Do not prune after a backup that found a configured path empty (closes #388)

snapshot create --prune of a snapshot whose path was an empty
directory, as the mount point of a volume that is not mounted is,
recorded an empty snapshot and then purged the older snapshots that
held the data, with their blobs.

The scan now counts the entries it finds in each configured path.
Under --prune, a run in which a path of a snapshot it made held
nothing skips the prune, keeps every older snapshot, names each such
path with its snapshot, and exits non-zero, as
#382 does for unreadable
files. A configured path that is a symlink whose target is gone fails
the run. Only the current run is looked at. Without --prune nothing
changes.

Model: opus-5-5

README.md

@@ -340,6 +340,8 @@ value where a list belongs or a word where a number belongs), is refused.
   does not exist, or when the local index holds snapshots and the
   destination has no `vaultik-destination.txt` (see
   [remote storage layout](#remote-storage-layout))
+* Fails, before anything is pruned, when a configured path is a symlink
+  whose target does not exist, as one into a volume that is not mounted
 
 **`snapshot list`**: Show every snapshot known to this host — the union
 of the local index and the backup destination store — with timestamps

TODO.md

@@ -28,7 +28,8 @@ owner's; a `v*` tag runs the release build in
   that found nothing in a configured path
   ([issue #388](https://git.eeqj.de/sneak/vaultik/issues/388)), as in
   the mount point of a volume that is not mounted. Such a prune could
-  delete every snapshot still holding that path's files.
+  delete every snapshot still holding that path's files. A configured
+  path that is a symlink into such a volume now fails the backup.
 
 - 2026-10-14: Stopped `snapshot create --prune` pruning after a backup
   that skipped files it could not read

internal/cli/entry_snapshot_create_prune_test.go

@@ -87,6 +87,36 @@ func TestEntrySnapshotCreateReportsPostBackupPruneFailure(t *testing.T) {
 		"the new snapshot is on the store beside the planted one")
 }
 
+// TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty backs up,
+// empties the source directory, as a volume that is not mounted leaves
+// its mount point, and runs `snapshot create --prune`. The command exits
+// non-zero and reports the backup as completed, not as a failed snapshot
+// creation. See https://git.eeqj.de/sneak/vaultik/issues/388.
+//
+//nolint:paralleltest // replaces os.Args, os.Stdout, os.Stderr and the xdg globals
+func TestEntrySnapshotCreatePruneExitsNonZeroWhenSourceIsEmpty(t *testing.T) {
+	configPath, storeDir := writeBackupConfig(t)
+
+	code, _, _ := runEntry(t, flagConfig, configPath, cmdSnapshot, cmdCreate)
+	require.Equal(t, 0, code)
+
+	// writeBackupConfig puts the source directory beside the config file.
+	require.NoError(t, os.Remove(
+		filepath.Join(filepath.Dir(configPath), "source", "file.txt")))
+
+	code, _, stderr := runEntry(t,
+		flagConfig, configPath, cmdSnapshot, cmdCreate, flagPrune)
+
+	assert.Equal(t, 1, code)
+	assert.Contains(t, stderr,
+		"Backup completed, but 1 source path(s) were empty")
+	assert.NotContains(t, stderr, "Snapshot creation failed")
+
+	snapshots, err := os.ReadDir(filepath.Join(storeDir, "metadata"))
+	require.NoError(t, err)
+	assert.Len(t, snapshots, 2, "the older snapshot is kept")
+}
+
 // writeBackupConfig builds a hermetic config with one small file in its
 // source directory and an empty destination store directory. Returns the
 // config path and the store directory.

