
+	child, e2 := WithSpan(root, "child")
+	childN := logAndGetTraceNode(t, "child", child)
+	assert.Equal(t, rootN, childN.parentTask)
+	assert.Equal(t, rootN, childN.parentSpan)
+
+	grandchild, e3 := WithSpan(child, "grandchild")
+	grandchildN := logAndGetTraceNode(t, "grandchild", grandchild)
+	assert.Equal(t, rootN, grandchildN.parentTask)
+	assert.Equal(t, childN, grandchildN.parentSpan)
+
+	gcTask, e4 := WithTask(grandchild, "grandchild-task")
+	gcTaskN := logAndGetTraceNode(t, "grandchild-task", gcTask)
+	assert.Equal(t, rootN, gcTaskN.parentTask)
+	assert.Nil(t, gcTaskN.parentSpan)
+
+	// it is allowed that a child task outlives the _span_ in which it was created
+	// (albeit not its parent task)
+	e3()
+	e2()
+	gcTaskSpan, e5 := WithSpan(gcTask, "granschild-task-span")
+	gcTaskSpanN := logAndGetTraceNode(t, "granschild-task-span", gcTaskSpan)
+	assert.Equal(t, gcTaskN, gcTaskSpanN.parentTask)
+	assert.Equal(t, gcTaskN, gcTaskSpanN.parentSpan)
+	e5()
+
+	e4()
+	e1()
+}
+
+func TestOrphanTasksBecomeNewRootWhenAllAncestorsAreDead(t *testing.T) {
+	parent, e1 := WithTask(context.Background(), "parent")
+	_ = logAndGetTraceNode(t, "parent-task", parent)
+	e1()
+
+	var child context.Context
+	var e2 DoneFunc
+	require.NotPanics(t, func() {
+		child, e2 = WithTask(parent, "child")
+	})
+	childN := logAndGetTraceNode(t, "child-task", child)
+	assert.Nil(t, childN.parentTask)
+
+	grandchild, _ := WithTask(child, "grandchild")
+	grandchildN := logAndGetTraceNode(t, "grandchild-task", grandchild)
+	assert.Equal(t, childN, grandchildN.parentTask)
+
+	require.Panics(t, func() { e2() }, "if the parent was alive at creation of child task, wrong termination order remains a panicable offense though")
+}
+
+func TestOrphanTaskBecomesChildToNearestLiveAncestor(t *testing.T) {
+	parent, e1 := WithTask(context.Background(), "parent")
+	parentN := logAndGetTraceNode(t, "parent-task", parent)
+
+	child, e2 := WithTask(parent, "child")
+	childN := logAndGetTraceNode(t, "child-task", child)
+	assert.Equal(t, parentN, childN.parentTask)
+	e2()
+
+	grandchild, e3 := WithTask(child, "grandchild")
+	grandchildN := logAndGetTraceNode(t, "grandchild-task", grandchild)
+	assert.Equal(t, parentN, grandchildN.parentTask) // child is already dead
+
+	require.NotPanics(t, func() {
+		e3()
+		e1()
+	})
+}

daemon/logging/trace/trace_unique_concurrent_task_namer.go

@@ -0,0 +1,53 @@
+package trace
+
+import (
+	"fmt"
+	"strings"
+	"sync"
+
+	"github.com/willf/bitset"
+)
+
+type uniqueConcurrentTaskNamer struct {
+	mtx    sync.Mutex
+	active map[string]*bitset.BitSet
+}
+
+// bitvecLengthGauge may be nil
+func newUniqueTaskNamer() *uniqueConcurrentTaskNamer {
+	return &uniqueConcurrentTaskNamer{
+		active: make(map[string]*bitset.BitSet),
+	}
+}
+
+// appends `#%d` to `name` such that until `done` is called,
+// it is guaranteed that `#%d` is not returned a second time for the same `name`
+func (namer *uniqueConcurrentTaskNamer) UniqueConcurrentTaskName(name string) (uniqueName string, done func()) {
+	if strings.Contains(name, "#") {
+		panic(name)
+	}
+	namer.mtx.Lock()
+	act, ok := namer.active[name]
+	if !ok {
+		act = bitset.New(64) // FIXME magic const
+		namer.active[name] = act
+	}
+	id, ok := act.NextClear(0)
+	if !ok {
+		// if !ok, all bits are 1 and act.Len() returns the next bit
+		id = act.Len()
+		// FIXME unbounded growth without reclamation
+	}
+	act.Set(id)
+	namer.mtx.Unlock()
+
+	return fmt.Sprintf("%s#%d", name, id), func() {
+		namer.mtx.Lock()
+		defer namer.mtx.Unlock()
+		act, ok := namer.active[name]
+		if !ok {
+			panic("must be initialized upon entry")
+		}
+		act.Clear(id)
+	}
+}

daemon/logging/trace/trace_unique_concurrent_task_namer_test.go

@@ -0,0 +1,58 @@
+package trace
+
+import (
+	"fmt"
+	"sync"
+	"sync/atomic"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"github.com/willf/bitset"
+)
+
+func TestBitsetFeaturesForUniqueConcurrentTaskNamer(t *testing.T) {
+	var b bitset.BitSet
+	require.Equal(t, uint(0), b.Len())
+	require.Equal(t, uint(0), b.Count())
+
+	b.Set(0)
+	require.Equal(t, uint(1), b.Len())
+	require.Equal(t, uint(1), b.Count())
+
+	b.Set(8)
+	require.Equal(t, uint(9), b.Len())
+	require.Equal(t, uint(2), b.Count())
+
+	b.Set(1)
+	require.Equal(t, uint(9), b.Len())
+	require.Equal(t, uint(3), b.Count())
+}
+
+func TestUniqueConcurrentTaskNamer(t *testing.T) {
+	namer := newUniqueTaskNamer()
+
+	var wg sync.WaitGroup
+
+	const N = 8128
+	const Q = 23
+
+	var fails uint32
+	var m sync.Map
+	wg.Add(N)
+	for i := 0; i < N; i++ {
+		go func(i int) {
+			defer wg.Done()
+			name := fmt.Sprintf("%d", i/Q)
+			uniqueName, done := namer.UniqueConcurrentTaskName(name)
+			act, _ := m.LoadOrStore(uniqueName, i)
+			if act.(int) != i {
+				atomic.AddUint32(&fails, 1)
+			}
+			m.Delete(uniqueName)
+			done()
+		}(i)
+	}
+	wg.Wait()
+
+	require.Equal(t, uint32(0), fails)
+}

daemon/main.go

@@ -1,6 +1,8 @@
 package daemon
 
 import (
+	"context"
+
 	"github.com/zrepl/zrepl/cli"
 	"github.com/zrepl/zrepl/logger"
 )
@@ -10,7 +12,7 @@ type Logger = logger.Logger
 var DaemonCmd = &cli.Subcommand{
 	Use:   "daemon",
 	Short: "run the zrepl daemon",
-	Run: func(subcommand *cli.Subcommand, args []string) error {
-		return Run(subcommand.Config())
+	Run: func(ctx context.Context, subcommand *cli.Subcommand, args []string) error {
+		return Run(ctx, subcommand.Config())
 	},
 }

daemon/pprof.go

@@ -9,7 +9,10 @@ import (
 	"net/http/pprof"
 
 	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"golang.org/x/net/websocket"
+
 	"github.com/zrepl/zrepl/daemon/job"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 )
 
 type pprofServer struct {
@@ -66,6 +69,7 @@ outer:
 			mux.Handle("/debug/pprof/symbol", http.HandlerFunc(pprof.Symbol))
 			mux.Handle("/debug/pprof/trace", http.HandlerFunc(pprof.Trace))
 			mux.Handle("/metrics", promhttp.Handler())
+			mux.Handle("/debug/zrepl/activity-trace", websocket.Handler(trace.ChrometraceClientWebsocketHandler))
 			go func() {
 				err := http.Serve(s.listener, mux)
 				if ctx.Err() != nil {

daemon/prometheus.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/job"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/endpoint"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/rpc/dataconn/frameconn"
@@ -86,16 +87,19 @@ func (j *prometheusJob) Run(ctx context.Context) {
 }
 
 type prometheusJobOutlet struct {
-	jobName string
 }
 
 var _ logger.Outlet = prometheusJobOutlet{}
 
-func newPrometheusLogOutlet(jobName string) prometheusJobOutlet {
-	return prometheusJobOutlet{jobName}
+func newPrometheusLogOutlet() prometheusJobOutlet {
+	return prometheusJobOutlet{}
 }
 
 func (o prometheusJobOutlet) WriteEntry(entry logger.Entry) error {
-	prom.taskLogEntries.WithLabelValues(o.jobName, entry.Level.String()).Inc()
+	jobFieldVal, ok := entry.Fields[logging.JobField].(string)
+	if !ok {
+		jobFieldVal = "_nojobid"
+	}
+	prom.taskLogEntries.WithLabelValues(jobFieldVal, entry.Level.String()).Inc()
 	return nil
 }

daemon/pruner/pruner.go

@@ -12,6 +12,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 
 	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/pruning"
 	"github.com/zrepl/zrepl/replication/logic/pdu"
@@ -35,17 +36,13 @@ type Logger = logger.Logger
 
 type contextKey int
 
-const contextKeyLogger contextKey = 0
-
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLogger, log)
-}
+const (
+	contextKeyPruneSide contextKey = 1 + iota
+)
 
 func GetLogger(ctx context.Context) Logger {
-	if l, ok := ctx.Value(contextKeyLogger).(Logger); ok {
-		return l
-	}
-	return logger.NewNullLogger()
+	pruneSide := ctx.Value(contextKeyPruneSide).(string)
+	return logging.GetLogger(ctx, logging.SubsysPruning).WithField("prune_side", pruneSide)
 }
 
 type args struct {
@@ -138,7 +135,7 @@ func NewPrunerFactory(in config.PruningSenderReceiver, promPruneSecs *prometheus
 func (f *PrunerFactory) BuildSenderPruner(ctx context.Context, target Target, receiver History) *Pruner {
 	p := &Pruner{
 		args: args{
-			WithLogger(ctx, GetLogger(ctx).WithField("prune_side", "sender")),
+			context.WithValue(ctx, contextKeyPruneSide, "sender"),
 			target,
 			receiver,
 			f.senderRules,
@@ -154,7 +151,7 @@ func (f *PrunerFactory) BuildSenderPruner(ctx context.Context, target Target, re
 func (f *PrunerFactory) BuildReceiverPruner(ctx context.Context, target Target, receiver History) *Pruner {
 	p := &Pruner{
 		args: args{
-			WithLogger(ctx, GetLogger(ctx).WithField("prune_side", "receiver")),
+			context.WithValue(ctx, contextKeyPruneSide, "receiver"),
 			target,
 			receiver,
 			f.receiverRules,
@@ -170,7 +167,7 @@ func (f *PrunerFactory) BuildReceiverPruner(ctx context.Context, target Target,
 func (f *LocalPrunerFactory) BuildLocalPruner(ctx context.Context, target Target, receiver History) *Pruner {
 	p := &Pruner{
 		args: args{
-			ctx,
+			context.WithValue(ctx, contextKeyPruneSide, "local"),
 			target,
 			receiver,
 			f.keepRules,

daemon/snapper/snapper.go

@@ -8,10 +8,12 @@ import (
 	"time"
 
