Do not prune when a backup misses an earlier mount point (closes #395)

snapshot create --prune purged the older snapshots holding a volume's
files when the volume, mounted below a configured path, was not
mounted: its empty mount point read as deleted files.

The scan now records in the local index each directory at or below a
configured path that holds an entry and is on a different device from
its parent directory, looked up once the walk has listed it, as listing
mounts an automounted volume. Under --prune, when an earlier snapshot
of that name recorded such a directory and this run does not, the run
skips the prune, keeps every older snapshot and exits non-zero, naming
the directory. For a volume retired on purpose, snapshot purge removes
the older snapshots.

Model: opus-5-5

README.md

@@ -338,16 +338,15 @@ value where a list belongs or a word where a number belongs), is refused.
   snapshots by hand with `snapshot purge`. When an earlier snapshot of the
   same name in the local index found a volume mounted at or below a
   configured path (a directory on a different filesystem from its parent,
-  with an entry in it that the backup does not exclude) and this backup does
+  with something in it) and this backup finds none mounted there, the prune
-  not, the prune does not run either: every older snapshot is kept, and the
+  does not run either: every older snapshot is kept, and the command names
-  command names the directory and exits non-zero, on every run until the
+  the directory and exits non-zero, on every run until the volume is mounted
-  volume is mounted again. A backup also does not find the volume when it
+  again. The same happens when the volume is empty, as an automounted volume
-  finds no such entry in its directory, as when an automounted volume cannot
+  that could not be mounted looks, or when that directory is no longer
-  be mounted, or when that directory is no longer backed up, because it is
+  backed up, because it is excluded or its configured path was removed. If
-  excluded or its configured path was removed. If any of this is on purpose,
+  any of this is on purpose, as for a retired volume, remove the older
-  as for a retired volume, remove the older snapshots by hand with
+  snapshots by hand with `snapshot purge`. A file removed during the backup
-  `snapshot purge`. A file removed during the backup does not stop the
+  does not stop the prune.
-  prune.
 * `--keep-newer-than <duration>`: With `--prune`, keep snapshots newer than
   this duration instead of only the latest (e.g. `4w`, `30d`, `6mo`, `1y`).
   Without `--prune`, or with a value that is not a valid duration, the

TODO.md

@@ -30,8 +30,7 @@ owner's; a `v*` tag runs the release build in
   ([issue #395](https://git.eeqj.de/sneak/vaultik/issues/395)). Such a
   prune could delete every snapshot still holding the volume's files.
   Each snapshot now records in the local index the directories it found
-  on a different filesystem from their parent and found an entry in that
+  on a different filesystem from their parent, with something in them.
-  the backup does not exclude.
 
 - 2026-10-14: Stopped `snapshot create --prune` pruning after a backup
   that found nothing in a configured path

docs/DATAMODEL.md

@@ -154,7 +154,7 @@ Maps snapshots to the blobs they reference.
 **Purpose:** Tracks blob dependencies for snapshots and enables manifest generation.
 
 ### 10. `snapshot_mount_points`
-The directories at or below a configured path, the configured path itself included, that a snapshot's backup found an entry in and, once it had listed them, found on a different filesystem from their parent directory (a different device number), as a mount point is. An automount point whose volume cannot be mounted is not recorded, because the backup finds no entry in it.
+The directories at or below a configured path, the configured path itself included, that a snapshot's backup found on a different filesystem from their parent directory (a different device number), as a mount point is, and found an entry in. An empty one, as an automount point whose volume could not be mounted, is left out.
 
 **Columns:**
 - `snapshot_id` (TEXT) - Snapshot ID (FK to snapshots.id)
@@ -183,7 +183,7 @@ Tracks blob upload metrics.
    - `INSERT OR REPLACE INTO files` - Update file metadata
    - `SELECT * FROM files WHERE path = ?` - Check if file has changed
    - `INSERT INTO snapshot_files` - Add file to current snapshot
-   - `INSERT OR IGNORE INTO snapshot_mount_points` - Record each directory on a different filesystem from its parent that the walk found an entry in
+   - `INSERT OR IGNORE INTO snapshot_mount_points` - Record each directory on a different filesystem from its parent that holds an entry
 
 2. **Chunking** (for changed files)
    - `INSERT OR IGNORE INTO chunks` - Store new chunks

internal/snapshot/mount_point_test.go

@@ -6,7 +6,6 @@ import (
 	"path/filepath"
 	"slices"
 	"strings"
-	"syscall"
 	"testing"
 
