Walk an operand nested on another filesystem with -x (closes #106)

With -x, an operand under another operand is kept and walked on its
own when the walk of the deepest kept operand above it cannot reach
it: some directory from it (from its directory, for a regular file) up
to but not including that operand is on a different filesystem than
that operand. Below a kept operand, the rule keeping records under a
directory -x did not enter covers only directories its own walk did
not enter, so records of files gone from it are deleted.

The new tests stub the devices and share their scan steps with the
test for #99. Comments and
README sentences on operands dropped before the walk now also hold
with -x.

Model: opus-5-5

README.md

@@ -399,9 +399,10 @@ scan and are enumerated concurrently: every operand seeds the shared walk worker
 pool. Overlapping operands are harmless — an operand that duplicates another or
 lies under another is dropped before walking, so every file is reached exactly
 once and produces one database record. With `-x`, though, an operand under
-another one is kept and walked on its own when it is a directory on a different
-filesystem than the operand covering it, or a regular file in such a directory,
-because the walk of the covering operand does not reach it.
+another one is kept and walked on its own when any directory from it (from its
+directory, for a regular file) up to but not including the operand covering it
+is on a different filesystem than that operand, because that operand's walk does
+not enter such a directory and so does not reach it.
 
 An operand that is a symlink (never followed, not even as an operand), socket,
 FIFO, or device node, or a directory named `.zfs`, is not scanned. `scan` prints
@@ -410,9 +411,8 @@ drops it from the scanned operands before reading the database. Another operand
 beneath it is still scanned. The records stored beneath it are not deleted: they
 are treated like any other record outside the scanned operands, including the
 content-phase exception below. If it lies under another operand, they are under
-that operand instead, and are deleted like any other record there that this scan
-did not verify. This is not an error: a scan whose every operand is dropped
-walks nothing and exits 0.
+that operand instead, and are treated like any other record there. This is not
+an error: a scan whose every operand is dropped walks nothing and exits 0.
 
 `scan` synchronizes the database with the filesystem state under the scanned
 operands:
@@ -522,17 +522,19 @@ Rules for the walk:
 - Filesystem boundaries are crossed by default. With `-x` (long form
   `--one-file-system`, following the GNU `du`/`rsync` convention), never descend
   into a directory on a different filesystem than its `PATH` operand; each
-  operand is bounded by its own filesystem. The records below such a directory
-  count as outside the scanned operands, as described in "`scan` mode" above:
-  they are not deleted, and their sizes join the size census.
+  operand is bounded by its own filesystem. The records below such a directory,
+  unless they lie below an operand at or inside that directory, count as outside
+  the scanned operands, as described in "`scan` mode" above: they are not
+  deleted, and their sizes join the size census.
 - On any per-path error (permission denied, file vanished between passes,
   unreadable): print a one-line warning to stderr, skip the path, and continue.
   Per-file errors never abort the run; the final summary reports how many were
   skipped. As specified above, a skipped path that has a database record from an
   earlier scan loses that record, unless it failed only in the content phase, or
-  is an operand dropped before the database was read that lies under no other
-  operand; an unreadable directory subtree likewise loses its records (accepted:
-  the database mirrors what the latest scan could actually verify).
+  is an operand dropped before the database was read whose record counts as
+  outside the scanned operands; an unreadable directory subtree likewise loses
+  its records (accepted: the database mirrors what the latest scan could
+  actually verify).
 
 Concurrency: the walk phase (which also stats files), the hash phase, and the
 content phase each use a worker pool of `--workers` workers (default

TODO.md

@@ -30,8 +30,9 @@
 # Completed Steps
 
 - with `-x`, an operand under another operand is walked on its own instead of
-  being dropped when it is a directory on a different filesystem than that
-  operand, or a regular file in such a directory, and the records of files gone
+  being dropped when a directory from it (from its directory, for a regular
+  file) up to but not including that operand is on a different filesystem than
+  that operand, whose walk then cannot reach it, and the records of files gone
   from below it are deleted (2026-10-14,
   https://git.eeqj.de/sneak/sfdupes/issues/106)
 

