Rewrite TODO.md sentence by sentence (closes #135)

Rewrites `TODO.md` from scratch for
#135, by the rules applied to
`README.md` in #140. It is the
project's own task list, so every completed step keeps its date, its
issue or branch and every fact that the old entry recorded, in short
sentences, with lists as bullets. An entry superseded later says so in
one sentence.

`TODO.md` records history, so its entries were not corrected against the
code. The 2026-10-17 release-rule entry is known to be stale. One product
name in the 2026-10-15 entry is replaced by a description.

This file carries the `TODO.md` entry of
#140 word for word, so the
conflict once that merges is resolved by taking this file whole.

Model: fable-5-1

README.md

@@ -455,29 +455,32 @@ 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. An operand under another one is kept,
-though, when the walk of the outer operand does not reach it. It is then walked
-on its own. The walk of the outer operand does not reach the inner one when it,
-or any directory between the two, is:
+though, 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 a default
+exclude, is not on a persistent filesystem (see "Rules for the walk"), or, with
+`-x`, 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.
 
-- named `.zfs`;
-- a symlink;
-- a default exclude;
-- not on a persistent filesystem (see "Rules for the walk");
-- with `-x`, on a different filesystem than the outer operand.
+An operand is not scanned when it is:
 
-For a regular-file operand, only the directories between the two count.
+- a symlink, which `scan` never follows, not even as an operand;
+- a socket, FIFO or device node;
+- at or inside a directory named `.zfs`.
 
-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
-a one-line warning naming the path and what it is, counts it as skipped, and
-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. If it lies under
-another operand, they are under 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. An operand that is not on a persistent filesystem
-(see "Rules for the walk") is dropped the same way. The line `scan` prints for
-it is not a warning, though, and it is not counted as skipped.
+An operand reached through a symlink into a `.zfs` directory is not scanned
+either.
+
+This holds even when the operand lies under another operand. `scan` prints a
+one-line warning naming the path and what it is, counts it as skipped, and drops
+it from the scanned operands before reading the database. An operand beneath a
+symlink operand is still scanned. The records stored beneath the dropped operand
+are not deleted: they are treated like any other record outside the scanned
+operands. If it lies under another operand, they are under 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. An operand that is not on a
+persistent filesystem (see "Rules for the walk") is dropped the same way. The
+line `scan` prints for it is not a warning, though, and it is not counted as
+skipped.
 
 `scan` synchronizes the database with the filesystem state under the scanned
 operands:

main_test.go

@@ -928,24 +928,21 @@ func TestRunScanWalksOperandUnderSymlinkOperand(t *testing.T) {
 }
 
 func TestRunScanSkipsZFSOperand(t *testing.T) {
-	path := testDBPath(t)
-	t.Setenv(databaseEnv, path)
-
 	zfs := filepath.Join(t.TempDir(), ".zfs")
-	snapshot := filepath.Join(zfs, "snapshot", "hourly")
-	f := writeFile(t, snapshot, "f", pattern(1, 10))
+	f := writeFile(t, zfs, "snapshot/hourly/f", pattern(1, 10))
 
-	// An operand beneath a .zfs directory is walked, because it is not
-	// itself named .zfs.
-	scanOK(t, snapshot)
+	// No scan records a file inside a .zfs directory, so the record
+	// beneath the operand is written straight into the database.
+	path := seedDatabase(t, []scanRec{{size: 10, path: f}})
+	t.Setenv(databaseEnv, path)
 
 	assertOperandSkipped(t, path, zfs, ".zfs directory", f)
 }
 
 // assertOperandSkipped scans operand alone and checks that it is skipped
 // as kind: a warning naming it, one skip in the summary, exit 0, and the
-// record for kept, which an earlier scan stored beneath operand, still
-// in the database at dbPath.
+// record for kept, stored beneath operand before this scan, still in the
+// database at dbPath.
 func assertOperandSkipped(t *testing.T, dbPath, operand, kind,
 	kept string,
 ) {
@@ -974,6 +971,121 @@ func assertOperandSkipped(t *testing.T, dbPath, operand, kind,
 	recordByPath(t, dbRecords(t, db), kept)
 }
 
