pre- and post-snapshot hooks

* stack-based execution model, documented in documentation
* circbuf for capturing hook output
* built-in hooks for postgres and mysql
* refactor docs, too much info on the jobs page, too difficult
  to discover snapshotting & hooks

Co-authored-by: Ross Williams <ross@ross-williams.net>
Co-authored-by: Christian Schwarz <me@cschwarz.com>

fixes #74

daemon/hooks/hook_exec.go

@@ -0,0 +1,292 @@
+package hooks
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/zrepl/zrepl/zfs"
+)
+
+// Re-export type here so that
+// every file in package hooks doesn't
+// have to import github.com/zrepl/zrepl/zfs
+type Filter zfs.DatasetFilter
+
+type Hook interface {
+	Filesystems() Filter
+
+	// If true and the Pre edge invocation of Run fails, Post edge will not run and other Pre edges will not run.
+	ErrIsFatal() bool
+
+	// Run is invoked by HookPlan for a Pre edge.
+	// If HookReport.HadError() == false, the Post edge will be invoked, too.
+	Run(ctx context.Context, edge Edge, phase Phase, dryRun bool, extra Env, state map[interface{}]interface{}) HookReport
+
+	String() string
+}
+
+type Phase string
+
+const (
+	PhaseSnapshot = Phase("snapshot")
+	PhaseTesting  = Phase("testing")
+)
+
+func (p Phase) String() string {
+	return string(p)
+}
+
+//go:generate stringer -type=Edge
+type Edge uint
+
+const (
+	Pre = Edge(1 << iota)
+	Callback
+	Post
+)
+
+func (e Edge) StringForPhase(phase Phase) string {
+	return fmt.Sprintf("%s_%s", e.String(), phase.String())
+}
+
+//go:generate enumer -type=StepStatus -trimprefix=Step
+type StepStatus int
+
+const (
+	StepPending StepStatus = 1 << iota
+	StepExec
+	StepOk
+	StepErr
+	StepSkippedDueToFatalErr
+	StepSkippedDueToPreErr
+)
+
+type HookReport interface {
+	String() string
+	HadError() bool
+	Error() string
+}
+
+type Step struct {
+	Hook       Hook
+	Edge       Edge
+	Status     StepStatus
+	Begin, End time.Time
+	// Report may be nil
+	// FIXME cannot serialize this for client status, but contains interesting info (like what error happened)
+	Report     HookReport
+	state      map[interface{}]interface{}
+}
+
+func (s Step) String() (out string) {
+	fatal := "~"
+	if s.Hook.ErrIsFatal() && s.Edge == Pre {
+		fatal = "!"
+	}
+	runTime := "..."
+	if s.Status != StepPending {
+		t := s.End.Sub(s.Begin)
+		runTime = t.Round(time.Millisecond).String()
+	}
+	return fmt.Sprintf("[%s] [%5s] %s [%s]  %s", s.Status, runTime, fatal, s.Edge, s.Hook)
+}
+
+type Plan struct {
+	mtx sync.RWMutex
+
+	steps []*Step
+	pre   []*Step // protected by mtx
+	cb    *Step
+	post  []*Step // not reversed, i.e. entry at index i corresponds to pre-edge in pre[i]
+
+	phase Phase
+	env   Env
+}
+
+func NewPlan(hooks *List, phase Phase, cb *CallbackHook, extra Env) (*Plan, error) {
+
+	var pre, post []*Step
+	// TODO sanity check unique name of hook?
+	for _, hook := range *hooks {
+		state := make(map[interface{}]interface{})
+		preE := &Step{
+			Hook:   hook,
+			Edge:   Pre,
+			Status: StepPending,
+			state:  state,
+		}
+		pre = append(pre, preE)
+		postE := &Step{
+			Hook:   hook,
+			Edge:   Post,
+			Status: StepPending,
+			state:  state,
+		}
+		post = append(post, postE)
+	}
+
+	cbE := &Step{
+		Hook:   cb,
+		Edge:   Callback,
+		Status: StepPending,
+	}
+
+	steps := make([]*Step, 0, len(pre)+len(post)+1)
+	steps = append(steps, pre...)
+	steps = append(steps, cbE)
+	for i := len(post) - 1; i >= 0; i-- {
+		steps = append(steps, post[i])
+	}
+
+	plan := &Plan{