 	"github.com/spf13/afero"
@@ -17,10 +16,9 @@ import (
 )
 
 // A scan records with the snapshot each directory on a different device
-// from its parent directory that the walk found an entry in, as a mount
+// from its parent directory that holds an entry, as a mount point is,
-// point is, the scanned path included. A filesystem that gives no device
+// the scanned path included. A filesystem that gives no device numbers,
-// numbers, as the in-memory one, records none. See
+// as the in-memory one, records none. See https://git.eeqj.de/sneak/vaultik/issues/395.
-// https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestScanRecordsMountPoints(t *testing.T) {
 	t.Parallel()
 
@@ -99,29 +97,23 @@ func TestScanRecordsMountPoints(t *testing.T) {
 	}
 }
 
-// automountFs is an in-memory filesystem with a volume at mountPoint
+// automountFs is an in-memory filesystem in which opening mountPoint to
-// that is mounted on first access: opening mountPoint to list it mounts
+// list it mounts a volume there, if canMount, as a systemd automount
-// the volume or, if it cannot be mounted, fails with "no such file or
+// does on first access. Stat records the device an entry is on at that
-// directory", as with the autofs daemon. Stat records the device an
+// moment, as a real stat does: the automounter's, 3, for mountPoint
-// entry is on at that moment, as a real stat does: unmountedDevice for
+// until the volume is mounted; the volume's, 2, for mountPoint once it
-// mountPoint until the volume is mounted; the volume's, 2, for
+// is and for everything in it; 1 for everything else.
-// mountPoint once it is and for everything in it; 1 for everything else.
 type automountFs struct {
 	afero.Fs
 
 	mountPoint string
-	unmountedDevice uint64
 	canMount   bool
 	mounted    bool
 }
 
 //nolint:ireturn // afero.Fs.Open is defined to return the interface.
 func (fs *automountFs) Open(name string) (afero.File, error) {
-	if name == fs.mountPoint && !fs.canMount {
+	if name == fs.mountPoint && fs.canMount {
-		return nil, &os.PathError{Op: "open", Path: name, Err: syscall.ENOENT}
-	}
-
-	if name == fs.mountPoint {
 		fs.mounted = true
 	}
 
@@ -138,7 +130,7 @@ func (fs *automountFs) Stat(name string) (os.FileInfo, error) {
 
 	switch {
 	case name == fs.mountPoint && !fs.mounted:
-		device = fs.unmountedDevice
+		device = 3
 	case name == fs.mountPoint || strings.HasPrefix(name, fs.mountPoint+"/"):
 		device = 2
 	}
@@ -162,13 +154,12 @@ func deviceAtLookup(info os.FileInfo) (uint64, bool) {
 }
 
 // A volume mounted on first access is mounted when the walk lists its
-// mount point. Until then the mount point is on the automounter's own
+// mount point; until then the mount point is on the automounter's own
-// device (a systemd automount) or on its parent's (an autofs map that
+// device. Only the mount point is recorded, by the backup that mounts
-// shows its mount points before mounting them). Either way only the
+// the volume and by a later one. An automount point whose volume cannot
-// mount point is recorded, by the backup that mounts the volume and by a
+// be mounted stays on the automounter's device and lists nothing; it is
-// later one that finds it mounted. Listing an automount point whose
+// not recorded, so it does not count as the volume found mounted. See
-// volume cannot be mounted fails; it is not recorded, because the walk
+// https://git.eeqj.de/sneak/vaultik/issues/395.
-// finds no entry in it. See https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestScanRecordsOnlyTheMountPointOfAnAutomountedVolume(t *testing.T) {
 	t.Parallel()
 
