Implement placeholder filesystems.

Note the docs on the placeholder user property introduced with this
commit. The solution is not really satisfying but couldn't think of a
better one OTOMH

cmd/replication.go

@@ -243,19 +243,13 @@ func doPull(pull PullContext) (err error) {
 		return
 	}
 
+	// build mapping (local->RemoteLocalMapping) + traversal datastructure
 	type RemoteLocalMapping struct {
-		Remote      zfs.DatasetPath
-		Local       zfs.DatasetPath
-		LocalExists bool
+		Remote zfs.DatasetPath
+		Local  zfs.DatasetPath
 	}
 	replMapping := make(map[string]RemoteLocalMapping, len(remoteFilesystems))
 	localTraversal := zfs.NewDatasetPathForest()
-	localExists, err := zfs.ZFSListFilesystemExists()
-	if err != nil {
-		log.Printf("cannot get local filesystems map: %s", err)
-		return err
-	}
-
 	{
 
 		log.Printf("mapping using %#v\n", pull.Mapping)
@@ -270,13 +264,20 @@ func doPull(pull PullContext) (err error) {
 				}
 				continue
 			}
-			m := RemoteLocalMapping{remoteFilesystems[fs], localFs, localExists(localFs)}
+			m := RemoteLocalMapping{remoteFilesystems[fs], localFs}
 			replMapping[m.Local.ToString()] = m
 			localTraversal.Add(m.Local)
 		}
 
 	}
 
+	// get info about local filesystems
+	localFilesystemState, err := zfs.ZFSListFilesystemState()
+	if err != nil {
+		log.Printf("cannot get local filesystems map: %s", err)
+		return err
+	}
+
 	log.Printf("remoteFilesystems: %#v\nreplMapping: %#v\n", remoteFilesystems, replMapping)
 
 	// per fs sync, assume sorted in top-down order TODO
@@ -284,12 +285,18 @@ func doPull(pull PullContext) (err error) {
 	localTraversal.WalkTopDown(func(v zfs.DatasetPathVisit) bool {
 
 		if v.FilledIn {
-			if localExists(v.Path) {
+			if _, exists := localFilesystemState[v.Path.ToString()]; exists {
+				// No need to verify if this is a placeholder or not. It is sufficient
+				// to know we can add child filesystems to it
 				return true
 			}
-			log.Printf("aborting, don't know how to create fill-in dataset %s", v.Path)
-			err = fmt.Errorf("aborting, don't know how to create fill-in dataset: %s", v.Path)
-			return false
+			log.Printf("creating placeholder filesystem %s", v.Path)
+			err = zfs.ZFSCreatePlaceholderFilesystem(v.Path)
+			if err != nil {
+				err = fmt.Errorf("aborting, cannot create placeholder filesystem %s: %s", v.Path, err)
+				return false
+			}
+			return true
 		}
 
 		m, ok := replMapping[v.Path.ToString()]
@@ -303,8 +310,16 @@ func doPull(pull PullContext) (err error) {
 
 		log("mapping: %#v\n", m)
 
+		localState, localExists := localFilesystemState[m.Local.ToString()]
+
 		var versions []zfs.FilesystemVersion
-		if m.LocalExists {
+		switch {
+		case !localExists:
+			log("local filesystem does not exist")
+		case localState.Placeholder:
+			log("local filesystem is marked as placeholder")
+		default:
+			log("local filesystem exists, retrieving versions for diff")
 			if versions, err = zfs.ZFSListFilesystemVersions(m.Local, nil); err != nil {
 				log("cannot get filesystem versions, stopping...: %v\n", m.Local.ToString(), m, err)
 				return false
@@ -323,6 +338,10 @@ func doPull(pull PullContext) (err error) {
 		diff := zfs.MakeFilesystemDiff(versions, theirVersions)
 		log("diff: %#v\n", diff)
 
+		if localState.Placeholder && diff.Conflict != zfs.ConflictAllRight {
+			panic("internal inconsistency: local placeholder implies ConflictAllRight")
+		}
+
 		switch diff.Conflict {
 		case zfs.ConflictAllRight:
 
@@ -364,7 +383,13 @@ func doPull(pull PullContext) (err error) {
 				log("progress on receive operation: %v bytes received", p.TotalRX)
 			})
 