scan.go

@@ -204,12 +204,12 @@ func resolveRoots(roots []string) ([]string, error) {
 
 // pruneRoots drops operands already covered by another operand:
 // duplicates and any operand lying under another one. With -x, an
-// operand under another one is kept when device puts it, or for a
-// regular file its directory, on a different filesystem than the
-// deepest kept operand above it, because that operand's walk then does
-// not reach it. Every remaining file is then reachable through exactly
-// one root, so walked paths are unique without keeping a scan-wide set
-// of every path seen.
+// operand under another one is kept when the walk of the deepest kept
+// operand above it cannot reach it: when device puts any directory from
+// it (from its directory, for a regular file) up to but not including
+// that operand on a different filesystem than that operand. Every
+// remaining file is then reachable through exactly one root, so walked
+// paths are unique without keeping a scan-wide set of every path seen.
 func pruneRoots(roots []string, oneFS bool,
 	device func(path string) (uint64, bool),
 ) []string {
@@ -239,10 +239,15 @@ func pruneRoots(roots []string, oneFS bool,
 			continue
 		}
 
+		aboveDev, aboveOK := device(above)
+		if !aboveOK {
+			continue
+		}
+
 		// The walk of above enters a directory only when it is on
 		// above's filesystem, and then reaches every regular file in it
 		// whatever device the file reports, so for a regular-file
-		// operand its directory decides.
+		// operand the directories checked start at its directory.
 		dir := r
 
 		fi, err := os.Lstat(r)
@@ -250,11 +255,13 @@ func pruneRoots(roots []string, oneFS bool,
 			dir = filepath.Dir(r)
 		}
 
-		dev, ok := device(dir)
-		aboveDev, aboveOK := device(above)
+		for ; dir != above; dir = filepath.Dir(dir) {
+			dev, ok := device(dir)
+			if ok && dev != aboveDev {
+				kept = append(kept, r)
 
-		if ok && aboveOK && dev != aboveDev {
-			kept = append(kept, r)
+				break
+			}
 		}
 	}
 
@@ -331,8 +338,8 @@ func syncScan(ctx context.Context, db *sql.DB, roots []string,
 // size, head, and tail match another record's). Records outside the
 // roots are never touched, except that the content phase fills in
 // their content hash. Operands the walk cannot start from are dropped
-// first, so the records beneath them count as outside the roots unless
-// they lie under another root.
+// first, so the records beneath them are treated as if those operands
+// had not been given.
 func (s *scanState) runPhases(ctx context.Context, roots []string,
 	workers int, oneFS bool,
 ) error {
@@ -374,10 +381,10 @@ func (s *scanState) runPhases(ctx context.Context, roots []string,
 // walkableRoots returns the operands the walk can start from: regular
 // files, and directories not named .zfs. Every other operand is warned
 // about, counted as skipped, and dropped. A dropped operand is no
-// longer a root, so the records stored beneath it count as outside the
-// roots and are not deleted as unverified, unless it lies under another
-// root. An operand that fails lstat here is kept, and the walk warns
-// about it.
+// longer a root, so the records stored beneath it are treated as if it
+// had not been given: those under no other root count as outside the
+// roots and are not deleted as unverified. An operand that fails lstat
+// here is kept, and the walk warns about it.
 func (s *scanState) walkableRoots(roots []string) []string {
 	kept := make([]string, 0, len(roots))
 
@@ -1032,7 +1039,8 @@ func operandWarning(root string, fi fs.FileInfo) string {
 // operand becomes an initial job. walkableRoots has already dropped
 // every other operand. One that has changed into something else since
 // is warned about and skipped here; it is still a root, so the records
-// stored beneath it are deleted as unverified.
+// stored beneath it, other than those under another root, are deleted
+// as unverified.
 func seedRoot(ctx context.Context, root string,
 	events chan<- walkEvent,
 ) []dirJob {
@@ -1232,7 +1240,8 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 // .zfs (ZFS snapshot pseudo-dirs would list every file once per
 // snapshot), and with -x never enter a directory on a different
 // filesystem than its operand, sending an event that names it so the
-// scan keeps the records below it.
+// scan keeps the records below it, except those below an operand at or
+// inside it.
 func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	parent dirJob, oneFS bool, events chan<- walkEvent,
 ) (dirJob, bool) {