 	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/filters"
 	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/util/envconst"
 	"github.com/zrepl/zrepl/zfs"
@@ -45,7 +47,6 @@ type snapProgress struct {
 
 type args struct {
 	ctx            context.Context
-	log            Logger
 	prefix         string
 	interval       time.Duration
 	fsf            *filters.DatasetMapFilter
@@ -102,23 +103,10 @@ func (s State) sf() state {
 type updater func(u func(*Snapper)) State
 type state func(a args, u updater) state
 
-type contextKey int
-
-const (
-	contextKeyLog contextKey = 0
-)
-
 type Logger = logger.Logger
 
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLog, log)
-}
-
 func getLogger(ctx context.Context) Logger {
-	if log, ok := ctx.Value(contextKeyLog).(Logger); ok {
-		return log
-	}
-	return logger.NewNullLogger()
+	return logging.GetLogger(ctx, logging.SubsysSnapshot)
 }
 
 func PeriodicFromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in *config.SnapshottingPeriodic) (*Snapper, error) {
@@ -146,13 +134,12 @@ func PeriodicFromConfig(g *config.Global, fsf *filters.DatasetMapFilter, in *con
 }
 
 func (s *Snapper) Run(ctx context.Context, snapshotsTaken chan<- struct{}) {
-
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	getLogger(ctx).Debug("start")
 	defer getLogger(ctx).Debug("stop")
 
 	s.args.snapshotsTaken = snapshotsTaken
 	s.args.ctx = ctx
-	s.args.log = getLogger(ctx)
 	s.args.dryRun = false // for future expansion
 
 	u := func(u func(*Snapper)) State {
@@ -190,7 +177,7 @@ func onErr(err error, u updater) state {
 		case Snapshotting:
 			s.state = ErrorWait
 		}
-		s.args.log.WithError(err).WithField("pre_state", preState).WithField("post_state", s.state).Error("snapshotting error")
+		getLogger(s.args.ctx).WithError(err).WithField("pre_state", preState).WithField("post_state", s.state).Error("snapshotting error")
 	}).sf()
 }
 
@@ -209,7 +196,7 @@ func syncUp(a args, u updater) state {
 	if err != nil {
 		return onErr(err, u)
 	}
-	syncPoint, err := findSyncPoint(a.log, fss, a.prefix, a.interval)
+	syncPoint, err := findSyncPoint(a.ctx, fss, a.prefix, a.interval)
 	if err != nil {
 		return onErr(err, u)
 	}
@@ -266,18 +253,18 @@ func snapshot(a args, u updater) state {
 		suffix := time.Now().In(time.UTC).Format("20060102_150405_000")
 		snapname := fmt.Sprintf("%s%s", a.prefix, suffix)
 
-		l := a.log.
-			WithField("fs", fs.ToString()).
-			WithField("snap", snapname)
+		ctx := logging.WithInjectedField(a.ctx, "fs", fs.ToString())
+		ctx = logging.WithInjectedField(ctx, "snap", snapname)
 
 		hookEnvExtra := hooks.Env{
 			hooks.EnvFS:       fs.ToString(),
 			hooks.EnvSnapshot: snapname,
 		}
 
-		jobCallback := hooks.NewCallbackHookForFilesystem("snapshot", fs, func(_ context.Context) (err error) {
+		jobCallback := hooks.NewCallbackHookForFilesystem("snapshot", fs, func(ctx context.Context) (err error) {
+			l := getLogger(ctx)
 			l.Debug("create snapshot")
-			err = zfs.ZFSSnapshot(a.ctx, fs, snapname, false) // TODO propagate context to ZFSSnapshot
+			err = zfs.ZFSSnapshot(ctx, fs, snapname, false) // TODO propagate context to ZFSSnapshot
 			if err != nil {
 				l.WithError(err).Error("cannot create snapshot")
 			}
@@ -290,7 +277,7 @@ func snapshot(a args, u updater) state {
 		{
 			filteredHooks, err := a.hooks.CopyFilteredForFilesystem(fs)
 			if err != nil {
-				l.WithError(err).Error("unexpected filter error")
+				getLogger(ctx).WithError(err).Error("unexpected filter error")
 				fsHadErr = true
 				goto updateFSState
 			}
@@ -303,7 +290,7 @@ func snapshot(a args, u updater) state {
 			plan, planErr = hooks.NewPlan(&filteredHooks, hooks.PhaseSnapshot, jobCallback, hookEnvExtra)
 			if planErr != nil {
 				fsHadErr = true
-				l.WithError(planErr).Error("cannot create job hook plan")
+				getLogger(ctx).WithError(planErr).Error("cannot create job hook plan")
 				goto updateFSState
 			}
 		}
@@ -314,15 +301,14 @@ func snapshot(a args, u updater) state {
 			progress.state = SnapStarted
 		})
 		{
-			l := hooks.GetLogger(a.ctx).WithField("fs", fs.ToString()).WithField("snap", snapname)
-			l.WithField("report", plan.Report().String()).Debug("begin run job plan")
-			plan.Run(hooks.WithLogger(a.ctx, l), a.dryRun)
+			getLogger(ctx).WithField("report", plan.Report().String()).Debug("begin run job plan")
+			plan.Run(ctx, a.dryRun)
 			planReport = plan.Report()
 			fsHadErr = planReport.HadError() // not just fatal errors
 			if fsHadErr {
-				l.WithField("report", planReport.String()).Error("end run job plan with error")
+				getLogger(ctx).WithField("report", planReport.String()).Error("end run job plan with error")
 			} else {
-				l.WithField("report", planReport.String()).Info("end run job plan successful")
+				getLogger(ctx).WithField("report", planReport.String()).Info("end run job plan successful")
 			}
 		}
 
@@ -342,7 +328,7 @@ func snapshot(a args, u updater) state {
 	case a.snapshotsTaken <- struct{}{}:
 	default:
 		if a.snapshotsTaken != nil {
-			a.log.Warn("callback channel is full, discarding snapshot update event")
+			getLogger(a.ctx).Warn("callback channel is full, discarding snapshot update event")
 		}
 	}
 
@@ -355,7 +341,7 @@ func snapshot(a args, u updater) state {
 					break
 				}
 			}
-			a.log.WithField("hook", h.String()).WithField("hook_number", hookIdx+1).Warn("hook did not match any snapshotted filesystems")
+			getLogger(a.ctx).WithField("hook", h.String()).WithField("hook_number", hookIdx+1).Warn("hook did not match any snapshotted filesystems")
 		}
 	}
 
@@ -376,7 +362,7 @@ func wait(a args, u updater) state {
 		lastTick := snapper.lastInvocation
 		snapper.sleepUntil = lastTick.Add(a.interval)
 		sleepUntil = snapper.sleepUntil
-		log := a.log.WithField("sleep_until", sleepUntil).WithField("duration", a.interval)
+		log := getLogger(a.ctx).WithField("sleep_until", sleepUntil).WithField("duration", a.interval)
 		logFunc := log.Debug
 		if snapper.state == ErrorWait || snapper.state == SyncUpErrWait {
 			logFunc = log.Error
@@ -404,7 +390,7 @@ func listFSes(ctx context.Context, mf *filters.DatasetMapFilter) (fss []*zfs.Dat
 var syncUpWarnNoSnapshotUntilSyncupMinDuration = envconst.Duration("ZREPL_SNAPPER_SYNCUP_WARN_MIN_DURATION", 1*time.Second)
 
 // see docs/snapshotting.rst
-func findSyncPoint(log Logger, fss []*zfs.DatasetPath, prefix string, interval time.Duration) (syncPoint time.Time, err error) {
+func findSyncPoint(ctx context.Context, fss []*zfs.DatasetPath, prefix string, interval time.Duration) (syncPoint time.Time, err error) {
 
 	const (
 		prioHasVersions int = iota
@@ -426,10 +412,10 @@ func findSyncPoint(log Logger, fss []*zfs.DatasetPath, prefix string, interval t
 
 	now := time.Now()
 
-	log.Debug("examine filesystem state to find sync point")
+	getLogger(ctx).Debug("examine filesystem state to find sync point")
 	for _, d := range fss {
-		l := log.WithField("fs", d.ToString())
-		syncPoint, err := findSyncPointFSNextOptimalSnapshotTime(l, now, interval, prefix, d)
+		ctx := logging.WithInjectedField(ctx, "fs", d.ToString())
+		syncPoint, err := findSyncPointFSNextOptimalSnapshotTime(ctx, now, interval, prefix, d)
 		if err == findSyncPointFSNoFilesystemVersionsErr {
 			snaptimes = append(snaptimes, snapTime{
 				ds:   d,
@@ -438,9 +424,9 @@ func findSyncPoint(log Logger, fss []*zfs.DatasetPath, prefix string, interval t
 			})
 		} else if err != nil {
 			hardErrs++
-			l.WithError(err).Error("cannot determine optimal sync point for this filesystem")
+			getLogger(ctx).WithError(err).Error("cannot determine optimal sync point for this filesystem")
 		} else {
-			l.WithField("syncPoint", syncPoint).Debug("found optimal sync point for this filesystem")
+			getLogger(ctx).WithField("syncPoint", syncPoint).Debug("found optimal sync point for this filesystem")
 			snaptimes = append(snaptimes, snapTime{
 				ds:   d,
 				prio: prioHasVersions,
@@ -467,7 +453,7 @@ func findSyncPoint(log Logger, fss []*zfs.DatasetPath, prefix string, interval t
 	})
 
 	winnerSyncPoint := snaptimes[0].time
-	l := log.WithField("syncPoint", winnerSyncPoint.String())
+	l := getLogger(ctx).WithField("syncPoint", winnerSyncPoint.String())
 	l.Info("determined sync point")
 	if winnerSyncPoint.Sub(now) > syncUpWarnNoSnapshotUntilSyncupMinDuration {
 		for _, st := range snaptimes {
@@ -483,9 +469,9 @@ func findSyncPoint(log Logger, fss []*zfs.DatasetPath, prefix string, interval t
 
 var findSyncPointFSNoFilesystemVersionsErr = fmt.Errorf("no filesystem versions")
 
-func findSyncPointFSNextOptimalSnapshotTime(l Logger, now time.Time, interval time.Duration, prefix string, d *zfs.DatasetPath) (time.Time, error) {
+func findSyncPointFSNextOptimalSnapshotTime(ctx context.Context, now time.Time, interval time.Duration, prefix string, d *zfs.DatasetPath) (time.Time, error) {
 