@@ -178,22 +169,11 @@ func TestScanRecordsOnlyTheMountPointOfAnAutomountedVolume(t *testing.T) {
 
 	tests := []struct {
 		name     string
-		unmountedDevice uint64
 		canMount bool
 		want     []string
 	}{
-		{
+		{name: "volume that is mounted", canMount: true, want: []string{mountPoint}},
-			name:            "on the automounter's device until listed",
+		{name: "volume that cannot be mounted", canMount: false, want: nil},
-			unmountedDevice: 3, canMount: true, want: []string{mountPoint},
-		},
-		{
-			name:            "on its parent's device until listed",
-			unmountedDevice: 1, canMount: true, want: []string{mountPoint},
-		},
-		{
-			name:            "volume that cannot be mounted",
-			unmountedDevice: 1, canMount: false, want: nil,
-		},
 	}
 
 	for _, tt := range tests {
@@ -202,16 +182,17 @@ func TestScanRecordsOnlyTheMountPointOfAnAutomountedVolume(t *testing.T) {
 
 			ctx := context.Background()
 			fs := &automountFs{
-				Fs: afero.NewMemMapFs(), mountPoint: mountPoint,
+				Fs: afero.NewMemMapFs(), mountPoint: mountPoint, canMount: tt.canMount,
-				unmountedDevice: tt.unmountedDevice, canMount: tt.canMount,
 			}
 
-			require.NoError(t, fs.MkdirAll(mountPoint+"/photos", 0o755))
+			require.NoError(t, fs.MkdirAll(mountPoint, 0o755))
+			require.NoError(t, afero.WriteFile(
+				fs, scannedPath+"/notes.txt", []byte("notes"), 0o644))
 
-			for _, path := range []string{
+			if tt.canMount {
-				scannedPath + "/notes.txt", mountPoint + "/photos/a.jpg",
+				require.NoError(t, fs.MkdirAll(mountPoint+"/photos", 0o755))
-			} {
+				require.NoError(t, afero.WriteFile(
-				require.NoError(t, afero.WriteFile(fs, path, []byte(path), 0o644))
+					fs, mountPoint+"/photos/a.jpg", []byte("photo"), 0o644))
 			}
 
 			db, err := database.NewTestDB()

internal/snapshot/scanner.go

@@ -888,10 +888,9 @@ type ScanPhaseResult struct {
 	FilesToProcess   []*FileToProcess
 	UnchangedFileIDs []types.FileID   // IDs of unchanged files to associate with snapshot
 	Directories      []*FileToProcess // Every directory found, to record
-	// MountPoints are the directories, the scanned path included, that
+	// MountPoints are the directories found on a different device from
-	// the walk found an entry in and, once it had listed them, on a
+	// their parent directory, the scanned path included, that the walk
-	// different device from their parent directory (see
+	// found an entry in.
-	// recordIfMountPoint).
 	MountPoints []string
 }
 
@@ -912,7 +911,7 @@ func (s *Scanner) scanPhase(
 	// estimate leaves them out.
 	estimatedTotal := countNonDirectories(knownFiles)
 
-	collector := &scanCollector{dirsWithEntries: make(map[string]struct{})}
+	collector := &scanCollector{mountPoints: make(map[string]bool)}
 
 	// Set up periodic status output
 	startTime := time.Now()
@@ -955,9 +954,9 @@ func (s *Scanner) scanPhase(
 		// The scanned directory itself is not an entry found in it.
 		if filePath != path || !info.IsDir() {
 			result.EntriesFound++
-		}
 
-		s.recordIfMountPoint(filePath, path, collector)
+			collector.foundEntryIn(filepath.Dir(filePath))
+		}
 
 		// Handle symlinks and directories
 		handled, err := s.recordSpecialEntry(
@@ -993,7 +992,7 @@ func (s *Scanner) scanPhase(
 		FilesToProcess:   collector.filesToProcess,
 		UnchangedFileIDs: collector.unchangedFileIDs,
 		Directories:      collector.directories,
-		MountPoints:      collector.mountPoints,
+		MountPoints:      collector.mountPointsWithEntries(),
 	}, nil
 }
 
@@ -1013,8 +1012,10 @@ func countNonDirectories(files map[string]*database.File) int64 {
 // recordSpecialEntry records symlinks and directories (which have no
 // data to chunk) and reports whether it handled the entry. A symlink is
 // counted and checked for changes as a regular file is; a directory is
-// set aside to be recorded and is not counted. For a symlink whose
+// set aside to be recorded, also as a mount point when it is on a
-// target cannot be read it returns handleWalkError's result.
+// different device from its parent directory, and is not counted. For a
+// symlink whose target cannot be read it returns handleWalkError's
+// result.
 func (s *Scanner) recordSpecialEntry(
 	filePath string, info os.FileInfo,
 	existingFiles map[string]struct{},
@@ -1043,6 +1044,10 @@ func (s *Scanner) recordSpecialEntry(
 		existingFiles[filePath] = struct{}{}
 		collector.addDirectory(filePath, info, file)
 