internal/vaultik/snapshot.go

@@ -154,23 +154,8 @@ func (v *Vaultik) CreateSnapshot(opts *SnapshotCreateOptions) error {
 	}
 
 	switch {
-	case opts.Prune && filesUnreadable > 0:
-		// An older snapshot may be the only one that still holds a file
-		// this run could not read.
-		log.Warn("Skipping post-backup prune",
-			"files_unreadable", filesUnreadable)
-		v.UI.Warningf("Skipped the post-backup prune because %d file(s) "+
-			"could not be read; older snapshots are kept.", filesUnreadable)
-	case opts.Prune && len(emptyPaths) > 0:
-		// An older snapshot may be the only one that still holds what an
-		// empty path held. The previous snapshot is no guide: after one
-		// run like this it is empty too.
-		log.Warn("Skipping post-backup prune", "empty_paths", emptyPaths)
-		v.UI.Warningf("Skipped the post-backup prune because nothing was "+
-			"found in %s; older snapshots are kept.",
-			strings.Join(emptyPaths, ", "))
-
-		return fmt.Errorf("%d %w", len(emptyPaths), ErrSourcePathEmpty)
+	case opts.Prune && (filesUnreadable > 0 || len(emptyPaths) > 0):
+		return v.skipPostBackupPrune(filesUnreadable, emptyPaths)
 	case opts.Prune:
 		err = v.runPostBackupPrune(snapshotNames, opts.KeepNewerThan)
 		if err != nil {
@@ -209,6 +194,33 @@ func (v *Vaultik) createNamedSnapshots(
 	return filesUnreadable, emptyPaths, nil
 }
 
+// skipPostBackupPrune says why the post-backup prune does not run, naming
+// every cause, and returns the error the run ends with. An older snapshot
+// may be the only one that still holds a file this run could not read,
+// or what an empty path held. The previous snapshot is no guide: after
+// one run with an empty path it is empty too.
+func (v *Vaultik) skipPostBackupPrune(
+	filesUnreadable int, emptyPaths []string,
+) error {
+	log.Warn("Skipping post-backup prune",
+		"files_unreadable", filesUnreadable, "empty_paths", emptyPaths)
+
+	if len(emptyPaths) > 0 {
+		v.UI.Warningf("Skipped the post-backup prune because nothing was "+
+			"found in %s; older snapshots are kept.",
+			strings.Join(emptyPaths, ", "))
+	}
+
+	if filesUnreadable > 0 {
+		v.UI.Warningf("Skipped the post-backup prune because %d file(s) "+
+			"could not be read; older snapshots are kept.", filesUnreadable)
+
+		return fmt.Errorf("%d %w", filesUnreadable, ErrFilesUnreadable)
+	}
+
+	return fmt.Errorf("%d %w", len(emptyPaths), ErrSourcePathEmpty)
+}
+
 // finishCreateSnapshot ends a run whose snapshots are all complete and
 // kept. Like finishRestore, it returns an error when --skip-errors
 // skipped anything it could not read or list; otherwise it prints the
@@ -402,7 +414,8 @@ func (v *Vaultik) previousSnapshotID(snapName string) (string, error) {
 }
 
 // resolveSnapshotPaths resolves source directories to absolute paths
-// with symlink resolution.
+// with symlink resolution. A path that is a symlink whose target does
+// not exist is an error.
 func (v *Vaultik) resolveSnapshotPaths(snapName string) ([]string, error) {
 	snapConfig := v.Config.Snapshots[snapName]
 	resolvedDirs := make([]string, 0, len(snapConfig.Paths))
@@ -415,12 +428,18 @@ func (v *Vaultik) resolveSnapshotPaths(snapName string) ([]string, error) {
 
 		resolvedPath, err := filepath.EvalSymlinks(absPath)
 		if err != nil {
-			if os.IsNotExist(err) {
-				resolvedPath = absPath
-			} else {
+			// A path that does not exist is left for the scan to report.
+			// One that exists is a symlink whose target is gone, as one
+			// into a volume that is not mounted: the scan would record
+			// only the link, and --prune would then drop the older
+			// snapshots that hold the files.
+			_, lstatErr := os.Lstat(absPath)
+			if !os.IsNotExist(err) || lstatErr == nil {
 				return nil, fmt.Errorf(
 					"failed to resolve symlinks for %s: %w", absPath, err)
 			}
+
+			resolvedPath = absPath
 		}
 
 		resolvedDirs = append(resolvedDirs, resolvedPath)

internal/vaultik/snapshot_prune_test.go

@@ -120,6 +120,99 @@ func TestBackupWithEmptySourcePathDoesNotPrune(t *testing.T) {
 	}
 }
 