-	fsvs, err := zfs.ZFSListFilesystemVersions(context.TODO(), d, zfs.ListFilesystemVersionsOptions{
+	fsvs, err := zfs.ZFSListFilesystemVersions(ctx, d, zfs.ListFilesystemVersionsOptions{
 		Types:           zfs.Snapshots,
 		ShortnamePrefix: prefix,
 	})
@@ -502,7 +488,7 @@ func findSyncPointFSNextOptimalSnapshotTime(l Logger, now time.Time, interval ti
 	})
 
 	latest := fsvs[len(fsvs)-1]
-	l.WithField("creation", latest.Creation).Debug("found latest snapshot")
+	getLogger(ctx).WithField("creation", latest.Creation).Debug("found latest snapshot")
 
 	since := now.Sub(latest.Creation)
 	if since < 0 {

endpoint/context.go

@@ -3,25 +3,18 @@ package endpoint
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
 type contextKey int
 
 const (
-	contextKeyLogger contextKey = iota
-	ClientIdentityKey
+	ClientIdentityKey contextKey = iota
 )
 
 type Logger = logger.Logger
 
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLogger, log)
-}
-
 func getLogger(ctx context.Context) Logger {
-	if l, ok := ctx.Value(contextKeyLogger).(Logger); ok {
-		return l
-	}
-	return logger.NewNullLogger()
+	return logging.GetLogger(ctx, logging.SubsysEndpoint)
 }

endpoint/endpoint.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 
 	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/replication/logic/pdu"
 	"github.com/zrepl/zrepl/util/chainedio"
@@ -73,6 +74,8 @@ func (s *Sender) filterCheckFS(fs string) (*zfs.DatasetPath, error) {
 }
 
 func (s *Sender) ListFilesystems(ctx context.Context, r *pdu.ListFilesystemReq) (*pdu.ListFilesystemRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	fss, err := zfs.ZFSListMapping(ctx, s.FSFilter)
 	if err != nil {
 		return nil, err
@@ -95,6 +98,8 @@ func (s *Sender) ListFilesystems(ctx context.Context, r *pdu.ListFilesystemReq)
 }
 
 func (s *Sender) ListFilesystemVersions(ctx context.Context, r *pdu.ListFilesystemVersionsReq) (*pdu.ListFilesystemVersionsRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	lp, err := s.filterCheckFS(r.GetFilesystem())
 	if err != nil {
 		return nil, err
@@ -113,6 +118,7 @@ func (s *Sender) ListFilesystemVersions(ctx context.Context, r *pdu.ListFilesyst
 }
 
 func (p *Sender) HintMostRecentCommonAncestor(ctx context.Context, r *pdu.HintMostRecentCommonAncestorReq) (*pdu.HintMostRecentCommonAncestorRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 
 	fsp, err := p.filterCheckFS(r.GetFilesystem())
 	if err != nil {
@@ -245,6 +251,7 @@ func sendArgsFromPDUAndValidateExistsAndGetVersion(ctx context.Context, fs strin
 }
 
 func (s *Sender) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.ReadCloser, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 
 	_, err := s.filterCheckFS(r.Filesystem)
 	if err != nil {
@@ -350,6 +357,7 @@ func (s *Sender) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.Rea
 }
 
 func (p *Sender) SendCompleted(ctx context.Context, r *pdu.SendCompletedReq) (*pdu.SendCompletedRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 
 	orig := r.GetOriginalReq() // may be nil, always use proto getters
 	fsp, err := p.filterCheckFS(orig.GetFilesystem())
@@ -371,27 +379,30 @@ func (p *Sender) SendCompleted(ctx context.Context, r *pdu.SendCompletedReq) (*p
 		return nil, errors.Wrap(err, "validate `to` exists")
 	}
 
-	log := getLogger(ctx).WithField("to_guid", to.Guid).
-		WithField("fs", fs).
-		WithField("to", to.RelName)
-	if from != nil {
-		log = log.WithField("from", from.RelName).WithField("from_guid", from.Guid)
+	log := func(ctx context.Context) Logger {
+		log := getLogger(ctx).WithField("to_guid", to.Guid).
+			WithField("fs", fs).
+			WithField("to", to.RelName)
+		if from != nil {
+			log = log.WithField("from", from.RelName).WithField("from_guid", from.Guid)
+		}
+		return log
 	}
 
-	log.Debug("move replication cursor to most recent common version")
+	log(ctx).Debug("move replication cursor to most recent common version")
 	destroyedCursors, err := MoveReplicationCursor(ctx, fs, to, p.jobId)
 	if err != nil {
 		if err == zfs.ErrBookmarkCloningNotSupported {
-			log.Debug("not setting replication cursor, bookmark cloning not supported")
+			log(ctx).Debug("not setting replication cursor, bookmark cloning not supported")
 		} else {
 			msg := "cannot move replication cursor, keeping hold on `to` until successful"
-			log.WithError(err).Error(msg)
+			log(ctx).WithError(err).Error(msg)
 			err = errors.Wrap(err, msg)
 			// it is correct to not release the hold if we can't move the cursor!
 			return &pdu.SendCompletedRes{}, err
 		}
 	} else {
-		log.Info("successfully moved replication cursor")
+		log(ctx).Info("successfully moved replication cursor")
 	}
 
 	// kick off releasing of step holds / bookmarks
@@ -401,21 +412,27 @@ func (p *Sender) SendCompleted(ctx context.Context, r *pdu.SendCompletedReq) (*p
 	wg.Add(2)
 	go func() {
 		defer wg.Done()
-		log.Debug("release step-hold of or step-bookmark on `to`")
+		ctx, endTask := trace.WithTask(ctx, "release-step-hold-to")
+		defer endTask()
+
+		log(ctx).Debug("release step-hold of or step-bookmark on `to`")
 		err = ReleaseStep(ctx, fs, to, p.jobId)
 		if err != nil {
-			log.WithError(err).Error("cannot release step-holds on or destroy step-bookmark of `to`")
+			log(ctx).WithError(err).Error("cannot release step-holds on or destroy step-bookmark of `to`")
 		} else {
-			log.Info("successfully released step-holds on or destroyed step-bookmark of `to`")
+			log(ctx).Info("successfully released step-holds on or destroyed step-bookmark of `to`")
 		}
 
 	}()
 	go func() {
 		defer wg.Done()
+		ctx, endTask := trace.WithTask(ctx, "release-step-hold-from")
+		defer endTask()
+
 		if from == nil {
 			return
 		}
-		log.Debug("release step-hold of or step-bookmark on `from`")
+		log(ctx).Debug("release step-hold of or step-bookmark on `from`")
 		err := ReleaseStep(ctx, fs, *from, p.jobId)
 		if err != nil {
 			if dne, ok := err.(*zfs.DatasetDoesNotExist); ok {
@@ -424,15 +441,15 @@ func (p *Sender) SendCompleted(ctx context.Context, r *pdu.SendCompletedReq) (*p
 				// In that case, nonexistence of `from` is not an error, otherwise it is.
 				for _, c := range destroyedCursors {
 					if c.GetFullPath() == dne.Path {
-						log.Info("`from` was a replication cursor and has already been destroyed")
+						log(ctx).Info("`from` was a replication cursor and has already been destroyed")
 						return
 					}
 				}
 				// fallthrough
 			}
-			log.WithError(err).Error("cannot release step-holds on or destroy step-bookmark of `from`")
+			log(ctx).WithError(err).Error("cannot release step-holds on or destroy step-bookmark of `from`")
 		} else {
-			log.Info("successfully released step-holds on or destroyed step-bookmark of `from`")
+			log(ctx).Info("successfully released step-holds on or destroyed step-bookmark of `from`")
 		}
 	}()
 	wg.Wait()
@@ -441,6 +458,8 @@ func (p *Sender) SendCompleted(ctx context.Context, r *pdu.SendCompletedReq) (*p
 }
 
 func (p *Sender) DestroySnapshots(ctx context.Context, req *pdu.DestroySnapshotsReq) (*pdu.DestroySnapshotsRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	dp, err := p.filterCheckFS(req.Filesystem)
 	if err != nil {
 		return nil, err
@@ -449,6 +468,8 @@ func (p *Sender) DestroySnapshots(ctx context.Context, req *pdu.DestroySnapshots
 }
 
 func (p *Sender) Ping(ctx context.Context, req *pdu.PingReq) (*pdu.PingRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	res := pdu.PingRes{
 		Echo: req.GetMessage(),
 	}
@@ -456,14 +477,20 @@ func (p *Sender) Ping(ctx context.Context, req *pdu.PingReq) (*pdu.PingRes, erro
 }
 
 func (p *Sender) PingDataconn(ctx context.Context, req *pdu.PingReq) (*pdu.PingRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	return p.Ping(ctx, req)
 }
 
 func (p *Sender) WaitForConnectivity(ctx context.Context) error {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	return nil
 }
 
 func (p *Sender) ReplicationCursor(ctx context.Context, req *pdu.ReplicationCursorReq) (*pdu.ReplicationCursorRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	dp, err := p.filterCheckFS(req.Filesystem)
 	if err != nil {
 		return nil, err
@@ -594,6 +621,8 @@ func (f subroot) MapToLocal(fs string) (*zfs.DatasetPath, error) {
 }
 
 func (s *Receiver) ListFilesystems(ctx context.Context, req *pdu.ListFilesystemReq) (*pdu.ListFilesystemRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	root := s.clientRootFromCtx(ctx)
 	filtered, err := zfs.ZFSListMapping(ctx, subroot{root})
 	if err != nil {
@@ -644,6 +673,8 @@ func (s *Receiver) ListFilesystems(ctx context.Context, req *pdu.ListFilesystemR
 }
 
 func (s *Receiver) ListFilesystemVersions(ctx context.Context, req *pdu.ListFilesystemVersionsReq) (*pdu.ListFilesystemVersionsRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	root := s.clientRootFromCtx(ctx)
 	lp, err := subroot{root}.MapToLocal(req.GetFilesystem())
 	if err != nil {
@@ -665,6 +696,8 @@ func (s *Receiver) ListFilesystemVersions(ctx context.Context, req *pdu.ListFile
 }
 
 func (s *Receiver) Ping(ctx context.Context, req *pdu.PingReq) (*pdu.PingRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	res := pdu.PingRes{
 		Echo: req.GetMessage(),
 	}
@@ -672,24 +705,30 @@ func (s *Receiver) Ping(ctx context.Context, req *pdu.PingReq) (*pdu.PingRes, er
 }
 
 func (s *Receiver) PingDataconn(ctx context.Context, req *pdu.PingReq) (*pdu.PingRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	return s.Ping(ctx, req)
 }
 
 func (s *Receiver) WaitForConnectivity(ctx context.Context) error {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	return nil
 }
 
-func (s *Receiver) ReplicationCursor(context.Context, *pdu.ReplicationCursorReq) (*pdu.ReplicationCursorRes, error) {
+func (s *Receiver) ReplicationCursor(ctx context.Context, _ *pdu.ReplicationCursorReq) (*pdu.ReplicationCursorRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	return nil, fmt.Errorf("ReplicationCursor not implemented for Receiver")
 }
 
 func (s *Receiver) Send(ctx context.Context, req *pdu.SendReq) (*pdu.SendRes, io.ReadCloser, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	return nil, nil, fmt.Errorf("receiver does not implement Send()")
 }
 
 var maxConcurrentZFSRecvSemaphore = semaphore.New(envconst.Int64("ZREPL_ENDPOINT_MAX_CONCURRENT_RECV", 10))
 
 func (s *Receiver) Receive(ctx context.Context, req *pdu.ReceiveReq, receive io.ReadCloser) (*pdu.ReceiveRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	getLogger(ctx).Debug("incoming Receive")
 	defer receive.Close()
 
@@ -918,6 +957,8 @@ func (s *Receiver) Receive(ctx context.Context, req *pdu.ReceiveReq, receive io.
 }
 
 func (s *Receiver) DestroySnapshots(ctx context.Context, req *pdu.DestroySnapshotsReq) (*pdu.DestroySnapshotsRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	root := s.clientRootFromCtx(ctx)
 	lp, err := subroot{root}.MapToLocal(req.Filesystem)
 	if err != nil {
@@ -927,6 +968,7 @@ func (s *Receiver) DestroySnapshots(ctx context.Context, req *pdu.DestroySnapsho
 }
 
 func (p *Receiver) HintMostRecentCommonAncestor(ctx context.Context, r *pdu.HintMostRecentCommonAncestorReq) (*pdu.HintMostRecentCommonAncestorRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	// we don't move last-received-hold as part of this hint
 	// because that wouldn't give us any benefit wrt resumability.
 	//
@@ -935,7 +977,9 @@ func (p *Receiver) HintMostRecentCommonAncestor(ctx context.Context, r *pdu.Hint
 	return &pdu.HintMostRecentCommonAncestorRes{}, nil
 }
 