+		if s.onOtherDeviceThanParent(filePath, info) {
+			collector.addMountPoint(filePath)
+		}
+
 		return true, nil
 	}
 
@@ -1120,29 +1125,46 @@ type scanCollector struct {
 	unchangedFileIDs []types.FileID // Just IDs - no new records needed
 	directories      []*FileToProcess
 	filesScanned     int64
-	// dirsWithEntries holds each directory the walk has found an entry in.
+	// mountPoints maps each directory found on a different device from
-	dirsWithEntries map[string]struct{}
+	// its parent directory to whether the walk found an entry in it.
-	mountPoints     []string
+	mountPoints map[string]bool
 }
 
-// firstEntryIn notes that the walk found an entry in directory dir and
+// addMountPoint sets aside a directory found on a different device from
-// reports whether it is the first one.
+// its parent directory, to be recorded if the walk finds an entry in it.
-func (c *scanCollector) firstEntryIn(dir string) bool {
+func (c *scanCollector) addMountPoint(path string) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	_, seen := c.dirsWithEntries[dir]
+	c.mountPoints[path] = false
-	c.dirsWithEntries[dir] = struct{}{}
-
-	return !seen
 }
 
-// addMountPoint sets a mount point aside to be recorded.
+// foundEntryIn notes that the walk found an entry in directory dir.
-func (c *scanCollector) addMountPoint(dir string) {
+func (c *scanCollector) foundEntryIn(dir string) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
-	c.mountPoints = append(c.mountPoints, dir)
+	if _, ok := c.mountPoints[dir]; ok {
+		c.mountPoints[dir] = true
+	}
+}
+
+// mountPointsWithEntries returns the mount points the walk found an
+// entry in. An empty one, as an automount point whose volume could not
+// be mounted, does not show a volume mounted there.
+func (c *scanCollector) mountPointsWithEntries() []string {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	var mountPoints []string
+
+	for dir, foundEntry := range c.mountPoints {
+		if foundEntry {
+			mountPoints = append(mountPoints, dir)
+		}
+	}
+
+	return mountPoints
 }
 
 // addDirectory sets a directory aside to be recorded. It is not counted
@@ -1369,53 +1391,32 @@ func (s *Scanner) recordMountPoints(
 	})
 }
 
-// recordIfMountPoint sets the directory that holds entry, a path the
+// onOtherDeviceThanParent reports whether the directory at path, which
-// walk found, aside to be recorded as a mount point if it is on a
+// info describes, is on a different device from its parent directory,
-// different device from its parent directory. The walk calls it for each
+// looked up now. The walk calls it on reaching the directory, by when it
-// entry it finds, and it decides on the first entry in a directory, so a
+// has listed the parent (the scanned path aside). Listing a directory
-// directory the walk found no entry in is never recorded. By then the
+// mounts a volume mounted there on first access (a systemd automount),
-// walk has listed the directory, which mounts a volume mounted there on
+// so the walk's own entry for the parent, looked up before the listing,
-// first access, so the directory and its parent are looked up again:
+// can show the automounter's device instead. A directory whose device
-// before the listing, the directory can be on the automounter's own
+// number, or whose parent's, cannot be read counts as on its parent's
-// device (a systemd automount) or on its parent's (an autofs map that
+// device: a mount point missed can only stop a prune, never let one run.
-// shows its mount points before mounting them). The scanned path is in
+func (s *Scanner) onOtherDeviceThanParent(path string, info os.FileInfo) bool {
-// no listing the walk made, so it is skipped as an entry. A directory
+	dirDevice, ok := s.deviceNumber(info)
-// that cannot be looked up then, or whose parent cannot, is not
-// recorded, so this snapshot does not stop the prune of a later backup
-// that misses its volume.
-func (s *Scanner) recordIfMountPoint(
-	entry, scannedPath string, collector *scanCollector,
-) {
-	dir := filepath.Dir(entry)
-
-	if entry == scannedPath || !collector.firstEntryIn(dir) {
-		return
-	}
-
-	dirDevice, ok := s.deviceNumberAt(dir)
 	if !ok {
-		return
+		return false
 	}
 
