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 is on a different device from its parent
directory. Under --prune, when an earlier snapshot of that name
recorded such a directory and this run does not find it on a different
device, the run skips the prune, keeps every older snapshot, names the
directory and exits non-zero, as for an empty configured path. For a
volume retired, or a directory no longer backed up, on purpose, the
older snapshots are removed with snapshot purge.

Model: opus-5-5

README.md

@@ -337,17 +337,15 @@ value where a list belongs or a word where a number belongs), is refused.
   path that is empty on purpose out of the config, or remove the older
   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
-  not, the prune does not run either: every older snapshot is kept, and the
-  command names the directory and exits non-zero, on every run until the
-  volume is mounted again. A backup also does not find the volume when it
-  finds no such entry in its directory, as when an automounted volume cannot
-  be mounted, or when that directory is no longer backed up, because it is
-  excluded or its configured path was removed. If any of this is on purpose,
-  as for a retired volume, remove the older snapshots by hand with
-  `snapshot purge`. A file removed during the backup does not stop the
-  prune.
+  configured path (a directory on a different filesystem from its parent)
+  and this backup finds none mounted there, the prune does not run either:
+  every older snapshot is kept, and the command names the directory and
+  exits non-zero, on every run until the volume is mounted again. The same
+  happens when that directory is no longer backed up, because it is excluded
+  or its configured path was removed. If the volume was retired or the
+  directory is no longer backed up on purpose, remove the older snapshots by
+  hand with `snapshot purge`. A file removed during the backup does not stop
+  the 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
-  the backup does not exclude.
+  on a different filesystem from their parent.
 
 - 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 that a snapshot's backup found on a different filesystem from their parent directory (a different device number), as a mount point is, the configured path itself included.
 
 **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 INTO snapshot_mount_points` - Record each directory on a different filesystem from its parent
 
 2. **Chunking** (for changed files)
    - `INSERT OR IGNORE INTO chunks` - Store new chunks

internal/database/schema/001.sql

@@ -125,7 +125,7 @@ CREATE INDEX IF NOT EXISTS idx_snapshot_blobs_blob_id ON snapshot_blobs(blob_id)
 
 -- Snapshot mount points table: the directories at or below a configured
 -- path that the snapshot's backup found on a different filesystem from
--- their parent directory, as a mount point is, and found an entry in
+-- their parent directory, as a mount point is
 CREATE TABLE IF NOT EXISTS snapshot_mount_points (
     snapshot_id TEXT NOT NULL,
     path TEXT NOT NULL,

internal/snapshot/device_number_test.go

@@ -10,9 +10,9 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-// The device number a real backup reads is the same for a directory and
-// a file in it, and differs between the temp directory and /dev.
-// Without it no mount point is ever found. See
+// The device number a real backup reads is the same for two entries of
+// one directory and differs between two filesystems. Without it no
+// mount point is ever found. See
 // https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestDeviceNumberFromStat(t *testing.T) {
 	t.Parallel()

internal/snapshot/mount_point_test.go

@@ -5,8 +5,6 @@ import (
 	"os"
 	"path/filepath"
 	"slices"
-	"strings"
-	"syscall"
 	"testing"
 
 	"github.com/spf13/afero"
@@ -17,10 +15,9 @@ 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
-// point is, the scanned path included. A filesystem that gives no device
-// numbers, as the in-memory one, records none. See
-// https://git.eeqj.de/sneak/vaultik/issues/395.
+// from its parent directory, as a mount point is, the scanned path
+// included. A filesystem that gives no device numbers, as the in-memory
+// one, records none. See https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestScanRecordsMountPoints(t *testing.T) {
 	t.Parallel()
 