-func (p *Receiver) SendCompleted(context.Context, *pdu.SendCompletedReq) (*pdu.SendCompletedRes, error) {
+func (p *Receiver) SendCompleted(ctx context.Context, _ *pdu.SendCompletedReq) (*pdu.SendCompletedRes, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
+
 	return &pdu.SendCompletedRes{}, nil
 }
 
@@ -957,7 +1001,7 @@ func doDestroySnapshots(ctx context.Context, lp *zfs.DatasetPath, snaps []*pdu.F
 			ErrOut:     &errs[i],
 		}
 	}
-	zfs.ZFSDestroyFilesystemVersions(reqs)
+	zfs.ZFSDestroyFilesystemVersions(ctx, reqs)
 	for i := range reqs {
 		if errs[i] != nil {
 			if de, ok := errs[i].(*zfs.DestroySnapshotsError); ok && len(de.Reason) == 1 {

endpoint/endpoint_zfs_abstraction.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/pkg/errors"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"github.com/zrepl/zrepl/util/envconst"
 	"github.com/zrepl/zrepl/util/semaphore"
 	"github.com/zrepl/zrepl/zfs"
@@ -529,15 +530,16 @@ func ListAbstractionsStreamed(ctx context.Context, query ListZFSHoldsAndBookmark
 	}
 
 	sem := semaphore.New(int64(query.Concurrency))
+	ctx, endTask := trace.WithTask(ctx, "list-abstractions-streamed-producer")
 	go func() {
+		defer endTask()
 		defer close(out)
 		defer close(outErrs)
-		var wg sync.WaitGroup
-		defer wg.Wait()
+
+		_, add, wait := trace.WithTaskGroup(ctx, "list-abstractions-impl-fs")
+		defer wait()
 		for i := range fss {
-			wg.Add(1)
-			go func(i int) {
-				defer wg.Done()
+			add(func(ctx context.Context) {
 				g, err := sem.Acquire(ctx)
 				if err != nil {
 					errCb(err, fss[i], err.Error())
@@ -547,7 +549,7 @@ func ListAbstractionsStreamed(ctx context.Context, query ListZFSHoldsAndBookmark
 					defer g.Release()
 					listAbstractionsImplFS(ctx, fss[i], &query, emitAbstraction, errCb)
 				}()
-			}(i)
+			})
 		}
 	}()
 

go.mod

@@ -5,6 +5,7 @@ go 1.12
 require (
 	github.com/fatih/color v1.7.0
 	github.com/gdamore/tcell v1.2.0
+	github.com/gitchander/permutation v0.0.0-20181107151852-9e56b92e9909
 	github.com/go-logfmt/logfmt v0.4.0
 	github.com/go-sql-driver/mysql v1.4.1-0.20190907122137-b2c03bcae3d4
 	github.com/golang/protobuf v1.3.2
@@ -23,10 +24,12 @@ require (
 	github.com/pkg/profile v1.2.1
 	github.com/problame/go-netssh v0.0.0-20191209123953-18d8aa6923c7
 	github.com/prometheus/client_golang v1.2.1
+	github.com/prometheus/common v0.7.0
 	github.com/sergi/go-diff v1.0.1-0.20180205163309-da645544ed44 // go1.12 thinks it needs this
 	github.com/spf13/cobra v0.0.2
 	github.com/spf13/pflag v1.0.5
 	github.com/stretchr/testify v1.4.0
+	github.com/willf/bitset v1.1.10
 	github.com/yudai/gojsondiff v0.0.0-20170107030110-7b1b7adf999d
 	github.com/yudai/golcs v0.0.0-20170316035057-ecda9a501e82 // go1.12 thinks it needs this
 	github.com/zrepl/yaml-config v0.0.0-20191220194647-cbb6b0cf4bdd

go.sum

@@ -7,8 +7,10 @@ github.com/OpenPeeDeeP/depguard v0.0.0-20181229194401-1f388ab2d810/go.mod h1:7/4
 github.com/StackExchange/wmi v0.0.0-20180116203802-5d049714c4a6/go.mod h1:3eOhrUMpNV+6aFIbp5/iudMxNCF27Vw2OZgy4xEx0Fg=
 github.com/ajstarks/svgo v0.0.0-20180226025133-644b8db467af/go.mod h1:K08gAheRH3/J6wwsYMMT4xOr94bZjxIelGM0+d/wbFw=
 github.com/alecthomas/template v0.0.0-20160405071501-a0175ee3bccc/go.mod h1:LOuyumcjzFXgccqObfd/Ljyb9UuFJ6TxHnclSeseNhc=
+github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751 h1:JYp7IbQjafoB+tBA3gMyHYHrpOtNuDiK/uB5uXxq5wM=
 github.com/alecthomas/template v0.0.0-20190718012654-fb15b899a751/go.mod h1:LOuyumcjzFXgccqObfd/Ljyb9UuFJ6TxHnclSeseNhc=
 github.com/alecthomas/units v0.0.0-20151022065526-2efee857e7cf/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
+github.com/alecthomas/units v0.0.0-20190717042225-c3de453c63f4 h1:Hs82Z41s6SdL1CELW+XaDYmOH4hkBN4/N9og/AsOv7E=
 github.com/alecthomas/units v0.0.0-20190717042225-c3de453c63f4/go.mod h1:ybxpYRFXyAe+OPACYpWeL0wqObRcbAqCMya13uyzqw0=
 github.com/alvaroloes/enumer v1.1.1/go.mod h1:FxrjvuXoDAx9isTJrv4c+T410zFi0DtXIT0m65DJ+Wo=
 github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
@@ -38,6 +40,8 @@ github.com/gdamore/encoding v1.0.0 h1:+7OoQ1Bc6eTm5niUzBa0Ctsh6JbMW6Ra+YNuAtDBdk
 github.com/gdamore/encoding v1.0.0/go.mod h1:alR0ol34c49FCSBLjhosxzcPHQbf2trDkoo5dl+VrEg=
 github.com/gdamore/tcell v1.2.0 h1:ikixzsxc8K8o3V2/CEmyoEW8mJZaNYQQ3NP3VIQdUe4=
 github.com/gdamore/tcell v1.2.0/go.mod h1:Hjvr+Ofd+gLglo7RYKxxnzCBmev3BzsS67MebKS4zMM=
+github.com/gitchander/permutation v0.0.0-20181107151852-9e56b92e9909 h1:9NC8seTx6/zRmMTAdsHj/uOMi0EGHGQtjyLafBjk77Q=
+github.com/gitchander/permutation v0.0.0-20181107151852-9e56b92e9909/go.mod h1:lP+DW8LR6Rw3ru9Vo2/y/3iiLaLWmofYql/va+7zJOk=
 github.com/go-critic/go-critic v0.3.4/go.mod h1:AHR42Lk/E/aOznsrYdMYeIQS5RH10HZHSqP+rD6AJrc=
 github.com/go-critic/go-critic v0.3.5-0.20190526074819-1df300866540/go.mod h1:+sE8vrLDS2M0pZkBk0wy6+nLdKexVDrl/jBqQOTDThA=
 github.com/go-kit/kit v0.8.0/go.mod h1:xBxKIO96dXMWWy0MnWVtmwkA9/13aqxPnvrjFYMA2as=
@@ -242,6 +246,7 @@ github.com/shurcooL/go-goon v0.0.0-20170922171312-37c2f522c041/go.mod h1:N5mDOms
 github.com/sirupsen/logrus v1.0.5/go.mod h1:pMByvHTf9Beacp5x1UXfOR9xyW/9antXMhjMPG0dEzc=
 github.com/sirupsen/logrus v1.2.0/go.mod h1:LxeOpSwHxABJmUn/MG1IvRgCAasNZTLOkJPxbbu5VWo=
 github.com/sirupsen/logrus v1.4.0/go.mod h1:LxeOpSwHxABJmUn/MG1IvRgCAasNZTLOkJPxbbu5VWo=
+github.com/sirupsen/logrus v1.4.2 h1:SPIRibHv4MatM3XXNO2BJeFLZwZ2LvZgfQ5+UNI2im4=
 github.com/sirupsen/logrus v1.4.2/go.mod h1:tLMulIdttU9McNUspp0xgXVQah82FyeX6MwdIuYE2rE=
 github.com/sourcegraph/go-diff v0.5.1/go.mod h1:j2dHj3m8aZgQO8lMTcTnBcXkRRRqi34cd2MNlA9u1mE=
 github.com/spf13/afero v1.1.0/go.mod h1:j4pytiNVoe2o6bmDsKpLACNPDBIoEAkihy7loJ1B0CQ=
@@ -277,6 +282,8 @@ github.com/valyala/bytebufferpool v1.0.0/go.mod h1:6bBcMArwyJ5K/AmCkWv1jt77kVWyC
 github.com/valyala/fasthttp v1.2.0/go.mod h1:4vX61m6KN+xDduDNwXrhIAVZaZaZiQ1luJk8LWSxF3s=
 github.com/valyala/quicktemplate v1.1.1/go.mod h1:EH+4AkTd43SvgIbQHYu59/cJyxDoOVRUAfrukLPuGJ4=
 github.com/valyala/tcplisten v0.0.0-20161114210144-ceec8f93295a/go.mod h1:v3UYOV9WzVtRmSR+PDvWpU/qWl4Wa5LApYYX4ZtKbio=
+github.com/willf/bitset v1.1.10 h1:NotGKqX0KwQ72NUzqrjZq5ipPNDQex9lo3WpaS8L2sc=
+github.com/willf/bitset v1.1.10/go.mod h1:RjeCKbqT1RxIR/KWY6phxZiaY1IyutSBfGjNPySAYV4=
 github.com/xordataexchange/crypt v0.0.3-0.20170626215501-b2862e3d0a77/go.mod h1:aYKd//L2LvnjZzWKhF00oedf4jCCReLcmhLdhm1A27Q=
 github.com/yudai/gojsondiff v0.0.0-20170107030110-7b1b7adf999d h1:yJIizrfO599ot2kQ6Af1enICnwBD3XoxgX3MrMwot2M=
 github.com/yudai/gojsondiff v0.0.0-20170107030110-7b1b7adf999d/go.mod h1:AY32+k2cwILAkW1fbgxQ5mUmMiZFgLIV+FBNExI05xg=
@@ -366,6 +373,7 @@ google.golang.org/grpc v1.16.0/go.mod h1:0JHn/cJsOMiMfNA9+DeHDlAU7KAAB5GDlYFpa9M
 google.golang.org/grpc v1.17.0 h1:TRJYBgMclJvGYn2rIMjj+h9KtMt5r1Ij7ODVRIZkwhk=
 google.golang.org/grpc v1.17.0/go.mod h1:6QZJwpn2B+Zp71q/5VxRsJ6NXXVCE5NRUHRo+f3cWCs=
 gopkg.in/airbrake/gobrake.v2 v2.0.9/go.mod h1:/h5ZAUhDkGaJfjzjKLSjv6zCL6O0LLBxU4K+aSYdM/U=
+gopkg.in/alecthomas/kingpin.v2 v2.2.6 h1:jMFz6MfLP0/4fUyZle81rXUoxOBFi19VUFKVDOQfozc=
 gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLkstjWtayDeSgw=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

platformtest/harness/harness.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/fatih/color"
 	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/logging"
@@ -68,7 +69,9 @@ func doMain() error {
 		logger.Error(err.Error())
 		panic(err)
 	}
-	ctx := platformtest.WithLogger(context.Background(), logger)
+	ctx := context.Background()
+	defer trace.WithTaskFromStackUpdateCtx(&ctx)()
+	ctx = logging.WithLoggers(ctx, logging.SubsystemLoggersWithUniversalLogger(logger))
 	ex := platformtest.NewEx(logger)
 
 	type invocation struct {

platformtest/platformtest_logging.go

@@ -3,22 +3,12 @@ package platformtest
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
 type Logger = logger.Logger
 