-	parentDevice, ok := s.deviceNumberAt(filepath.Dir(dir))
+	parent, err := s.fs.Stat(filepath.Dir(path))
-	if ok && dirDevice != parentDevice {
-		collector.addMountPoint(dir)
-	}
-}
-
-// deviceNumberAt looks path up now and returns the number of the device
-// that holds it, and false when the lookup fails (logged) or gives no
-// device number.
-func (s *Scanner) deviceNumberAt(path string) (uint64, bool) {
-	info, err := s.fs.Stat(path)
 	if err != nil {
-		log.Warn("Could not look up a directory to check for a mount point",
+		log.Warn("Could not look up the parent of a directory "+
-			"path", path, "error", err)
+			"to check for a mount point", "path", path, "error", err)
 
-		return 0, false
+		return false
 	}
 
-	return s.deviceNumber(info)
+	parentDevice, ok := s.deviceNumber(parent)
+
+	return ok && dirDevice != parentDevice
 }
 
 // deviceNumberFromStat returns the number of the device that holds the

internal/vaultik/snapshot.go

@@ -56,11 +56,11 @@ var ErrSourcePathEmpty = errors.New("source path(s) were empty")
 // ErrVolumeNotMounted is returned when --prune skipped the post-backup
 // prune because a directory that an earlier snapshot of the same name
 // found as a mount point was not found as one in this run: the volume
-// mounted there is not mounted, the run found no entry in it, or the
+// mounted there is not mounted or is empty, or the directory is no
-// directory is no longer backed up. The snapshots are complete and
+// longer backed up. The snapshots are complete and kept, so the caller
-// kept, so the caller can report it apart from a failed backup. A run
+// can report it apart from a failed backup. A run that also skipped
-// that also skipped unreadable files or found a configured path empty
+// unreadable files or found a configured path empty returns
-// returns ErrFilesUnreadable or ErrSourcePathEmpty instead.
+// ErrFilesUnreadable or ErrSourcePathEmpty instead.
 var ErrVolumeNotMounted = errors.New(
 	"mount point(s) an earlier snapshot recorded were not found")
 
@@ -237,11 +237,10 @@ func (v *Vaultik) skipPostBackupPrune(
 	if len(unmountedPaths) > 0 {
 		v.UI.Warningf("Skipped the post-backup prune because this backup "+
 			"did not find a mount point at %s, where an earlier snapshot "+
-			"found one: the volume is not mounted, this backup found no "+
+			"found one: the volume is not mounted or is empty, or the "+
-			"entry in it, or the directory is no longer backed up. Older "+
+			"directory is no longer backed up. Older snapshots are kept. "+
-			"snapshots are kept. Mount the volume again, or, if that is on "+
+			"Mount the volume again, or, if that is on purpose, remove the "+
-			"purpose, remove the older snapshots by hand with "+
+			"older snapshots by hand with 'vaultik snapshot purge'.",
-			"'vaultik snapshot purge'.",
 			strings.Join(unmountedPaths, ", "))
 	}
 

internal/vaultik/snapshot_prune_test.go

@@ -4,10 +4,8 @@ import (
 	"context"
 	"os"
 	"path/filepath"
-	"syscall"
 	"testing"
 
-	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"sneak.berlin/go/vaultik/internal/config"
@@ -263,67 +261,42 @@ func TestBackupWithNoEmptySourcePathPrunes(t *testing.T) {
 	}
 }
 
-// automountFs is the real filesystem with a volume at volumeDir that is
-// mounted on first access, as by an autofs map that shows its mount
-// points before mounting them: listing volumeDir mounts the volume or,
-// while cannotMount, fails with "no such file or directory".
-type automountFs struct {
-	afero.OsFs
-
-	volumeDir   string
-	cannotMount bool
-	mounted     bool
-}
-
-//nolint:ireturn // afero.Fs.Open is defined to return the interface.
-func (f *automountFs) Open(name string) (afero.File, error) {
-	if name == f.volumeDir && f.cannotMount {
-		return nil, &os.PathError{Op: "open", Path: name, Err: syscall.ENOENT}
-	}
-
-	if name == f.volumeDir {
-		f.mounted = true
-	}
-
-	return f.OsFs.Open(name)
-}
-
 // A backup under --prune in which a volume that an earlier snapshot found
-// mounted below the configured path is missing does not prune: run after
+// mounted below the configured path is not mounted does not prune: run
-// run, the snapshot that holds the volume's files is kept with everything
+// after run, the snapshot that holds the volume's files is kept with
-// it stored, and the run fails. Once the volume is back, the prune runs.
+// everything it stored, and the run fails. Once the volume is mounted
-// The volume is mounted on first access and every backup starts with it
+// again, the prune runs. The volume's directory is left either as a
-// not mounted, so its directory is on its parent's device until the walk
+// plain directory or as an automount point whose volume cannot be
-// lists it. While the volume is missing, its directory is left empty, or
+// mounted, which stays empty on the automounter's own device. See
-// listing it fails with "no such file or directory", as when an autofs
+// https://git.eeqj.de/sneak/vaultik/issues/395.
-// volume cannot be mounted, which the walk counts as a directory removed
-// during the backup. See https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestBackupWithVolumeNotMountedDoesNotPrune(t *testing.T) {
 	log.Initialize(log.Config{})
 	t.Parallel()
 