+// TestRunScanSkipsOperandInsideZFS scans a directory inside a .zfs
+// directory, first alone and then under another operand. Each time it
+// is warned about and counted as skipped, and nothing in it is walked.
+// The other operand is scanned as usual.
+func TestRunScanSkipsOperandInsideZFS(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := t.TempDir()
+	hourly := filepath.Join(dir, ".zfs", "snapshot", "hourly")
+	writeFile(t, hourly, "f", pattern(1, 10))
+	f := writeFile(t, dir, "f", pattern(2, 10))
+
+	warning := "walk " + hourly + ": skipping operand inside a .zfs directory\n"
+
+	for _, tc := range []struct {
+		operands []string
+		summary  string
+	}{
+		{
+			[]string{hourly},
+			"scan: 0 files seen (0 added, 0 updated, 0 removed, " +
+				"0 unchanged), 1 skipped\n",
+		},
+		{
+			[]string{dir, hourly},
+			"scan: 1 files seen (1 added, 0 updated, 0 removed, " +
+				"0 unchanged), 1 skipped\n",
+		},
+	} {
+		stderr := scanOK(t, tc.operands...)
+		if !strings.Contains(stderr, warning) ||
+			!strings.Contains(stderr, tc.summary) {
+			t.Errorf("scan %q: stderr = %q, want %q and %q", tc.operands,
+				stderr, warning, tc.summary)
+		}
+	}
+
+	db, err := openDB(path, reportParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	if got := recordPaths(dbRecords(t, db)); !slices.Equal(got, []string{f}) {
+		t.Errorf("records = %q, want %q", got, []string{f})
+	}
+}
+
+// TestRunScanSkipsOperandThroughSymlinkIntoZFS scans snaps/daily, where
+// snaps is a symlink to pool/.zfs/snapshot. The operand is warned about
+// by the path given, counted as skipped, and not walked.
+func TestRunScanSkipsOperandThroughSymlinkIntoZFS(t *testing.T) {
+	t.Setenv(databaseEnv, testDBPath(t))
+
+	dir := t.TempDir()
+	writeFile(t, dir, "pool/.zfs/snapshot/daily/f", pattern(1, 10))
+
+	snaps := filepath.Join(dir, "snaps")
+
+	err := os.Symlink(filepath.Join(dir, "pool", ".zfs", "snapshot"), snaps)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	daily := filepath.Join(snaps, "daily")
+	stderr := scanOK(t, daily)
+
+	warning := "walk " + daily + ": skipping operand inside a .zfs directory\n"
+	summary := "scan: 0 files seen (0 added, 0 updated, 0 removed, " +
+		"0 unchanged), 1 skipped\n"
+
+	if !strings.Contains(stderr, warning) ||
+		!strings.Contains(stderr, summary) {
+		t.Errorf("stderr = %q, want %q and %q", stderr, warning, summary)
+	}
+}
+
+// TestRunScanWalksOperandThroughSymlink scans data/daily, where data is
+// a symlink to a directory outside any .zfs directory. The operand is
+// scanned as usual, and its file is recorded by the path given.
+func TestRunScanWalksOperandThroughSymlink(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := t.TempDir()
+	writeFile(t, dir, "pool/data/daily/f", pattern(1, 10))
+
+	data := filepath.Join(dir, "data")
+
+	err := os.Symlink(filepath.Join(dir, "pool", "data"), data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	daily := filepath.Join(data, "daily")
+	stderr := scanOK(t, daily)
+
+	summary := "scan: 1 files seen (1 added, 0 updated, 0 removed, " +
+		"0 unchanged), 0 skipped\n"
+	if !strings.Contains(stderr, summary) {
+		t.Errorf("stderr = %q, want %q", stderr, summary)
+	}
+
+	db, err := openDB(path, reportParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	recordByPath(t, dbRecords(t, db), filepath.Join(daily, "f"))
+}
+
 func TestRunReportSucceeds(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)

scan.go

@@ -48,11 +48,6 @@ const sampleWindow = 1024 * 1024
 // and hash worker pools.
 const workQueueDepth = 1024
 