-type contextKey int
-
-const (
-	contextKeyLogger contextKey = iota
-)
-
-func WithLogger(ctx context.Context, logger Logger) context.Context {
-	ctx = context.WithValue(ctx, contextKeyLogger, logger)
-	return ctx
-}
-
 func GetLog(ctx context.Context) Logger {
-	return ctx.Value(contextKeyLogger).(Logger)
+	return logging.GetLogger(ctx, logging.SubsysPlatformtest)
 }

platformtest/tests/batchDestroy.go

@@ -32,7 +32,7 @@ func BatchDestroy(ctx *platformtest.Context) {
 			Name:       "2",
 		},
 	}
-	zfs.ZFSDestroyFilesystemVersions(reqs)
+	zfs.ZFSDestroyFilesystemVersions(ctx, reqs)
 	if *reqs[0].ErrOut != nil {
 		panic("expecting no error")
 	}

replication/driver/replication_driver.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
 
@@ -287,6 +288,17 @@ func Do(ctx context.Context, planner Planner) (ReportFunc, WaitFunc) {
 }
 
 func (a *attempt) do(ctx context.Context, prev *attempt) {
+	prevs := a.doGlobalPlanning(ctx, prev)
+	if prevs == nil {
+		return
+	}
+	a.doFilesystems(ctx, prevs)
+}
+
+// if no error occurs, returns a map that maps this attempt's a.fss to `prev`'s a.fss
+func (a *attempt) doGlobalPlanning(ctx context.Context, prev *attempt) map[*fs]*fs {
+	ctx, endSpan := trace.WithSpan(ctx, "plan")
+	defer endSpan()
 	pfss, err := a.planner.Plan(ctx)
 	errTime := time.Now()
 	defer a.l.Lock().Unlock()
@@ -294,7 +306,7 @@ func (a *attempt) do(ctx context.Context, prev *attempt) {
 		a.planErr = newTimedError(err, errTime)
 		a.fss = nil
 		a.finishedAt = time.Now()
-		return
+		return nil
 	}
 
 	for _, pfs := range pfss {
@@ -351,7 +363,7 @@ func (a *attempt) do(ctx context.Context, prev *attempt) {
 			a.planErr = newTimedError(errors.New(msg.String()), now)
 			a.fss = nil
 			a.finishedAt = now
-			return
+			return nil
 		}
 		for cur, fss := range prevFSs {
 			if len(fss) > 0 {
@@ -373,6 +385,15 @@ func (a *attempt) do(ctx context.Context, prev *attempt) {
 		}
 	}
 
+	return prevs
+}
+
+func (a *attempt) doFilesystems(ctx context.Context, prevs map[*fs]*fs) {
+	ctx, endSpan := trace.WithSpan(ctx, "do-repl")
+	defer endSpan()
+
+	defer a.l.Lock().Unlock()
+
 	stepQueue := newStepQueue()
 	defer stepQueue.Start(envconst.Int("ZREPL_REPLICATION_EXPERIMENTAL_REPLICATION_CONCURRENCY", 1))() // TODO parallel replication
 	var fssesDone sync.WaitGroup
@@ -380,6 +401,9 @@ func (a *attempt) do(ctx context.Context, prev *attempt) {
 		fssesDone.Add(1)
 		go func(f *fs) {
 			defer fssesDone.Done()
+			// avoid explosion of tasks with name f.report().Info.Name
+			ctx, endTask := trace.WithTaskAndSpan(ctx, "repl-fs", f.report().Info.Name)
+			defer endTask()
 			f.do(ctx, stepQueue, prevs[f])
 		}(f)
 	}
@@ -423,7 +447,7 @@ func (f *fs) do(ctx context.Context, pq *stepQueue, prev *fs) {
 		// TODO hacky
 		// choose target time that is earlier than any snapshot, so fs planning is always prioritized
 		targetDate := time.Unix(0, 0)
-		defer pq.WaitReady(f, targetDate)()
+		defer pq.WaitReady(ctx, f, targetDate)()
 		psteps, err = f.fs.PlanFS(ctx) // no shadow
 		errTime = time.Now()           // no shadow
 	})
@@ -584,8 +608,10 @@ func (f *fs) do(ctx context.Context, pq *stepQueue, prev *fs) {
 		f.l.DropWhile(func() {
 			// wait for parallel replication
 			targetDate := s.step.TargetDate()
-			defer pq.WaitReady(f, targetDate)()
+			defer pq.WaitReady(ctx, f, targetDate)()
 			// do the step
+			ctx, endSpan := trace.WithSpan(ctx, fmt.Sprintf("%#v", s.step.ReportInfo()))
+			defer endSpan()
 			err, errTime = s.step.Step(ctx), time.Now() // no shadow
 		})
 

replication/driver/replication_driver_logging.go

@@ -3,23 +3,10 @@ package driver
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
-type Logger = logger.Logger
-
-type contextKey int
-
-const contextKeyLogger contextKey = iota + 1
-
-func getLog(ctx context.Context) Logger {
-	l, ok := ctx.Value(contextKeyLogger).(Logger)
-	if !ok {
-		l = logger.NewNullLogger()
-	}
-	return l
-}
-
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLogger, log)
+func getLog(ctx context.Context) logger.Logger {
+	return logging.GetLogger(ctx, logging.SubsysReplication)
 }

replication/driver/replication_driver_test.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/require"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
 	"github.com/zrepl/zrepl/replication/report"
 
@@ -149,6 +150,7 @@ func (f *mockStep) ReportInfo() *report.StepInfo {
 func TestReplication(t *testing.T) {
 
 	ctx := context.Background()
+	defer trace.WithTaskFromStackUpdateCtx(&ctx)()
 
 	mp := &mockPlanner{}
 	getReport, wait := Do(ctx, mp)

replication/driver/replication_stepqueue.go

@@ -2,8 +2,10 @@ package driver
 
 import (
 	"container/heap"
+	"context"
 	"time"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"github.com/zrepl/zrepl/util/chainlock"
 )
 
@@ -155,7 +157,8 @@ func (q *stepQueue) sendAndWaitForWakeup(ident interface{}, targetDate time.Time
 }
 
 // Wait for the ident with targetDate to be selected to run.
-func (q *stepQueue) WaitReady(ident interface{}, targetDate time.Time) StepCompletedFunc {
+func (q *stepQueue) WaitReady(ctx context.Context, ident interface{}, targetDate time.Time) StepCompletedFunc {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	if targetDate.IsZero() {
 		panic("targetDate of zero is reserved for marking Done")
 	}

replication/driver/replication_stepqueue_test.go

@@ -1,6 +1,7 @@
 package driver
 
 import (
+	"context"
 	"fmt"
 	"math"
 	"sort"
@@ -11,18 +12,23 @@ import (
 
 	"github.com/montanaflynn/stats"
 	"github.com/stretchr/testify/assert"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 )
 
 // FIXME: this test relies on timing and is thus rather flaky
 // (relies on scheduler responsiveness of < 500ms)
 func TestPqNotconcurrent(t *testing.T) {
+	ctx, end := trace.WithTaskFromStack(context.Background())
+	defer end()
 	var ctr uint32
 	q := newStepQueue()
 	var wg sync.WaitGroup
 	wg.Add(4)
 	go func() {
+		ctx, end := trace.WithTaskFromStack(ctx)
+		defer end()
 		defer wg.Done()
-		defer q.WaitReady("1", time.Unix(9999, 0))()
+		defer q.WaitReady(ctx, "1", time.Unix(9999, 0))()
 		ret := atomic.AddUint32(&ctr, 1)
 		assert.Equal(t, uint32(1), ret)
 		time.Sleep(1 * time.Second)
@@ -34,20 +40,26 @@ func TestPqNotconcurrent(t *testing.T) {
 
 	// while "1" is still running, queue in "2", "3" and "4"
 	go func() {
+		ctx, end := trace.WithTaskFromStack(ctx)
+		defer end()
 		defer wg.Done()
-		defer q.WaitReady("2", time.Unix(2, 0))()
+		defer q.WaitReady(ctx, "2", time.Unix(2, 0))()
 		ret := atomic.AddUint32(&ctr, 1)
 		assert.Equal(t, uint32(2), ret)
 	}()
 	go func() {
+		ctx, end := trace.WithTaskFromStack(ctx)
+		defer end()
 		defer wg.Done()
-		defer q.WaitReady("3", time.Unix(3, 0))()
+		defer q.WaitReady(ctx, "3", time.Unix(3, 0))()
 		ret := atomic.AddUint32(&ctr, 1)
 		assert.Equal(t, uint32(3), ret)
 	}()
 	go func() {
+		ctx, end := trace.WithTaskFromStack(ctx)
+		defer end()
 		defer wg.Done()
-		defer q.WaitReady("4", time.Unix(4, 0))()
+		defer q.WaitReady(ctx, "4", time.Unix(4, 0))()
 		ret := atomic.AddUint32(&ctr, 1)
 		assert.Equal(t, uint32(4), ret)
 	}()
@@ -77,6 +89,8 @@ func (r record) String() string {
 // Hence, perform some statistics on the wakeup times and assert that the mean wakeup
 // times for each step are close together.
 func TestPqConcurrent(t *testing.T) {
+	ctx, end := trace.WithTaskFromStack(context.Background())
+	defer end()
 
 	q := newStepQueue()
 	var wg sync.WaitGroup
@@ -90,12 +104,14 @@ func TestPqConcurrent(t *testing.T) {
 	records := make(chan []record, filesystems)
 	for fs := 0; fs < filesystems; fs++ {
 		go func(fs int) {
+			ctx, end := trace.WithTaskFromStack(ctx)
+			defer end()
 			defer wg.Done()
 			recs := make([]record, 0)
 			for step := 0; step < stepsPerFS; step++ {
 				pos := atomic.AddUint32(&globalCtr, 1)
 				t := time.Unix(int64(step), 0)
-				done := q.WaitReady(fs, t)
+				done := q.WaitReady(ctx, fs, t)
 				wakeAt := time.Since(begin)
 				time.Sleep(sleepTimePerStep)
 				done()

replication/logic/replication_logic.go

@@ -9,7 +9,9 @@ import (
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 
+	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/replication/driver"
 	. "github.com/zrepl/zrepl/replication/logic/diff"
 	"github.com/zrepl/zrepl/replication/logic/pdu"
@@ -80,6 +82,8 @@ func (p *Planner) WaitForConnectivity(ctx context.Context) error {
 	var wg sync.WaitGroup
 	doPing := func(endpoint Endpoint, errOut *error) {
 		defer wg.Done()
+		ctx, endTask := trace.WithTaskFromStack(ctx)
+		defer endTask()
 		err := endpoint.WaitForConnectivity(ctx)
 		if err != nil {
 			*errOut = err
@@ -303,9 +307,11 @@ func (p *Planner) doPlanning(ctx context.Context) ([]*Filesystem, error) {
 
 func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 
-	log := getLogger(ctx).WithField("filesystem", fs.Path)
+	log := func(ctx context.Context) logger.Logger {
+		return getLogger(ctx).WithField("filesystem", fs.Path)
+	}
 