+	// unmountedDevice is the device number of the volume's directory
+	// while the volume is not mounted. Its parent directory is on device
+	// 1, the mounted volume on device 2.
 	tests := []struct {
 		name            string
-		listingFails bool
+		unmountedDevice uint64
 	}{
-		{name: "directory left empty", listingFails: false},
+		{name: "plain directory", unmountedDevice: 1},
-		{name: "listing fails", listingFails: true},
+		{name: "automount point that cannot be mounted", unmountedDevice: 3},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			backUpWithVolumeNotMounted(t, tt.listingFails)
+			backUpWithVolumeNotMounted(t, tt.unmountedDevice)
 		})
 	}
 }
 
 // backUpWithVolumeNotMounted runs the backups of
-// TestBackupWithVolumeNotMountedDoesNotPrune. While the volume is
+// TestBackupWithVolumeNotMountedDoesNotPrune, with the volume's directory
-// missing, listing its directory fails if listingFails; otherwise the
+// on device unmountedDevice while the volume is not mounted.
-// directory is empty.
+func backUpWithVolumeNotMounted(t *testing.T, unmountedDevice uint64) {
-func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
 	t.Helper()
 
 	// volume is the directory, below the configured path, where the
@@ -338,22 +311,21 @@ func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
 		volume + "/a.txt": []byte("file a"),
 		"notes.txt":       []byte("notes"),
 	})
-	env.setSnapshot(snapshotName, "")
+	env.v.Config.Snapshots[snapshotName] = config.SnapshotConfig{
+		Paths: []string{env.src},
+	}
 	prune := &vaultik.SnapshotCreateOptions{
 		Prune: true, Snapshots: []string{snapshotName},
 	}
 
-	// The volume's directory is on the volume's device, 2, while the
+	// volumeDevice is the device of the volume's directory, 2 while the
 	// volume is mounted. Everything else is on device 1.
-	volumeDir := filepath.Join(env.src, volume)
+	volumeDevice := uint64(2)
-	fs := &automountFs{volumeDir: volumeDir}
 	newScanner := env.v.ScannerFactory
 	env.v.ScannerFactory = func(params snapshot.ScannerParams) *snapshot.Scanner {
-		fs.mounted = false
-		params.Fs = fs
 		params.DeviceNumber = func(info os.FileInfo) (uint64, bool) {
-			if info.Name() == volume && fs.mounted {
+			if info.Name() == volume {
-				return 2, true
+				return volumeDevice, true
 			}
 
 			return 1, true
@@ -368,13 +340,13 @@ func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
 	stored, err := env.v.Storage.List(ctx, "")
 	require.NoError(t, err)
 
-	// The volume goes missing.
+	// A volume that is not mounted leaves its mount point behind as an
-	fs.cannotMount = listingFails
+	// empty directory.
-
+	volumeDir := filepath.Join(env.src, volume)
-	if !listingFails {
 	require.NoError(t, os.RemoveAll(volumeDir))
 	require.NoError(t, os.Mkdir(volumeDir, 0o750))
-	}
+
+	volumeDevice = unmountedDevice
 
 	for range 2 {
 		env.out.Reset()
@@ -399,11 +371,12 @@ func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
 		require.NoError(t, err, "%s is kept", key)
 	}
 
-	// The volume is back.
+	// The volume is mounted again.
-	fs.cannotMount = false
-
 	require.NoError(t, os.WriteFile(
 		filepath.Join(volumeDir, "a.txt"), []byte("file a"), 0o600))
+
+	volumeDevice = 2
+
 	require.NoError(t, env.v.CreateSnapshot(prune))
 
 	snapshots, err = env.v.Repositories.Snapshots.ListRecent(