+// A backup under --prune of a configured path that is a symlink whose
+// target is gone, as one into a volume that is not mounted, fails before
+// anything is pruned: the snapshot that holds the files is kept with
+// everything it stored. See https://git.eeqj.de/sneak/vaultik/issues/388.
+func TestBackupThroughSymlinkWhoseTargetIsGoneDoesNotPrune(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	const snapshotName = "photos"
+
+	ctx := context.Background()
+	env := newDeletedCountEnv(t, map[string][]byte{
+		"volume/a.jpg": []byte("photo a"),
+		"volume/b.jpg": []byte("photo b"),
+	})
+
+	target := filepath.Join(env.src, "volume")
+	link := filepath.Join(env.src, "photos")
+	require.NoError(t, os.Symlink(target, link))
+	env.setSnapshot(snapshotName, "photos")
+	env.backUp(t, snapshotName)
+
+	firstID := localSnapshotID(ctx, t, env.v.Repositories, snapshotName)
+	stored, err := env.v.Storage.List(ctx, "")
+	require.NoError(t, err)
+
+	// The volume is not mounted, so the symlink points at nothing.
+	require.NoError(t, os.RemoveAll(target))
+
+	env.out.Reset()
+	err = env.v.CreateSnapshot(&vaultik.SnapshotCreateOptions{
+		Prune:     true,
+		Snapshots: []string{snapshotName},
+	})
+	require.ErrorIs(t, err, os.ErrNotExist)
+	assert.Contains(t, err.Error(), link)
+	assert.NotContains(t, env.out.String(), "Running post-backup prune.")
+
+	snapshots, err := env.v.Repositories.Snapshots.ListRecent(
+		ctx, listRecentTestLimit)
+	require.NoError(t, err)
+	require.Len(t, snapshots, 1)
+	assert.Equal(t, firstID, snapshots[0].ID.String(),
+		"the snapshot that holds the files is kept")
+
+	for _, key := range stored {
+		_, err := env.v.Storage.Stat(ctx, key)
+		require.NoError(t, err, "%s is kept", key)
+	}
+}
+
+// A backup under --skip-errors --prune that both skipped a file it could
+// not read and found nothing in a configured path names the empty path
+// too when it skips the prune. See
+// https://git.eeqj.de/sneak/vaultik/issues/388.
+func TestBackupWithEmptySourcePathAndUnreadableFileNamesBoth(t *testing.T) {
+	log.Initialize(log.Config{})
+	t.Parallel()
+
+	const snapshotName = "media"
+
+	env := newDeletedCountEnv(t, map[string][]byte{
+		"docs/notes.txt": []byte("notes"),
+	})
+	require.NoError(t, os.Mkdir(filepath.Join(env.src, "music"), 0o750))
+	env.v.Config.Snapshots[snapshotName] = config.SnapshotConfig{
+		Paths: []string{
+			filepath.Join(env.src, "docs"),
+			filepath.Join(env.src, "music"),
+		},
+	}
+
+	// A mode does not lock out root, who runs the tests, so opening the
+	// file is made to fail instead.
+	env.v.Fs = &openFailFs{
+		path: filepath.Join(env.src, "docs", "notes.txt"),
+		err:  os.ErrPermission,
+	}
+
+	err := env.v.CreateSnapshot(&vaultik.SnapshotCreateOptions{
+		Prune:      true,
+		SkipErrors: true,
+		Snapshots:  []string{snapshotName},
+	})
+	require.ErrorIs(t, err, vaultik.ErrFilesUnreadable)
+	assert.Contains(t, env.out.String(),
+		"Skipped the post-backup prune because 1 file(s) could not be read")
+	assert.Contains(t, env.out.String(),
+		"Skipped the post-backup prune because nothing was found in ")
+	assert.Contains(t, env.out.String(), "music of snapshot "+snapshotName+";")
+	assert.NotContains(t, env.out.String(), "Running post-backup prune.")
+}
+
 // A backup under --prune that finds something in each configured path
 // prunes the previous snapshot, also when a path is a single file. See
 // https://git.eeqj.de/sneak/vaultik/issues/388.