Do not prune after a backup that found a volume not mounted (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, a run that does not find such a directory
recorded by an earlier snapshot of that name skips the prune, keeps
every older snapshot, names the directory and its snapshot, and exits
non-zero, as for an empty configured path. A volume retired on purpose
is cleared by removing the older snapshots with snapshot purge.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -73,6 +73,9 @@ type Scanner struct {
 	skipErrors bool
 	// ui is the user-facing output; never nil (defaults to a discarding writer).
 	ui *ui.Writer
+	// deviceNumber returns the number of the device that holds a file;
+	// never nil (defaults to deviceNumberFromStat).
+	deviceNumber func(info os.FileInfo) (uint64, bool)
 
 	// In-memory cache of known chunk hashes for fast existence checks
 	knownChunks   map[string]struct{}
@@ -136,6 +139,10 @@ type ScannerConfig struct {
 	// PreviousSnapshotID is the most recent completed snapshot of the
 	// same name, "" for none.
 	PreviousSnapshotID string
+	// DeviceNumber returns the number of the device that holds a file,
+	// and false when there is none to read. nil reads it from the file's
+	// stat; tests replace it, since the in-memory test filesystem has none.
+	DeviceNumber func(info os.FileInfo) (uint64, bool)
 }
 
 // ScanResult contains the results of a scan operation. Files and bytes
@@ -214,6 +221,11 @@ func NewScanner(cfg ScannerConfig) *Scanner {
 		uiw = ui.NewWithColor(io.Discard, false)
 	}
 
+	deviceNumber := cfg.DeviceNumber
+	if deviceNumber == nil {
+		deviceNumber = deviceNumberFromStat
+	}
+
 	return &Scanner{
 		fs:                 cfg.FS,
 		chunker:            chunker.NewChunker(cfg.ChunkSize),
@@ -229,6 +241,7 @@ func NewScanner(cfg ScannerConfig) *Scanner {
 		skipErrors:         cfg.SkipErrors,
 		previousSnapshotID: cfg.PreviousSnapshotID,
 		ui:                 uiw,
+		deviceNumber:       deviceNumber,
 		pendingChunkHashes: make(map[string]struct{}),
 	}
 }
@@ -1290,9 +1303,9 @@ func (s *Scanner) buildDirectoryEntry(path string, info os.FileInfo) *database.F
 	}
 }
 
-// recordDirectories records every directory the walk found. A directory
-// has no data to back up, so it is not among the files counted and
-// backed up.
+// recordDirectories records every directory the walk found, and its
+// 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, directories []*FileToProcess,
 ) error {
@@ -1303,7 +1316,97 @@ func (s *Scanner) recordDirectories(
 		}
 	}
 
-	return nil
+	return s.recordMountPoints(ctx, directories)
+}
+
+// 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, 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)
+
+			err := s.repos.Snapshots.AddMountPoint(txCtx, tx, s.snapshotID, dir)
+			if err != nil {
+				return fmt.Errorf("recording mount point %s: %w", dir, err)
+			}
+		}
+
+		return nil
+	})
+}
+
+// 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
+	}
+
+	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)
+		}
+	}
+
+	return mountPoints, nil
+}
+
+// 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
+	}
+
+	parentDevice, ok := s.deviceNumber(parent)
+	if !ok {
+		return false
+	}
+
+	return dirDevice != parentDevice
+}
+
+// deviceNumberFromStat returns the number of the device that holds the
+// file info describes, and false when info carries no stat, as from the
+// in-memory test filesystem.
+func deviceNumberFromStat(info os.FileInfo) (uint64, bool) {
+	stat, ok := info.Sys().(*syscall.Stat_t)
+	if !ok {
+		return 0, false
+	}
+
+	return uint64(stat.Dev), true //nolint:unconvert // Dev is an int32 on darwin
 }
 
 // recordNonRegularFile writes a symlink or directory entry to the database