@@ -98,148 +95,3 @@ func TestScanRecordsMountPoints(t *testing.T) {
 		})
 	}
 }
-
-// automountFs is an in-memory filesystem with a volume at mountPoint
-// that is mounted on first access: opening mountPoint to list it mounts
-// the volume or, if it cannot be mounted, fails with "no such file or
-// directory", as with the autofs daemon. Stat records the device an
-// entry is on at that moment, as a real stat does: unmountedDevice for
-// mountPoint until the volume is mounted; the volume's, 2, for
-// 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 {
-		return nil, &os.PathError{Op: "open", Path: name, Err: syscall.ENOENT}
-	}
-
-	if name == fs.mountPoint {
-		fs.mounted = true
-	}
-
-	return fs.Fs.Open(name)
-}
-
-func (fs *automountFs) Stat(name string) (os.FileInfo, error) {
-	info, err := fs.Fs.Stat(name)
-	if err != nil {
-		return nil, err
-	}
-
-	device := uint64(1)
-
-	switch {
-	case name == fs.mountPoint && !fs.mounted:
-		device = fs.unmountedDevice
-	case name == fs.mountPoint || strings.HasPrefix(name, fs.mountPoint+"/"):
-		device = 2
-	}
-
-	return statWithDevice{FileInfo: info, device: device}, nil
-}
-
-// statWithDevice is an entry automountFs.Stat looked up, with the device
-// it was on then.
-type statWithDevice struct {
-	os.FileInfo
-
-	device uint64
-}
-
-// deviceAtLookup returns the device automountFs.Stat recorded for info.
-func deviceAtLookup(info os.FileInfo) (uint64, bool) {
-	stat, ok := info.(statWithDevice)
-
-	return stat.device, ok
-}
-
-// 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
-// device (a systemd automount) or on its parent's (an autofs map that
-// shows its mount points before mounting them). Either way only the
-// mount point is recorded, by the backup that mounts the volume and by a
-// later one that finds it mounted. Listing an automount point whose
-// volume cannot be mounted fails; it is not recorded, because the walk
-// finds no entry in it. See https://git.eeqj.de/sneak/vaultik/issues/395.
-func TestScanRecordsOnlyTheMountPointOfAnAutomountedVolume(t *testing.T) {
-	t.Parallel()
-
-	const scannedPath = "/data/source"
-
-	mountPoint := scannedPath + "/nas"
-
-	tests := []struct {
-		name            string
-		unmountedDevice uint64
-		canMount        bool
-		want            []string
-	}{
-		{
-			name:            "on the automounter's device until listed",
-			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 {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			ctx := context.Background()
-			fs := &automountFs{
-				Fs: afero.NewMemMapFs(), mountPoint: mountPoint,
-				unmountedDevice: tt.unmountedDevice, canMount: tt.canMount,
-			}
-
-			require.NoError(t, fs.MkdirAll(mountPoint+"/photos", 0o755))
-
-			for _, path := range []string{
-				scannedPath + "/notes.txt", mountPoint + "/photos/a.jpg",
-			} {
-				require.NoError(t, afero.WriteFile(fs, path, []byte(path), 0o644))
-			}
-
-			db, err := database.NewTestDB()
-			require.NoError(t, err)
-			t.Cleanup(func() { _ = db.Close() })
-
-			repos := database.NewRepositories(db)
-
-			for _, snapshotID := range []string{"first", "later"} {
-				createTestSnapshotRecord(ctx, t, repos, snapshotID)
-
-				scanner := snapshot.NewScanner(snapshot.ScannerConfig{
-					FS:               fs,
-					ChunkSize:        int64(1024 * 16),
-					Repositories:     repos,
-					MaxBlobSize:      int64(1024 * 1024),
-					CompressionLevel: 3,
-					AgeRecipients:    []string{testAgePublicKey},
-					DeviceNumber:     deviceAtLookup,
-				})
-
-				_, err = scanner.Scan(ctx, scannedPath, snapshotID)
-				require.NoError(t, err)
-
-				got, err := repos.Snapshots.GetMountPoints(ctx, snapshotID)
-				require.NoError(t, err)
-				assert.Equal(t, tt.want, got, "mount points of the %s backup", snapshotID)
-			}
-		})
-	}
-}

internal/snapshot/scanner.go

@@ -310,7 +310,7 @@ func (s *Scanner) Scan(
 		}
 	}
 
-	err = s.recordDirectories(ctx, scanResult)
+	err = s.recordDirectories(ctx, scanResult.Directories)
 	if err != nil {
 		return nil, err
 	}
@@ -888,11 +888,6 @@ 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
-	// the walk found an entry in and, once it had listed them, on a
-	// different device from their parent directory (see
-	// recordIfMountPoint).
-	MountPoints []string
 }
 
 // scanPhase performs the initial directory scan to identify files to process
@@ -912,7 +907,7 @@ func (s *Scanner) scanPhase(
 	// estimate leaves them out.
 	estimatedTotal := countNonDirectories(knownFiles)
 
-	collector := &scanCollector{dirsWithEntries: make(map[string]struct{})}
+	collector := &scanCollector{}
 
 	// Set up periodic status output
 	startTime := time.Now()
@@ -957,8 +952,6 @@ func (s *Scanner) scanPhase(
 			result.EntriesFound++
 		}
 
-		s.recordIfMountPoint(filePath, path, collector)
-
 		// Handle symlinks and directories
 		handled, err := s.recordSpecialEntry(
 			filePath, info, existingFiles, knownFiles, collector, result)
@@ -993,7 +986,6 @@ func (s *Scanner) scanPhase(
 		FilesToProcess:   collector.filesToProcess,
 		UnchangedFileIDs: collector.unchangedFileIDs,
 		Directories:      collector.directories,
-		MountPoints:      collector.mountPoints,
 	}, nil
 }
 
