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 it is a directory on a different filesystem than the deepest
kept operand above it, or a regular file in such a directory, since
that operand's walk does not reach it. 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, whose comment no
longer claims it runs every phase. The startWalk comment is rewrapped.

Model: opus-5-5

README.md

@@ -399,10 +399,9 @@ 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 any directory from it (from its
+another one is kept and walked on its own when it is a directory on a different
-directory, for a regular file) up to but not including the operand covering it
+filesystem than the operand covering it, or a regular file in such a directory,
-is on a different filesystem than that operand, because that operand's walk does
+because the walk of the covering operand does not reach it.
-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
@@ -411,8 +410,9 @@ 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 treated like any other record there. This is not
+that operand instead, and are deleted like any other record there that this scan
-an error: a scan whose every operand is dropped walks nothing and exits 0.
+did not verify. 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:
@@ -531,10 +531,9 @@ Rules for the walk:
   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 whose record counts as
+  is an operand dropped before the database was read that lies under no other
-  outside the scanned operands; an unreadable directory subtree likewise loses
+  operand; an unreadable directory subtree likewise loses its records (accepted:
-  its records (accepted: the database mirrors what the latest scan could
+  the database mirrors what the latest scan could actually verify).
-  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

@@ -30,9 +30,8 @@
 # Completed Steps
 
 - with `-x`, an operand under another operand is walked on its own instead of
-  being dropped when a directory from it (from its directory, for a regular
+  being dropped when it is a directory on a different filesystem than that
-  file) up to but not including that operand is on a different filesystem than
+  operand, or a regular file in such a directory, and the records of files gone
-  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 the walk of the deepest kept
+// operand under another one is kept when device puts it, or for a
-// operand above it cannot reach it: when device puts any directory from
+// regular file its directory, on a different filesystem than the
-// it (from its directory, for a regular file) up to but not including
+// deepest kept operand above it, because that operand's walk then does
-// that operand on a different filesystem than that operand. Every
+// not reach it. Every remaining file is then reachable through exactly
-// remaining file is then reachable through exactly one root, so walked
+// one root, so walked paths are unique without keeping a scan-wide set
-// paths are unique without keeping a scan-wide set of every path seen.
+// of every path seen.
 func pruneRoots(roots []string, oneFS bool,
 	device func(path string) (uint64, bool),
 ) []string {
@@ -239,15 +239,10 @@ 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 the directories checked start at its directory.
+		// operand its directory decides.
 		dir := r
 
 		fi, err := os.Lstat(r)
@@ -255,13 +250,11 @@ func pruneRoots(roots []string, oneFS bool,
 			dir = filepath.Dir(r)
 		}
 
-		for ; dir != above; dir = filepath.Dir(dir) {
+		dev, ok := device(dir)
-			dev, ok := device(dir)
+		aboveDev, aboveOK := device(above)
-			if ok && dev != aboveDev {
-				kept = append(kept, r)
 
-				break
+		if ok && aboveOK && dev != aboveDev {
-			}
+			kept = append(kept, r)
 		}
 	}
 
@@ -338,8 +331,8 @@ 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 are treated as if those operands
+// first, so the records beneath them count as outside the roots unless
-// had not been given.
+// they lie under another root.
 func (s *scanState) runPhases(ctx context.Context, roots []string,
 	workers int, oneFS bool,
 ) error {
@@ -381,10 +374,10 @@ 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 are treated as if it
+// longer a root, so the records stored beneath it count as outside the
-// had not been given: those under no other root count as outside the
+// roots and are not deleted as unverified, unless it lies under another
-// roots and are not deleted as unverified. An operand that fails lstat
+// root. An operand that fails lstat here is kept, and the walk warns
-// here is kept, and the walk warns about it.
+// about it.
 func (s *scanState) walkableRoots(roots []string) []string {
 	kept := make([]string, 0, len(roots))
 
@@ -1039,8 +1032,7 @@ 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, other than those under another root, are deleted
+// stored beneath it are deleted as unverified.
-// as unverified.
 func seedRoot(ctx context.Context, root string,
 	events chan<- walkEvent,
 ) []dirJob {
@@ -1240,8 +1232,7 @@ 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, except those below an operand at or
+// scan keeps the records below it.
-// inside it.
 func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	parent dirJob, oneFS bool, events chan<- walkEvent,
 ) (dirJob, bool) {

scan_test.go

@@ -1136,11 +1136,10 @@ func TestSubdirJobStatError(t *testing.T) {
 }
 
 // syncOneFilesystem scans roots into db with -x and returns the scan
-// stats. Every device pruneRoots looks up, and the device each
+// stats, taking the device of each operand, and of a regular-file
-// operand's walk compares its subdirectories with, comes from devices
+// operand's directory, from devices instead of from lstat, so a test
-// instead of from lstat, so a test can put a path on a filesystem that
+// can put an operand on a filesystem that no directory here is on. It
-// no directory here is on. Unlike runPhases, it skips walkableRoots,
+// runs the steps of runPhases except the content phase.
-// 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 {
@@ -1303,64 +1302,6 @@ 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