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

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
+}