-	log.Debug("assessing filesystem")
+	log(ctx).Debug("assessing filesystem")
 
 	if fs.policy.EncryptedSend == True && !fs.senderFS.GetIsEncrypted() {
 		return nil, fmt.Errorf("sender filesystem is not encrypted but policy mandates encrypted send")
@@ -313,14 +319,14 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 
 	sfsvsres, err := fs.sender.ListFilesystemVersions(ctx, &pdu.ListFilesystemVersionsReq{Filesystem: fs.Path})
 	if err != nil {
-		log.WithError(err).Error("cannot get remote filesystem versions")
+		log(ctx).WithError(err).Error("cannot get remote filesystem versions")
 		return nil, err
 	}
 	sfsvs := sfsvsres.GetVersions()
 
 	if len(sfsvs) < 1 {
 		err := errors.New("sender does not have any versions")
-		log.Error(err.Error())
+		log(ctx).Error(err.Error())
 		return nil, err
 	}
 
@@ -328,7 +334,7 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 	if fs.receiverFS != nil && !fs.receiverFS.GetIsPlaceholder() {
 		rfsvsres, err := fs.receiver.ListFilesystemVersions(ctx, &pdu.ListFilesystemVersionsReq{Filesystem: fs.Path})
 		if err != nil {
-			log.WithError(err).Error("receiver error")
+			log(ctx).WithError(err).Error("receiver error")
 			return nil, err
 		}
 		rfsvs = rfsvsres.GetVersions()
@@ -340,17 +346,17 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 	var resumeTokenRaw string
 	if fs.receiverFS != nil && fs.receiverFS.ResumeToken != "" {
 		resumeTokenRaw = fs.receiverFS.ResumeToken // shadow
-		log.WithField("receiverFS.ResumeToken", resumeTokenRaw).Debug("decode receiver fs resume token")
+		log(ctx).WithField("receiverFS.ResumeToken", resumeTokenRaw).Debug("decode receiver fs resume token")
 		resumeToken, err = zfs.ParseResumeToken(ctx, resumeTokenRaw) // shadow
 		if err != nil {
 			// TODO in theory, we could do replication without resume token, but that would mean that
 			// we need to discard the resumable state on the receiver's side.
 			// Would be easy by setting UsedResumeToken=false in the RecvReq ...
 			// FIXME / CHECK semantics UsedResumeToken if SendReq.ResumeToken == ""
-			log.WithError(err).Error("cannot decode resume token, aborting")
+			log(ctx).WithError(err).Error("cannot decode resume token, aborting")
 			return nil, err
 		}
-		log.WithField("token", resumeToken).Debug("decode resume token")
+		log(ctx).WithField("token", resumeToken).Debug("decode resume token")
 	}
 
 	// give both sides a hint about how far prior replication attempts got
@@ -369,7 +375,10 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 		var wg sync.WaitGroup
 		doHint := func(ep Endpoint, name string) {
 			defer wg.Done()
-			log := log.WithField("to_side", name).
+			ctx, endTask := trace.WithTask(ctx, "hint-mrca-"+name)
+			defer endTask()
+
+			log := log(ctx).WithField("to_side", name).
 				WithField("sender_mrca", sender_mrca.String())
 			log.Debug("hint most recent common ancestor")
 			hint := &pdu.HintMostRecentCommonAncestorReq{
@@ -428,7 +437,7 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 			encryptionMatches = true
 		}
 
-		log.WithField("fromVersion", fromVersion).
+		log(ctx).WithField("fromVersion", fromVersion).
 			WithField("toVersion", toVersion).
 			WithField("encryptionMatches", encryptionMatches).
 			Debug("result of resume-token-matching to sender's versions")
@@ -484,11 +493,11 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 			var msg string
 			path, msg = resolveConflict(conflict) // no shadowing allowed!
 			if path != nil {
-				log.WithField("conflict", conflict).Info("conflict")
-				log.WithField("resolution", msg).Info("automatically resolved")
+				log(ctx).WithField("conflict", conflict).Info("conflict")
+				log(ctx).WithField("resolution", msg).Info("automatically resolved")
 			} else {
-				log.WithField("conflict", conflict).Error("conflict")
-				log.WithField("problem", msg).Error("cannot resolve conflict")
+				log(ctx).WithField("conflict", conflict).Error("conflict")
+				log(ctx).WithField("problem", msg).Error("cannot resolve conflict")
 			}
 		}
 		if len(path) == 0 {
@@ -522,37 +531,35 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 	}
 
 	if len(steps) == 0 {
-		log.Info("planning determined that no replication steps are required")
+		log(ctx).Info("planning determined that no replication steps are required")
 	}
 
-	log.Debug("compute send size estimate")
+	log(ctx).Debug("compute send size estimate")
 	errs := make(chan error, len(steps))
-	var wg sync.WaitGroup
 	fanOutCtx, fanOutCancel := context.WithCancel(ctx)
+	_, fanOutAdd, fanOutWait := trace.WithTaskGroup(fanOutCtx, "compute-size-estimate")
 	defer fanOutCancel()
 	for _, step := range steps {
-		wg.Add(1)
-		go func(step *Step) {
-			defer wg.Done()
-
+		step := step // local copy that is moved into the closure
+		fanOutAdd(func(ctx context.Context) {
 			// TODO instead of the semaphore, rely on resource-exhaustion signaled by the remote endpoint to limit size-estimate requests
 			// Send is handled over rpc/dataconn ATM, which doesn't support the resource exhaustion status codes that gRPC defines
-			guard, err := fs.sizeEstimateRequestSem.Acquire(fanOutCtx)
+			guard, err := fs.sizeEstimateRequestSem.Acquire(ctx)
 			if err != nil {
 				fanOutCancel()
 				return
 			}
 			defer guard.Release()
 
-			err = step.updateSizeEstimate(fanOutCtx)
+			err = step.updateSizeEstimate(ctx)
 			if err != nil {
-				log.WithError(err).WithField("step", step).Error("error computing size estimate")
+				log(ctx).WithError(err).WithField("step", step).Error("error computing size estimate")
 				fanOutCancel()
 			}
 			errs <- err
-		}(step)
+		})
 	}
-	wg.Wait()
+	fanOutWait()
 	close(errs)
 	var significantErr error = nil
 	for err := range errs {
@@ -566,7 +573,7 @@ func (fs *Filesystem) doPlanning(ctx context.Context) ([]*Step, error) {
 		return nil, significantErr
 	}
 
-	log.Debug("filesystem planning finished")
+	log(ctx).Debug("filesystem planning finished")
 	return steps, nil
 }
 

replication/logic/replication_logic_context.go

@@ -3,26 +3,10 @@ package logic
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
-type contextKey int
-
-const (
-	contextKeyLog contextKey = iota
-)
-
-type Logger = logger.Logger
-
-func WithLogger(ctx context.Context, l Logger) context.Context {
-	ctx = context.WithValue(ctx, contextKeyLog, l)
-	return ctx
-}
-
-func getLogger(ctx context.Context) Logger {
-	l, ok := ctx.Value(contextKeyLog).(Logger)
-	if !ok {
-		l = logger.NewNullLogger()
-	}
-	return l
+func getLogger(ctx context.Context) logger.Logger {
+	return logging.GetLogger(ctx, logging.SubsysReplication)
 }

rpc/dataconn/dataconn_client.go

@@ -143,7 +143,6 @@ func (c *Client) ReqSend(ctx context.Context, req *pdu.SendReq) (*pdu.SendRes, i
 }
 
 func (c *Client) ReqRecv(ctx context.Context, req *pdu.ReceiveReq, stream io.ReadCloser) (*pdu.ReceiveRes, error) {
-
 	defer c.log.Debug("ReqRecv returns")
 	conn, err := c.getWire(ctx)
 	if err != nil {

rpc/dataconn/dataconn_server.go

@@ -33,16 +33,33 @@ type Handler interface {
 
 type Logger = logger.Logger
 
+type ContextInterceptorData interface {
+	FullMethod() string
+	ClientIdentity() string
+}
+
+type ContextInterceptor = func(ctx context.Context, data ContextInterceptorData, handler func(ctx context.Context))
+
 type Server struct {
 	h   Handler
 	wi  WireInterceptor
+	ci  ContextInterceptor
 	log Logger
 }
 
-func NewServer(wi WireInterceptor, logger Logger, handler Handler) *Server {
+var noopContextInteceptor = func(ctx context.Context, _ ContextInterceptorData, handler func(context.Context)) {
+	handler(ctx)
+}
+
+// wi and ci may be nil
+func NewServer(wi WireInterceptor, ci ContextInterceptor, logger Logger, handler Handler) *Server {
+	if ci == nil {
+		ci = noopContextInteceptor
+	}
 	return &Server{
 		h:   handler,
 		wi:  wi,
+		ci:  ci,
 		log: logger,
 	}
 }
@@ -93,6 +110,14 @@ func (s *Server) Serve(ctx context.Context, l transport.AuthenticatedListener) {
 	}
 }
 
+type contextInterceptorData struct {
+	fullMethod     string
+	clientIdentity string
+}
+
+func (d contextInterceptorData) FullMethod() string     { return d.fullMethod }
+func (d contextInterceptorData) ClientIdentity() string { return d.clientIdentity }
+
 func (s *Server) serveConn(nc *transport.AuthConn) {
 	s.log.Debug("serveConn begin")
 	defer s.log.Debug("serveConn done")
@@ -117,6 +142,17 @@ func (s *Server) serveConn(nc *transport.AuthConn) {
 	}
 	endpoint := string(header)
 
+	data := contextInterceptorData{
+		fullMethod:     endpoint,
+		clientIdentity: nc.ClientIdentity(),
+	}
+	s.ci(ctx, data, func(ctx context.Context) {
+		s.serveConnRequest(ctx, endpoint, c)
+	})
+}
+
+func (s *Server) serveConnRequest(ctx context.Context, endpoint string, c *stream.Conn) {
+
 	reqStructured, err := c.ReadStreamedMessage(ctx, RequestStructuredMaxSize, ReqStructured)
 	if err != nil {
 		s.log.WithError(err).Error("error reading structured part")

rpc/dataconn/microbenchmark/microbenchmark.go

@@ -112,7 +112,7 @@ func server() {
 	orDie(err)
 	l := tcpListener{nl.(*net.TCPListener), "fakeclientidentity"}
 
-	srv := dataconn.NewServer(nil, logger.NewStderrDebugLogger(), devNullHandler{})
+	srv := dataconn.NewServer(nil, nil, logger.NewStderrDebugLogger(), devNullHandler{})
 
 	ctx := context.Background()
 

rpc/dataconn/stream/stream.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"net"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"unicode/utf8"
 
@@ -80,9 +81,13 @@ func doWriteStream(ctx context.Context, c *heartbeatconn.Conn, stream io.Reader,
 		err error
 	}
 