-// zfsDirName is the name of the directory in which ZFS shows a
-// filesystem's snapshots. The walk never enters one, since it would
-// list every file once per snapshot.
-const zfsDirName = ".zfs"
-
 // errInterrupted reports a scan stopped by SIGINT or SIGTERM. runScan
 // has already printed its line, so run prints nothing more.
 var errInterrupted = errors.New("scan interrupted")
@@ -277,10 +272,9 @@ func pruneRoots(roots []string, oneFS bool, rules walkRules,
 }
 
 // walkReaches reports whether the walk of operand above reaches r, an
-// operand under it. That walk never follows a symlink. It does not
-// enter a directory named .zfs, nor a default exclude in rules, nor a
-// directory that device puts on a filesystem in rules that is not
-// persistent, nor, with -x, one that device puts on a different
+// operand under it. That walk does not enter a default exclude in
+// rules, nor a directory that device puts on a filesystem in rules that
+// is not persistent, nor, with -x, one that device puts on a different
 // filesystem than above, when device knows above's. It reaches every
 // regular file in a directory it enters, so for a regular-file operand
 // the directories checked start at its directory; they end below above.
@@ -301,15 +295,6 @@ func walkReaches(above, r string, oneFS bool, rules walkRules,
 			return false
 		}
 
-		if filepath.Base(dir) == zfsDirName {
-			return false
-		}
-
-		fi, err = os.Lstat(dir)
-		if err == nil && fi.Mode()&fs.ModeSymlink != 0 {
-			return false
-		}
-
 		dev, ok := device(dir)
 		if !ok {
 			continue
@@ -398,7 +383,8 @@ func (s *scanState) runPhases(ctx context.Context, roots []string,
 ) error {
 	// Types and filesystems are checked before pruning so that an
 	// operand under a dropped one is still scanned, not dropped as lying
-	// under it.
+	// under it, and so that an operand inside a .zfs directory gets its
+	// warning even when it lies under another operand.
 	roots = pruneRoots(s.walkableRoots(roots, rules), oneFS, rules,
 		deviceOfPath)
 
@@ -426,15 +412,15 @@ 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
-// had not been given: those under no other root count as outside the
-// roots and are not deleted as unverified. An operand on a filesystem
-// in rules that is not persistent is dropped the same way, with a line
-// saying so that is not a warning, and is not counted as skipped. An
-// operand that fails lstat here is kept, and the walk warns about it
-// unless it no longer exists.
+// files, and directories not named .zfs, none of them inside a
+// directory 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 had not been given:
+// those under no other root count as outside the roots and are not
+// deleted as unverified. An operand on a filesystem in rules that is not
+// persistent is dropped the same way, with a line saying so that is not
+// a warning, and is not counted as skipped. An operand that fails lstat
+// here is kept, and the walk warns about it unless it no longer exists.
 func (s *scanState) walkableRoots(roots []string, rules walkRules) []string {
 	kept := make([]string, 0, len(roots))
 
@@ -889,16 +875,31 @@ func sendEvent(ctx context.Context, events chan<- walkEvent,
 
 // operandWarning returns the one-line warning for an operand the walk
 // does not start from, naming the path and what it is, or "" for one it
-// does: a regular file, or a directory not named .zfs. Symlinks are
-// never followed, including as operands.
+// does: a regular file, or a directory not named .zfs, that is not
+// inside a directory named .zfs, neither as given nor with its symlinks
+// resolved. Symlinks are never followed, including as operands.
 func operandWarning(root string, fi fs.FileInfo) string {
+	// The walk never enters a .zfs directory, so it starts from nothing
+	// inside one either, not even through a symlink. An operand whose
+	// symlinks cannot be resolved is checked as given.
+	resolved, err := filepath.EvalSymlinks(root)
+	if err != nil {
+		resolved = root
+	}
+
+	if slices.Contains(strings.Split(filepath.Dir(root), "/"), ".zfs") ||
+		slices.Contains(strings.Split(filepath.Dir(resolved), "/"), ".zfs") {
+		return fmt.Sprintf("walk %s: skipping operand inside a .zfs directory",
+			root)
+	}
+
 	var kind string
 
 	switch mode := fi.Mode(); {
 	case mode.IsRegular():
 		return ""
 	case mode.IsDir():
-		if filepath.Base(root) != zfsDirName {
+		if filepath.Base(root) != ".zfs" {
 			return ""
 		}
 