scan_test.go

@@ -1136,10 +1136,11 @@ func TestSubdirJobStatError(t *testing.T) {
 }
 
 // syncOneFilesystem scans roots into db with -x and returns the scan
-// stats, taking the device of each operand, and of a regular-file
-// operand's directory, from devices instead of from lstat, so a test
-// can put an operand on a filesystem that no directory here is on. It
-// runs the steps of runPhases except the content phase.
+// stats. Every device pruneRoots looks up, and the device each
+// operand's walk compares its subdirectories with, comes from devices
+// instead of from lstat, so a test can put a path on a filesystem that
+// no directory here is on. Unlike runPhases, it skips walkableRoots,
+// the check for a cancelled walk, and the content phase.
 func syncOneFilesystem(t *testing.T, db *sql.DB, roots []string,
 	devices map[string]uint64,
 ) scanStats {
@@ -1302,6 +1303,64 @@ func TestScanOneFilesystemWalksNestedOperand(t *testing.T) {
 	}
 }
 
+// TestScanOneFilesystemWalksOperandBelowOtherFilesystem checks that
+// with -x an operand on the same filesystem as the operand covering it
+// is walked on its own when a directory between them is on another
+// filesystem, since the walk of the covering operand does not enter
+// that directory: a file added below the nested operand is recorded,
+// and the record of a file gone from below it is deleted. A record
+// below that directory but outside the nested operand is still kept.
+// Both operands are given one device that no directory here is on,
+// while mnt keeps its own.
+func TestScanOneFilesystemWalksOperandBelowOtherFilesystem(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	db := openTestDB(t)
+
+	// A scan without -x records both files below mnt.
+	gone := writeFile(t, dir, "mnt/back/gone", pattern(1, 10))
+	kept := writeFile(t, dir, "mnt/kept", pattern(2, 20))
+	syncTree(t, db, dir)
+
+	err := os.Remove(gone)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	added := writeFile(t, dir, "mnt/back/added", pattern(3, 30))
+
+	info, err := os.Lstat(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dev, ok := deviceOfInfo(info)
+	if !ok {
+		t.Skip("platform exposes no device id")
+	}
+
+	mnt := filepath.Join(dir, "mnt")
+	back := filepath.Join(mnt, "back")
+	syncOneFilesystem(t, db, []string{dir, back},
+		map[string]uint64{dir: dev + 1, mnt: dev, back: dev + 1})
+
+	paths := recordPaths(dbRecords(t, db))
+
+	if !slices.Contains(paths, added) {
+		t.Errorf("no record for %s: the walk did not reach %s", added, back)
+	}
+
+	if slices.Contains(paths, gone) {
+		t.Errorf("record for %s kept: the file is gone", gone)
+	}
+
+	if !slices.Contains(paths, kept) {
+		t.Errorf("record for %s deleted: the walk of %s did not enter %s",
+			kept, dir, mnt)
+	}
+}
+
 // TestScanOneFilesystemRecordsNestedFileOnce checks that with -x a
 // regular-file operand under another operand is dropped when the file
 // reports a different device than that operand but its directory does
@@ -1334,6 +1393,38 @@ func TestScanOneFilesystemRecordsNestedFileOnce(t *testing.T) {
 	}
 }
 
+// TestScanOneFilesystemWalksNestedFileOperand checks that with -x a
+// regular-file operand under another operand is kept when its directory
+// is on a different filesystem than that operand: the walk of the other
+// operand does not enter the directory, so the file is recorded once,
+// as its own operand. The other operand is given a device that no
+// directory here is on.
+func TestScanOneFilesystemWalksNestedFileOperand(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	db := openTestDB(t)
+	file := writeFile(t, dir, "sub/f", pattern(1, 10))
+
+	info, err := os.Lstat(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	dev, ok := deviceOfInfo(info)
+	if !ok {
+		t.Skip("platform exposes no device id")
+	}
+
+	st := syncOneFilesystem(t, db, []string{dir, file}, map[string]uint64{
+		dir: dev + 1, filepath.Dir(file): dev,
+	})
+
+	if st.added != 1 {
+		t.Errorf("%s recorded %d times, want once", file, st.added)
+	}
+}
+
 // buildSmokeTree recreates the README smoke-test filesystem layout
 // with deterministic content and returns the tree root.
 func buildSmokeTree(t *testing.T) string {