+		phase: phase,
+		env:   extra,
+		steps: steps,
+		pre:   pre,
+		post:  post,
+		cb:    cbE,
+	}
+
+	return plan, nil
+}
+
+type PlanReport []Step
+
+func (p *Plan) Report() PlanReport {
+	p.mtx.RLock()
+	defer p.mtx.RUnlock()
+	rep := make([]Step, len(p.steps))
+	for i := range rep {
+		rep[i] = *p.steps[i]
+	}
+	return rep
+}
+
+func (r PlanReport) HadError() bool {
+	for _, e := range r {
+		if e.Status == StepErr {
+			return true
+		}
+	}
+	return false
+}
+
+func (r PlanReport) HadFatalError() bool {
+	for _, e := range r {
+		if e.Status == StepSkippedDueToFatalErr {
+			return true
+		}
+	}
+	return false
+}
+
+func (r PlanReport) String() string {
+	stepStrings := make([]string, len(r))
+	for i, e := range r {
+		stepStrings[i] = fmt.Sprintf("%02d %s", i+1, e)
+	}
+	return strings.Join(stepStrings, "\n")
+}
+
+func (p *Plan) Run(ctx context.Context, dryRun bool) {
+	p.mtx.RLock()
+	defer p.mtx.RUnlock()
+	w := func(f func()) {
+		p.mtx.RUnlock()
+		defer p.mtx.RLock()
+		p.mtx.Lock()
+		defer p.mtx.Unlock()
+		f()
+	}
+	runHook := func(s *Step, ctx context.Context, edge Edge) HookReport {
+		w(func() { s.Status = StepExec })
+		begin := time.Now()
+		r := s.Hook.Run(ctx, edge, p.phase, dryRun, p.env, s.state)
+		end := time.Now()
+		w(func() {
+			s.Report = r
+			s.Status = StepOk
+			if r.HadError() {
+				s.Status = StepErr
+			}
+			s.Begin, s.End = begin, end
+		})
+		return r
+	}
+
+	l := getLogger(ctx)
+
+	// it's a stack, execute until we reach the end of the list (last item in)
+	// or fail inbetween
+	l.Info("run pre-edges in configuration order")
+	next := 0
+	for ; next < len(p.pre); next++ {
+		e := p.pre[next]
+		l := l.WithField("hook", e.Hook)
+		r := runHook(e, ctx, Pre)
+		if r.HadError() {
+			l.WithError(r).Error("hook invocation failed for pre-edge")
+			if e.Hook.ErrIsFatal() {
+				l.Error("the hook run was aborted due to a fatal error in this hook")
+				break
+			}
+		}
+	}
+
+	hadFatalErr := next != len(p.pre)
+	if hadFatalErr {
+		l.Error("fatal error in a pre-snapshot hook invocation")
+		l.Error("no snapshot will be taken")
+		l.Error("only running post-edges for successful pre-edges")
+		w(func() {
+			p.post[next].Status = StepSkippedDueToFatalErr
+			for i := next + 1; i < len(p.pre); i++ {
+				p.pre[i].Status = StepSkippedDueToFatalErr
+				p.post[i].Status = StepSkippedDueToFatalErr
+			}
+			p.cb.Status = StepSkippedDueToFatalErr
+		})
+		return
+	}
+
+	l.Info("running callback")
+	cbR := runHook(p.cb, ctx, Callback)
+	if cbR.HadError() {
+		l.WithError(cbR).Error("callback failed")
+	}
+
+	l.Info("run post-edges for successful pre-edges in reverse configuration order")
+
+	// the constructor produces pre and post entries
+	// post is NOT reversed
+	next-- // now at index of last executed pre-edge
+	for ; next >= 0; next-- {
+		e := p.post[next]
+		l := l.WithField("hook", e.Hook)
+
+		if p.pre[next].Status != StepOk {
+			if p.pre[next].Status != StepErr {
+				panic(fmt.Sprintf("expecting a pre-edge hook report to be either Ok or Err, got %s", p.pre[next].Status))
+			}
+			l.Info("skip post-edge because pre-edge failed")
+			w(func() {
+				e.Status = StepSkippedDueToPreErr
+			})
+			continue
+		}
+
+		report := runHook(e, ctx, Post)
+
+		if report.HadError() {
+			l.WithError(report).Error("hook invocation failed for post-edge")
+			l.Error("subsequent post-edges run regardless of this post-edge failure")
+		}
+
+		// ErrIsFatal is only relevant for Pre
+
+	}
+
+}