+	var wg sync.WaitGroup
+	defer wg.Wait()
 	reads := make(chan read, 5)
 	var stopReading uint32
+	wg.Add(1)
 	go func() {
+		defer wg.Done()
 		defer close(reads)
 		for atomic.LoadUint32(&stopReading) == 0 {
 			buffer := bufpool.Get(1 << FramePayloadShift)

rpc/grpcclientidentity/authlistener_grpc_adaptor.go

@@ -99,10 +99,23 @@ func (*transportCredentials) OverrideServerName(string) error {
 	panic("not implemented")
 }
 
-type ContextInterceptor = func(ctx context.Context) context.Context
+type ContextInterceptorData interface {
+	FullMethod() string
+	ClientIdentity() string
+}
 
-func NewInterceptors(logger Logger, clientIdentityKey interface{}, ctxInterceptor ContextInterceptor) (unary grpc.UnaryServerInterceptor, stream grpc.StreamServerInterceptor) {
-	unary = func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (resp interface{}, err error) {
+type contextInterceptorData struct {
+	fullMethod     string
+	clientIdentity string
+}
+
+func (d contextInterceptorData) FullMethod() string     { return d.fullMethod }
+func (d contextInterceptorData) ClientIdentity() string { return d.clientIdentity }
+
+type Interceptor = func(ctx context.Context, data ContextInterceptorData, handler func(ctx context.Context))
+
+func NewInterceptors(logger Logger, clientIdentityKey interface{}, interceptor Interceptor) (unary grpc.UnaryServerInterceptor, stream grpc.StreamServerInterceptor) {
+	unary = func(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
 		logger.WithField("fullMethod", info.FullMethod).Debug("request")
 		p, ok := peer.FromContext(ctx)
 		if !ok {
@@ -115,10 +128,18 @@ func NewInterceptors(logger Logger, clientIdentityKey interface{}, ctxIntercepto
 		}
 		logger.WithField("peer_client_identity", a.clientIdentity).Debug("peer client identity")
 		ctx = context.WithValue(ctx, clientIdentityKey, a.clientIdentity)
-		if ctxInterceptor != nil {
-			ctx = ctxInterceptor(ctx)
+		data := contextInterceptorData{
+			fullMethod:     info.FullMethod,
+			clientIdentity: a.clientIdentity,
 		}
-		return handler(ctx, req)
+		var (
+			resp interface{}
+			err  error
+		)
+		interceptor(ctx, data, func(ctx context.Context) {
+			resp, err = handler(ctx, req) // no-shadow
+		})
+		return resp, err
 	}
 	stream = func(srv interface{}, ss grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
 		panic("unimplemented")

rpc/grpcclientidentity/grpchelper/authlistener_grpc_adaptor_wrapper.go

@@ -50,7 +50,7 @@ func ClientConn(cn transport.Connecter, log Logger) *grpc.ClientConn {
 }
 
 // NewServer is a convenience interface around the TransportCredentials and Interceptors interface.
-func NewServer(authListener transport.AuthenticatedListener, clientIdentityKey interface{}, logger grpcclientidentity.Logger, ctxInterceptor grpcclientidentity.ContextInterceptor) (srv *grpc.Server, serve func() error) {
+func NewServer(authListener transport.AuthenticatedListener, clientIdentityKey interface{}, logger grpcclientidentity.Logger, ctxInterceptor grpcclientidentity.Interceptor) (srv *grpc.Server, serve func() error) {
 	ka := grpc.KeepaliveParams(keepalive.ServerParameters{
 		Time:    StartKeepalivesAfterInactivityDuration,
 		Timeout: KeepalivePeerTimeout,

rpc/rpc_client.go

@@ -10,6 +10,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"google.golang.org/grpc"
 
 	"github.com/google/uuid"
@@ -83,6 +84,9 @@ func (c *Client) Close() {
 // callers must ensure that the returned io.ReadCloser is closed
 // TODO expose dataClient interface to the outside world
 func (c *Client) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.ReadCloser, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.Send")
+	defer endSpan()
+
 	// TODO the returned sendStream may return a read error created by the remote side
 	res, stream, err := c.dataClient.ReqSend(ctx, r)
 	if err != nil {
@@ -97,34 +101,58 @@ func (c *Client) Send(ctx context.Context, r *pdu.SendReq) (*pdu.SendRes, io.Rea
 }
 
 func (c *Client) Receive(ctx context.Context, req *pdu.ReceiveReq, stream io.ReadCloser) (*pdu.ReceiveRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.Receive")
+	defer endSpan()
+
 	return c.dataClient.ReqRecv(ctx, req, stream)
 }
 
 func (c *Client) ListFilesystems(ctx context.Context, in *pdu.ListFilesystemReq) (*pdu.ListFilesystemRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.ListFilesystems")
+	defer endSpan()
+
 	return c.controlClient.ListFilesystems(ctx, in)
 }
 
 func (c *Client) ListFilesystemVersions(ctx context.Context, in *pdu.ListFilesystemVersionsReq) (*pdu.ListFilesystemVersionsRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.ListFilesystemVersions")
+	defer endSpan()
+
 	return c.controlClient.ListFilesystemVersions(ctx, in)
 }
 
 func (c *Client) DestroySnapshots(ctx context.Context, in *pdu.DestroySnapshotsReq) (*pdu.DestroySnapshotsRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.DestroySnapshots")
+	defer endSpan()
+
 	return c.controlClient.DestroySnapshots(ctx, in)
 }
 
 func (c *Client) ReplicationCursor(ctx context.Context, in *pdu.ReplicationCursorReq) (*pdu.ReplicationCursorRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.ReplicationCursor")
+	defer endSpan()
+
 	return c.controlClient.ReplicationCursor(ctx, in)
 }
 
 func (c *Client) SendCompleted(ctx context.Context, in *pdu.SendCompletedReq) (*pdu.SendCompletedRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.SendCompleted")
+	defer endSpan()
+
 	return c.controlClient.SendCompleted(ctx, in)
 }
 
 func (c *Client) HintMostRecentCommonAncestor(ctx context.Context, in *pdu.HintMostRecentCommonAncestorReq) (*pdu.HintMostRecentCommonAncestorRes, error) {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.HintMostRecentCommonAncestor")
+	defer endSpan()
+
 	return c.controlClient.HintMostRecentCommonAncestor(ctx, in)
 }
 
 func (c *Client) WaitForConnectivity(ctx context.Context) error {
+	ctx, endSpan := trace.WithSpan(ctx, "rpc.client.WaitForConnectivity")
+	defer endSpan()
+
 	ctx, cancel := context.WithCancel(ctx)
 	defer cancel()
 	msg := uuid.New().String()

rpc/rpc_logging.go

@@ -3,17 +3,12 @@ package rpc
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
 type Logger = logger.Logger
 
-type contextKey int
-
-const (
-	contextKeyLoggers contextKey = iota
-)
-
 /// All fields must be non-nil
 type Loggers struct {
 	General Logger
@@ -21,11 +16,10 @@ type Loggers struct {
 	Data    Logger
 }
 
-func WithLoggers(ctx context.Context, loggers Loggers) context.Context {
-	ctx = context.WithValue(ctx, contextKeyLoggers, loggers)
-	return ctx
-}
-
 func GetLoggersOrPanic(ctx context.Context) Loggers {
-	return ctx.Value(contextKeyLoggers).(Loggers)
+	return Loggers{
+		General: logging.GetLogger(ctx, logging.SubsysRPC),
+		Control: logging.GetLogger(ctx, logging.SubsysRPCControl),
+		Data:    logging.GetLogger(ctx, logging.SubsysRPCData),
+	}
 }

rpc/rpc_server.go

@@ -7,6 +7,7 @@ import (
 	"github.com/zrepl/zrepl/endpoint"
 	"github.com/zrepl/zrepl/replication/logic/pdu"
 	"github.com/zrepl/zrepl/rpc/dataconn"
+	"github.com/zrepl/zrepl/rpc/grpcclientidentity"
 	"github.com/zrepl/zrepl/rpc/grpcclientidentity/grpchelper"
 	"github.com/zrepl/zrepl/rpc/versionhandshake"
 	"github.com/zrepl/zrepl/transport"
@@ -30,7 +31,20 @@ type Server struct {
 	dataServerServe    serveFunc
 }
 
-type HandlerContextInterceptor func(ctx context.Context) context.Context
+type HandlerContextInterceptorData interface {
+	FullMethod() string
+	ClientIdentity() string
+}
+
+type interceptorData struct {
+	prefixMethod string
+	wrapped      HandlerContextInterceptorData
+}
+
+func (d interceptorData) ClientIdentity() string { return d.wrapped.ClientIdentity() }
+func (d interceptorData) FullMethod() string     { return d.prefixMethod + d.wrapped.FullMethod() }
+
+type HandlerContextInterceptor func(ctx context.Context, data HandlerContextInterceptorData, handler func(ctx context.Context))
 
 // config must be valid (use its Validate function).
 func NewServer(handler Handler, loggers Loggers, ctxInterceptor HandlerContextInterceptor) *Server {
@@ -38,7 +52,10 @@ func NewServer(handler Handler, loggers Loggers, ctxInterceptor HandlerContextIn
 	// setup control server
 	controlServerServe := func(ctx context.Context, controlListener transport.AuthenticatedListener, errOut chan<- error) {
 
-		controlServer, serve := grpchelper.NewServer(controlListener, endpoint.ClientIdentityKey, loggers.Control, ctxInterceptor)
+		var controlCtxInterceptor grpcclientidentity.Interceptor = func(ctx context.Context, data grpcclientidentity.ContextInterceptorData, handler func(ctx context.Context)) {
+			ctxInterceptor(ctx, interceptorData{"control://", data}, handler)
+		}
+		controlServer, serve := grpchelper.NewServer(controlListener, endpoint.ClientIdentityKey, loggers.Control, controlCtxInterceptor)
 		pdu.RegisterReplicationServer(controlServer, handler)
 
 		// give time for graceful stop until deadline expires, then hard stop
@@ -59,12 +76,12 @@ func NewServer(handler Handler, loggers Loggers, ctxInterceptor HandlerContextIn
 	dataServerClientIdentitySetter := func(ctx context.Context, wire *transport.AuthConn) (context.Context, *transport.AuthConn) {
 		ci := wire.ClientIdentity()
 		ctx = context.WithValue(ctx, endpoint.ClientIdentityKey, ci)
-		if ctxInterceptor != nil {
-			ctx = ctxInterceptor(ctx) // SHADOWING
-		}
 		return ctx, wire
 	}
-	dataServer := dataconn.NewServer(dataServerClientIdentitySetter, loggers.Data, handler)
+	var dataCtxInterceptor dataconn.ContextInterceptor = func(ctx context.Context, data dataconn.ContextInterceptorData, handler func(ctx context.Context)) {
+		ctxInterceptor(ctx, interceptorData{"data://", data}, handler)
+	}
+	dataServer := dataconn.NewServer(dataServerClientIdentitySetter, dataCtxInterceptor, loggers.Data, handler)
 	dataServerServe := func(ctx context.Context, dataListener transport.AuthenticatedListener, errOut chan<- error) {
 		dataServer.Serve(ctx, dataListener)
 		errOut <- nil // TODO bad design of dataServer?

rpc/transportmux/transportmux.go

@@ -15,27 +15,15 @@ import (
 	"net"
 	"time"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/transport"
 )
 