@@ -1120,29 +1112,6 @@ 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.
-	dirsWithEntries map[string]struct{}
-	mountPoints     []string
-}
-
-// firstEntryIn notes that the walk found an entry in directory dir and
-// reports whether it is the first one.
-func (c *scanCollector) firstEntryIn(dir string) bool {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-
-	_, seen := c.dirsWithEntries[dir]
-	c.dirsWithEntries[dir] = struct{}{}
-
-	return !seen
-}
-
-// addMountPoint sets a mount point aside to be recorded.
-func (c *scanCollector) addMountPoint(dir string) {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-
-	c.mountPoints = append(c.mountPoints, dir)
 }
 
 // addDirectory sets a directory aside to be recorded. It is not counted
@@ -1338,23 +1307,29 @@ func (s *Scanner) buildDirectoryEntry(path string, info os.FileInfo) *database.F
 // mount points (see recordMountPoints). A directory has no data to back
 // up, so it is not among the files counted and backed up.
 func (s *Scanner) recordDirectories(
-	ctx context.Context, scanned *ScanPhaseResult,
+	ctx context.Context, directories []*FileToProcess,
 ) error {
-	for _, dir := range scanned.Directories {
+	for _, dir := range directories {
 		err := s.recordNonRegularFile(ctx, dir)
 		if err != nil {
 			return fmt.Errorf("recording directory %s: %w", dir.Path, err)
 		}
 	}
 
-	return s.recordMountPoints(ctx, scanned.MountPoints)
+	return s.recordMountPoints(ctx, directories)
 }
 
-// recordMountPoints records with the snapshot the mount points the walk
-// found (see ScanPhaseResult.MountPoints).
+// recordMountPoints records with the snapshot each directory the walk
+// found on a different device from its parent directory, as a mount
+// point is, the scanned path included.
 func (s *Scanner) recordMountPoints(
-	ctx context.Context, mountPoints []string,
+	ctx context.Context, directories []*FileToProcess,
 ) error {
+	mountPoints, err := s.findMountPoints(directories)
+	if err != nil {
+		return err
+	}
+
 	return s.repos.WithTx(ctx, func(txCtx context.Context, tx *sql.Tx) error {
 		for _, dir := range mountPoints {
 			log.Info("Found a mount point", "path", dir)
@@ -1369,53 +1344,57 @@ func (s *Scanner) recordMountPoints(
 	})
 }
 
-// recordIfMountPoint sets the directory that holds entry, a path the
-// walk found, aside to be recorded as a mount point if it is on a
-// different device from its parent directory. The walk calls it for each
-// entry it finds, and it decides on the first entry in a directory, so a
-// directory the walk found no entry in is never recorded. By then the
-// walk has listed the directory, which mounts a volume mounted there on
-// first access, so the directory and its parent are looked up again:
-// before the listing, the directory can be on the automounter's own
-// device (a systemd automount) or on its parent's (an autofs map that
-// shows its mount points before mounting them). The scanned path is in
-// no listing the walk made, so it is skipped as an entry. A directory
-// 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
+// findMountPoints returns the directories, among those the walk found,
+// that are on a different device from their parent directory. The
+// scanned path is compared with the directory that holds it, which the
+// walk did not reach.
+func (s *Scanner) findMountPoints(
+	directories []*FileToProcess,
+) ([]string, error) {
+	walked := make(map[string]os.FileInfo, len(directories))
+	for _, dir := range directories {
+		walked[dir.Path] = dir.FileInfo
 	}
 
-	dirDevice, ok := s.deviceNumberAt(dir)
-	if !ok {
-		return
+	var mountPoints []string
+
+	for _, dir := range directories {
+		parentPath := filepath.Dir(dir.Path)
+
+		parent, ok := walked[parentPath]
+		if !ok {
+			var err error
+
+			parent, err = s.fs.Stat(parentPath)
+			if err != nil {
+				return nil, fmt.Errorf(
+					"looking up the parent of %s: %w", dir.Path, err)
+			}
+		}
+
+		if s.onOtherDevice(dir.FileInfo, parent) {
+			mountPoints = append(mountPoints, dir.Path)
+		}
 	}
 
-	parentDevice, ok := s.deviceNumberAt(filepath.Dir(dir))
-	if ok && dirDevice != parentDevice {
-		collector.addMountPoint(dir)
-	}
+	return mountPoints, nil
 }
 
-// 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",
-			"path", path, "error", err)
-
-		return 0, false
+// onOtherDevice reports whether dir is on a different device from
+// parent. A file whose device number cannot be read counts as on the
+// same device as its parent.
+func (s *Scanner) onOtherDevice(dir, parent os.FileInfo) bool {
+	dirDevice, ok := s.deviceNumber(dir)
+	if !ok {
+		return false
 	}
 
