zfs: use channel as iterator for ZFSList results

The old approach with ZFSList would keep the two-dimensional array of
lines and their fields in memory (for a short time), which could easily
consume 100s of MiB with > 10000 snapshots / bookmarks (see #34)

fixes #61

docs/changelog.rst

@@ -19,6 +19,8 @@ Developers should consult the git commit log or GitHub issue tracker.
   * Make sure to understand the meaning bookmarks have for :ref:`maximum replication downtime <replication-downtime>`.
   * Example: :sampleconf:`pullbackup/productionhost.yml`
 
+* |bugfix| :issue:`61`: fix excessive memory usage
+
 0.0.2
 -----
 

zfs/mapping.go

@@ -1,6 +1,9 @@
 package zfs
 
-import "fmt"
+import (
+	"context"
+	"fmt"
+)
 
 type DatasetFilter interface {
 	Filter(p *DatasetPath) (pass bool, err error)
@@ -12,15 +15,21 @@ func ZFSListMapping(filter DatasetFilter) (datasets []*DatasetPath, err error) {
 		panic("filter must not be nil")
 	}
 
-	var lines [][]string
-	lines, err = ZFSList([]string{"name"}, "-r", "-t", "filesystem,volume")
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	rchan := make(chan ZFSListResult)
+	go ZFSListChan(ctx, rchan, []string{"name"}, "-r", "-t", "filesystem,volume")
 
-	datasets = make([]*DatasetPath, 0, len(lines))
+	datasets = make([]*DatasetPath, 0)
+	for r := range rchan {
 
-	for _, line := range lines {
+		if r.err != nil {
+			err = r.err
+			return
+		}
 
 		var path *DatasetPath
-		if path, err = NewDatasetPath(line[0]); err != nil {
+		if path, err = NewDatasetPath(r.fields[0]); err != nil {
 			return
 		}
 

zfs/versions.go

@@ -2,6 +2,7 @@ package zfs
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"strconv"
@@ -61,17 +62,23 @@ type FilesystemVersionFilter interface {
 }
 
 func ZFSListFilesystemVersions(fs *DatasetPath, filter FilesystemVersionFilter) (res []FilesystemVersion, err error) {
-	var fieldLines [][]string
-	fieldLines, err = ZFSList(
+	listResults := make(chan ZFSListResult)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	go ZFSListChan(ctx, listResults,
 		[]string{"name", "guid", "createtxg", "creation"},
 		"-r", "-d", "1",
 		"-t", "bookmark,snapshot",
 		"-s", "createtxg", fs.ToString())
-	if err != nil {
-		return
-	}
-	res = make([]FilesystemVersion, 0, len(fieldLines))
-	for _, line := range fieldLines {
+
+	res = make([]FilesystemVersion, 0)
+	for listResult := range listResults {
+		if listResult.err != nil {
+			return nil, listResult.err
+		}
+
+		line := listResult.fields
 
 		if len(line[0]) < 3 {
 			err = errors.New(fmt.Sprintf("snapshot or bookmark name implausibly short: %s", line[0]))

zfs/zfs.go

@@ -10,6 +10,8 @@ import (
 	"os/exec"
 	"strings"
 
+	"context"
+	"github.com/problame/go-rwccmd"
 	"github.com/zrepl/zrepl/util"
 )
 
@@ -185,6 +187,68 @@ func ZFSList(properties []string, zfsArgs ...string) (res [][]string, err error)
 	return
 }
 
+type ZFSListResult struct {
+	fields []string
+	err    error
+}
+
+// ZFSListChan executes `zfs list` and sends the results to the `out` channel.
+// The `out` channel is always closed by ZFSListChan:
+// If an error occurs, it is closed after sending a result with the err field set.
+// If no error occurs, it is just closed.
+// If the operation is cancelled via context, the channel is just closed.
+//
+// However, if callers do not drain `out` or cancel via `ctx`, the process will leak either running because
+// IO is pending or as a zombie.
+func ZFSListChan(ctx context.Context, out chan ZFSListResult, properties []string, zfsArgs ...string) {
+	defer close(out)
+
+	args := make([]string, 0, 4+len(zfsArgs))
+	args = append(args,
+		"list", "-H", "-p",
+		"-o", strings.Join(properties, ","))
+	args = append(args, zfsArgs...)
+
+	sendResult := func(fields []string, err error) (done bool) {
+		select {
+		case <-ctx.Done():
+			return true
+		case out <- ZFSListResult{fields, err}:
+			return false
+		}
+	}
+
+	cmd, err := rwccmd.CommandContext(ctx, ZFS_BINARY, args, []string{})
+	if err != nil {
+		sendResult(nil, err)
+		return
+	}
+	if err = cmd.Start(); err != nil {
+		sendResult(nil, err)
+		return
+	}
+	defer cmd.Close()
+
+	s := bufio.NewScanner(cmd)
+	buf := make([]byte, 1024) // max line length
+	s.Buffer(buf, 0)
+
+	for s.Scan() {
+		fields := strings.SplitN(s.Text(), "\t", len(properties))
+		if len(fields) != len(properties) {
+			sendResult(nil, errors.New("unexpected output"))
+			return
+		}
+		if sendResult(fields, nil) {
+			return
+		}
+	}
+	if s.Err() != nil {
+		sendResult(nil, s.Err())
+	}
+	return
+}
+
 func ZFSSend(fs *DatasetPath, from, to *FilesystemVersion) (stream io.Reader, err error) {
 
 	args := make([]string, 0)