-			if err = zfs.ZFSRecv(m.Local, &watcher, "-u"); err != nil {
+			recvArgs := []string{"-u"}
+			if localState.Placeholder {
+				log("receive with forced rollback to replace placeholder filesystem")
+				recvArgs = append(recvArgs, "-F")
+			}
+
+			if err = zfs.ZFSRecv(m.Local, &watcher, recvArgs...); err != nil {
 				log("error receiving stream, stopping...: %s", err)
 				return false
 			}

zfs/diff.go

@@ -1,6 +1,11 @@
 package zfs
 
 import (
+	"bytes"
+	"crypto/sha512"
+	"encoding/hex"
+	"fmt"
+	"os/exec"
 	"sort"
 )
 
@@ -158,23 +163,86 @@ outer:
 	return
 }
 
+const ZREPL_PLACEHOLDER_PROPERTY_NAME string = "zrepl:placeholder"
+
+type FilesystemState struct {
+	Placeholder bool
+	// TODO extend with resume token when that feature is finally added
+}
+
 // A somewhat efficient way to determine if a filesystem exists on this host.
 // Particularly useful if exists is called more than once (will only fork exec once and cache the result)
-func ZFSListFilesystemExists() (exists func(p DatasetPath) bool, err error) {
+func ZFSListFilesystemState() (localState map[string]FilesystemState, err error) {
 
 	var actual [][]string
-	if actual, err = ZFSList([]string{"name"}, "-t", "filesystem,volume"); err != nil {
+	if actual, err = ZFSList([]string{"name", ZREPL_PLACEHOLDER_PROPERTY_NAME}, "-t", "filesystem,volume"); err != nil {
 		return
 	}
 
-	filesystems := make(map[string]bool, len(actual))
+	localState = make(map[string]FilesystemState, len(actual))
 	for _, e := range actual {
-		filesystems[e[0]] = true
-	}
-
-	exists = func(p DatasetPath) bool {
-		return filesystems[p.ToString()]
+		dp, err := NewDatasetPath(e[0])
+		if err != nil {
+			fmt.Errorf("ZFS does not return parseable dataset path: %s", e[0])
+		}
+		placeholder, _ := IsPlaceholder(dp, e[1])
+		localState[e[0]] = FilesystemState{
+			placeholder,
+		}
 	}
 	return
 
 }
+
+// Computes the value for the ZREPL_PLACEHOLDER_PROPERTY_NAME ZFS user property
+// to mark the given DatasetPath p as a placeholder
+//
+// We cannot simply use booleans here since user properties are always
+// inherited.
+//
+// We hash the DatasetPath and use it to check for a given path if it is the
+// one originally marked as placeholder.
+//
+// However, this prohibits moving datasets around via `zfs rename`. The
+// placeholder attribute must be re-computed for the dataset path after the
+// move.
+//
+// TODO better solution available?
+func PlaceholderPropertyValue(p DatasetPath) string {
+	ps := []byte(p.ToString())
+	sum := sha512.Sum512_256(ps)
+	return hex.EncodeToString(sum[:])
+}
+
+func IsPlaceholder(p DatasetPath, placeholderPropertyValue string) (isPlaceholder bool, err error) {
+	expected := PlaceholderPropertyValue(p)
+	isPlaceholder = expected == placeholderPropertyValue
+	if !isPlaceholder {
+		err = fmt.Errorf("expected %s, has %s", expected, placeholderPropertyValue)
+	}
+	return
+}
+
+func ZFSCreatePlaceholderFilesystem(p DatasetPath) (err error) {
+	v := PlaceholderPropertyValue(p)
+	cmd := exec.Command(ZFS_BINARY, "create",
+		"-o", fmt.Sprintf("%s=%s", ZREPL_PLACEHOLDER_PROPERTY_NAME, v),
+		"-o", "mountpoint=none",
+		p.ToString())
+
+	stderr := bytes.NewBuffer(make([]byte, 0, 1024))
+	cmd.Stderr = stderr
+
+	if err = cmd.Start(); err != nil {
+		return err
+	}
+
+	if err = cmd.Wait(); err != nil {
+		err = ZFSError{
+			Stderr:  stderr.Bytes(),
+			WaitErr: err,
+		}
+	}
+
+	return
+}