-	return s.deviceNumber(info)
+	parentDevice, ok := s.deviceNumber(parent)
+	if !ok {
+		return false
+	}
+
+	return 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
-// directory is no longer backed up. The snapshots are complete and
-// kept, so the caller can report it apart from a failed backup. A run
-// that also skipped unreadable files or found a configured path empty
-// returns ErrFilesUnreadable or ErrSourcePathEmpty instead.
+// mounted there is not mounted, or the directory is no longer backed up.
+// The snapshots are complete and kept, so the caller can report it apart
+// from a failed backup. A run that also skipped unreadable files or
+// found a configured path empty returns ErrFilesUnreadable or
+// ErrSourcePathEmpty instead.
 var ErrVolumeNotMounted = errors.New(
 	"mount point(s) an earlier snapshot recorded were not found")
 
@@ -237,11 +237,11 @@ 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 "+
-			"entry in it, or the directory is no longer backed up. Older "+
-			"snapshots are kept. Mount the volume again, or, if that is on "+
-			"purpose, remove the older snapshots by hand with "+
-			"'vaultik snapshot purge'.",
+			"found one: either the volume is not mounted or the directory "+
+			"is no longer backed up. Older snapshots are kept. Mount the "+
+			"volume again, or, if the volume was retired or the directory "+
+			"is no longer backed up on purpose, remove the older snapshots "+
+			"by hand with '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"
@@ -263,69 +261,15 @@ 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
-// run, the snapshot that holds the volume's files is kept with everything
-// it stored, and the run fails. Once the volume is back, the prune runs.
-// The volume is mounted on first access and every backup starts with it
-// not mounted, so its directory is on its parent's device until the walk
-// lists it. While the volume is missing, its directory is left empty, or
-// listing it fails with "no such file or directory", as when an autofs
-// volume cannot be mounted, which the walk counts as a directory removed
-// during the backup. See https://git.eeqj.de/sneak/vaultik/issues/395.
+// mounted below the configured path is not mounted does not prune: run
+// after run, the snapshot that holds the volume's files is kept with
+// everything it stored, and the run fails. Once the volume is mounted
+// again, the prune runs. See https://git.eeqj.de/sneak/vaultik/issues/395.
 func TestBackupWithVolumeNotMountedDoesNotPrune(t *testing.T) {
 	log.Initialize(log.Config{})
 	t.Parallel()
 
-	tests := []struct {
-		name         string
-		listingFails bool
-	}{
-		{name: "directory left empty", listingFails: false},
-		{name: "listing fails", listingFails: true},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			backUpWithVolumeNotMounted(t, tt.listingFails)
-		})
-	}
-}
-
-// backUpWithVolumeNotMounted runs the backups of
-// TestBackupWithVolumeNotMountedDoesNotPrune. While the volume is
-// missing, listing its directory fails if listingFails; otherwise the
-// directory is empty.
-func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
-	t.Helper()
-
 	// volume is the directory, below the configured path, where the
 	// volume is mounted.
 	const (
@@ -338,21 +282,20 @@ 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
-	// volume is mounted. Everything else is on device 1.
-	volumeDir := filepath.Join(env.src, volume)
-	fs := &automountFs{volumeDir: volumeDir}
+	// While the volume is mounted, its directory is on a device of its
+	// own.
+	mounted := true
 	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 mounted && info.Name() == volume {
 				return 2, true
 			}
 
@@ -368,13 +311,13 @@ func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
 	stored, err := env.v.Storage.List(ctx, "")
 	require.NoError(t, err)
 
-	// The volume goes missing.
-	fs.cannotMount = listingFails
+	// A volume that is not mounted leaves its mount point behind as an
+	// empty directory on its parent's device.
+	volumeDir := filepath.Join(env.src, volume)
+	require.NoError(t, os.RemoveAll(volumeDir))
+	require.NoError(t, os.Mkdir(volumeDir, 0o750))
 
-	if !listingFails {
-		require.NoError(t, os.RemoveAll(volumeDir))
-		require.NoError(t, os.Mkdir(volumeDir, 0o750))
-	}
+	mounted = false
 
 	for range 2 {
 		env.out.Reset()
@@ -399,11 +342,12 @@ func backUpWithVolumeNotMounted(t *testing.T, listingFails bool) {
 		require.NoError(t, err, "%s is kept", key)
 	}
 
-	// The volume is back.
-	fs.cannotMount = false
-
+	// The volume is mounted again.
 	require.NoError(t, os.WriteFile(
 		filepath.Join(volumeDir, "a.txt"), []byte("file a"), 0o600))
+
+	mounted = true
+
 	require.NoError(t, env.v.CreateSnapshot(prune))
 
 	snapshots, err = env.v.Repositories.Snapshots.ListRecent(