@@ -1168,7 +1169,7 @@ func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	parent dirJob, oneFS bool, rules walkRules,
 	events chan<- walkEvent,
 ) (dirJob, bool) {
-	if e.Name() == zfsDirName {
+	if e.Name() == ".zfs" {
 		return dirJob{}, false
 	}
 

scan_test.go

@@ -1672,81 +1672,6 @@ func TestSyncScanOverlappingRoots(t *testing.T) {
 	}
 }
 
-// TestScanWalksOperandInZFSDirectory checks that an operand inside a
-// directory named .zfs below another operand is walked on its own,
-// since the walk of the other operand does not enter .zfs. The file
-// under it is recorded, and a second scan keeps the record.
-func TestScanWalksOperandInZFSDirectory(t *testing.T) {
-	t.Parallel()
-
-	dir := t.TempDir()
-	db := openTestDB(t)
-	f := writeFile(t, dir, "pool/.zfs/snapshot/daily/f", pattern(1, 10))
-	daily := filepath.Dir(f)
-
-	syncTree(t, db, dir, daily)
-	recordByPath(t, dbRecords(t, db), f)
-
-	syncTree(t, db, dir, daily)
-	recordByPath(t, dbRecords(t, db), f)
-}
-
-// TestScanWalksOperandBehindSymlink checks that an operand reached
-// through a symlinked directory below another operand is walked on its
-// own, since the walk of the other operand does not follow the symlink.
-// The file under it is recorded, and a second scan keeps the record.
-func TestScanWalksOperandBehindSymlink(t *testing.T) {
-	t.Parallel()
-
-	dir := t.TempDir()
-	target := t.TempDir()
-	db := openTestDB(t)
-	writeFile(t, target, "sub/f", pattern(1, 10))
-
-	link := filepath.Join(dir, "link")
-
-	err := os.Symlink(target, link)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	sub := filepath.Join(link, "sub")
-	f := filepath.Join(sub, "f")
-
-	syncTree(t, db, dir, sub)
-	recordByPath(t, dbRecords(t, db), f)
-
-	syncTree(t, db, dir, sub)
-	recordByPath(t, dbRecords(t, db), f)
-}
-
-// TestScanDropsOperandReachedBehindSymlink checks that an operand under
-// another one is still dropped when both lie behind the same symlinked
-// directory: the walk of the other operand starts past the symlink and
-// reaches it, so the file under it is walked once.
-func TestScanDropsOperandReachedBehindSymlink(t *testing.T) {
-	t.Parallel()
-
-	dir := t.TempDir()
-	target := t.TempDir()
-	db := openTestDB(t)
-	writeFile(t, target, "sub/deeper/f", pattern(1, 10))
-
-	link := filepath.Join(dir, "link")
-
-	err := os.Symlink(target, link)
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	sub := filepath.Join(link, "sub")
-
-	st := syncTree(t, db, sub, filepath.Join(sub, "deeper"))
-	if st != (scanStats{walked: 1, added: 1}) {
-		t.Fatalf("stats = %+v, want 1 walked and added", st)
-	}
-}
-
 // TestScanHashesUniqueSize checks that a file whose size no other file
 // shares is hashed like every other file.
 func TestScanHashesUniqueSize(t *testing.T) {
@@ -2160,16 +2085,6 @@ func TestPruneRoots(t *testing.T) {
 		t.Fatalf("pruneRoots with %s not persistent = %q, want %q",
 			pqRoot, got, want)
 	}
-
-	// Nor does it enter a directory named .zfs, so /p/.zfs/s is kept. The
-	// walk of /p/.zfs/s reaches /p/.zfs/s/t, which is dropped.
-	got = pruneRoots([]string{"/p", "/p/.zfs/s", "/p/.zfs/s/t"}, false,
-		walkRules{}, device)
-
-	want = []string{"/p", "/p/.zfs/s"}
-	if !slices.Equal(got, want) {
-		t.Fatalf("pruneRoots with .zfs = %q, want %q", got, want)
-	}
 }
 
 func TestReportsNeverTouchFilesystem(t *testing.T) {