-type contextKey int
-
-const (
-	contextKeyLog contextKey = 1 + iota
-)
-
 type Logger = logger.Logger
 
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLog, log)
-}
-
 func getLog(ctx context.Context) Logger {
-	if l, ok := ctx.Value(contextKeyLog).(Logger); ok {
-		return l
-	}
-	return logger.NewNullLogger()
+	return logging.GetLogger(ctx, logging.SubsysTransportMux)
 }
 
 type acceptRes struct {

transport/transport.go

@@ -8,6 +8,7 @@ import (
 	"syscall"
 
 	"github.com/pkg/errors"
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 	"github.com/zrepl/zrepl/rpc/dataconn/timeoutconn"
 	"github.com/zrepl/zrepl/zfs"
@@ -63,19 +64,8 @@ func ValidateClientIdentity(in string) error {
 	return nil
 }
 
-type contextKey int
-
-const contextKeyLog contextKey = 0
-
 type Logger = logger.Logger
 
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLog, log)
-}
-
 func GetLogger(ctx context.Context) Logger {
-	if log, ok := ctx.Value(contextKeyLog).(Logger); ok {
-		return log
-	}
-	return logger.NewNullLogger()
+	return logging.GetLogger(ctx, logging.SubsysTransport)
 }

util/semaphore/semaphore.go

@@ -3,6 +3,7 @@ package semaphore
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	wsemaphore "golang.org/x/sync/semaphore"
 )
 
@@ -21,6 +22,7 @@ type AcquireGuard struct {
 
 // The returned AcquireGuard is not goroutine-safe.
 func (s *S) Acquire(ctx context.Context) (*AcquireGuard, error) {
+	defer trace.WithSpanFromStackUpdateCtx(&ctx)()
 	if err := s.ws.Acquire(ctx, 1); err != nil {
 		return nil, err
 	} else if err := ctx.Err(); err != nil {

util/semaphore/semaphore_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 )
 
 func TestSemaphore(t *testing.T) {
@@ -24,12 +25,17 @@ func TestSemaphore(t *testing.T) {
 		beforeT, afterT uint32
 	}
 
+	ctx := context.Background()
+	defer trace.WithTaskFromStackUpdateCtx(&ctx)()
+
 	var wg sync.WaitGroup
 	wg.Add(numGoroutines)
 	for i := 0; i < numGoroutines; i++ {
 		go func() {
+			ctx, end := trace.WithTaskFromStack(ctx)
+			defer end()
 			defer wg.Done()
-			res, err := sem.Acquire(context.Background())
+			res, err := sem.Acquire(ctx)
 			require.NoError(t, err)
 			defer res.Release()
 			if time.Since(begin) > sleepTime {

zfs/versions_destroy.go

@@ -38,8 +38,8 @@ func (o *DestroySnapOp) String() string {
 	return fmt.Sprintf("destroy operation %s@%s", o.Filesystem, o.Name)
 }
 
-func ZFSDestroyFilesystemVersions(reqs []*DestroySnapOp) {
-	doDestroy(context.TODO(), reqs, destroyerSingleton)
+func ZFSDestroyFilesystemVersions(ctx context.Context, reqs []*DestroySnapOp) {
+	doDestroy(ctx, reqs, destroyerSingleton)
 }
 
 func setDestroySnapOpErr(b []*DestroySnapOp, err error) {

zfs/zfs_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+// FIXME make this a platformtest
 func TestZFSListHandlesProducesZFSErrorOnNonZeroExit(t *testing.T) {
 	t.SkipNow() // FIXME ZFS_BINARY does not work if tests run in parallel
 

zfs/zfscmd/zfscmd-logging-scraper/zfscmd_logging_scraper.go

@@ -23,7 +23,7 @@ type RuntimeLine struct {
 	Error                        string
 }
 
-var humanFormatterLineRE = regexp.MustCompile(`^(\[[^\]]+\]){2}\[zfs.cmd\]:\s+command\s+exited\s+(with|without)\s+error\s+(.+)`)
+var humanFormatterLineRE = regexp.MustCompile(`^(\[[^\]]+\]){2}\[zfs.cmd\]\[[^\]]+\]:\s+command\s+exited\s+(with|without)\s+error\s+(.+)`)
 
 func parseSecs(s string) (time.Duration, error) {
 	d, err := time.ParseDuration(s + "s")

zfs/zfscmd/zfscmd-logging-scraper/zfscmd_logging_scrapter_test.go

@@ -30,7 +30,7 @@ func TestParseHumanFormatter(t *testing.T) {
 	tcs := []testCase{
 		{
 			Name:  "human-formatter-noerror",
-			Input: `2020-04-04T00:00:05+02:00 [DEBG][jobname][zfs.cmd]: command exited without error usertime_s="0.008445" cmd="zfs list -H -p -o name -r -t filesystem,volume" systemtime_s="0.033783" invocation="84" total_time_s="0.037828619"`,
+			Input: `2020-04-04T00:00:05+02:00 [DEBG][jobname][zfs.cmd][task$stack$span.stack]: command exited without error usertime_s="0.008445" cmd="zfs list -H -p -o name -r -t filesystem,volume" systemtime_s="0.033783" invocation="84" total_time_s="0.037828619"`,
 			Expect: &RuntimeLine{
 				Cmd:       "zfs list -H -p -o name -r -t filesystem,volume",
 				TotalTime: secs("0.037828619"),
@@ -42,7 +42,7 @@ func TestParseHumanFormatter(t *testing.T) {
 		},
 		{
 			Name:  "human-formatter-witherror",
-			Input: `2020-04-04T00:00:05+02:00 [DEBG][jobname][zfs.cmd]: command exited with error usertime_s="0.008445" cmd="zfs list -H -p -o name -r -t filesystem,volume" systemtime_s="0.033783" invocation="84" total_time_s="0.037828619" err="some error"`,
+			Input: `2020-04-04T00:00:05+02:00 [DEBG][jobname][zfs.cmd][task$stack$span.stack]: command exited with error usertime_s="0.008445" cmd="zfs list -H -p -o name -r -t filesystem,volume" systemtime_s="0.033783" invocation="84" total_time_s="0.037828619" err="some error"`,
 			Expect: &RuntimeLine{
 				Cmd:       "zfs list -H -p -o name -r -t filesystem,volume",
 				TotalTime: secs("0.037828619"),
@@ -54,7 +54,7 @@ func TestParseHumanFormatter(t *testing.T) {
 		},
 		{
 			Name:  "from graylog",
-			Input: `2020-04-04T00:00:05+02:00 [DEBG][csnas][zfs.cmd]:  command  exited  without  error  usertime_s="0"  cmd="zfs  send  -i  zroot/ezjail/synapse-12@zrepl_20200329_095518_000  zroot/ezjail/synapse-12@zrepl_20200329_102454_000"  total_time_s="0.101598591"  invocation="85"  systemtime_s="0.041581"`,
+			Input: `2020-04-04T00:00:05+02:00 [DEBG][csnas][zfs.cmd][task$stack$span.stack]:  command  exited  without  error  usertime_s="0"  cmd="zfs  send  -i  zroot/ezjail/synapse-12@zrepl_20200329_095518_000  zroot/ezjail/synapse-12@zrepl_20200329_102454_000"  total_time_s="0.101598591"  invocation="85"  systemtime_s="0.041581"`,
 			Expect: &RuntimeLine{
 				Cmd:       "zfs  send  -i  zroot/ezjail/synapse-12@zrepl_20200329_095518_000  zroot/ezjail/synapse-12@zrepl_20200329_102454_000",
 				TotalTime: secs("0.101598591"),

zfs/zfscmd/zfscmd.go

@@ -14,6 +14,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/zrepl/zrepl/daemon/logging/trace"
 	"github.com/zrepl/zrepl/util/circlog"
 )
 
@@ -22,6 +23,7 @@ type Cmd struct {
 	ctx                                      context.Context
 	mtx                                      sync.RWMutex
 	startedAt, waitStartedAt, waitReturnedAt time.Time
+	waitReturnEndSpanCb                      trace.DoneFunc
 }
 
 func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
@@ -31,7 +33,7 @@ func CommandContext(ctx context.Context, name string, arg ...string) *Cmd {
 
 // err.(*exec.ExitError).Stderr will NOT be set
 func (c *Cmd) CombinedOutput() (o []byte, err error) {
-	c.startPre()
+	c.startPre(false)
 	c.startPost(nil)
 	c.waitPre()
 	o, err = c.cmd.CombinedOutput()
@@ -41,7 +43,7 @@ func (c *Cmd) CombinedOutput() (o []byte, err error) {
 
 // err.(*exec.ExitError).Stderr will be set
 func (c *Cmd) Output() (o []byte, err error) {
-	c.startPre()
+	c.startPre(false)
 	c.startPost(nil)
 	c.waitPre()
 	o, err = c.cmd.Output()
@@ -78,7 +80,7 @@ func (c *Cmd) log() Logger {
 }
 
 func (c *Cmd) Start() (err error) {
-	c.startPre()
+	c.startPre(true)
 	err = c.cmd.Start()
 	c.startPost(err)
 	return err
@@ -95,15 +97,17 @@ func (c *Cmd) Process() *os.Process {
 func (c *Cmd) Wait() (err error) {
 	c.waitPre()
 	err = c.cmd.Wait()
-	if !c.waitReturnedAt.IsZero() {
-		// ignore duplicate waits
-		return err
-	}
 	c.waitPost(err)
 	return err
 }
 
-func (c *Cmd) startPre() {
+func (c *Cmd) startPre(newTask bool) {
+	if newTask {
+		// avoid explosion of tasks with name c.String()
+		c.ctx, c.waitReturnEndSpanCb = trace.WithTaskAndSpan(c.ctx, "zfscmd", c.String())
+	} else {
+		c.ctx, c.waitReturnEndSpanCb = trace.WithSpan(c.ctx, c.String())
+	}
 	startPreLogging(c, time.Now())
 }
 
@@ -178,6 +182,9 @@ func (c *Cmd) waitPost(err error) {
 	waitPostReport(c, u, now)
 	waitPostLogging(c, u, err, now)
 	waitPostPrometheus(c, u, err, now)
+
+	// must be last because c.ctx might be used by other waitPost calls
+	c.waitReturnEndSpanCb()
 }
 
 // returns 0 if the command did not yet finish

zfs/zfscmd/zfscmd_context.go

@@ -3,14 +3,14 @@ package zfscmd
 import (
 	"context"
 
+	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/logger"
 )
 
 type contextKey int
 
 const (
-	contextKeyLogger contextKey = iota
-	contextKeyJobID
+	contextKeyJobID contextKey = 1 + iota
 )
 
 type Logger = logger.Logger
@@ -27,13 +27,6 @@ func getJobIDOrDefault(ctx context.Context, def string) string {
 	return ret
 }
 
-func WithLogger(ctx context.Context, log Logger) context.Context {
-	return context.WithValue(ctx, contextKeyLogger, log)
-}
-
 func getLogger(ctx context.Context) Logger {
-	if l, ok := ctx.Value(contextKeyLogger).(Logger); ok {
-		return l
-	}
-	return logger.NewNullLogger()
+	return logging.GetLogger(ctx, logging.SubsysZFSCmd)
 }

zfs/zfscmd/zfscmd_platform_test.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
 	"github.com/zrepl/zrepl/util/circlog"
 )