Warn about and skip non-regular and .zfs operands, keeping their records (closes #9)

A symlink, socket, FIFO or device-node operand, or a directory operand
named .zfs, was silently ignored yet stayed in the scanned operands, so
the update phase deleted every record stored beneath it. Such an
operand now gets a one-line warning, counts as skipped, and is dropped
before overlapping operands are pruned and the database index is
loaded: another operand beneath it is still scanned, and the records
beneath it count as outside the scanned operands and are not deleted,
unless it lies under another operand. The exit status stays 0. An
operand that turns into one of these after that check is warned about
and skipped by the walk instead. README "scan mode" and "Rules for the
walk" say so.

Model: opus-5-5

README.md

@@ -267,13 +267,13 @@ 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. 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.
+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.
 
 `scan` synchronizes the database with the filesystem state under the
 scanned operands:

main_test.go

@@ -423,6 +423,34 @@ func TestRunScanSkipsSymlinkOperand(t *testing.T) {
 		filepath.Join(link, "sub", "f"))
 }
 
+func TestRunScanWalksOperandUnderSymlinkOperand(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := t.TempDir()
+	writeFile(t, dir, "target/sub/f", pattern(1, 10))
+
+	link := filepath.Join(dir, "link")
+
+	err := os.Symlink(filepath.Join(dir, "target"), link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// link is dropped as a symlink, but link/sub must still be scanned,
+	// not dropped as lying under link.
+	scanOK(t, link, filepath.Join(link, "sub"))
+
+	db, err := openDB(path, reportParams)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Cleanup(func() { _ = db.Close() })
+
+	recordByPath(t, dbRecords(t, db), filepath.Join(link, "sub", "f"))
+}
+
 func TestRunScanSkipsZFSOperand(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)

scan.go

@@ -219,6 +219,8 @@ func syncScan(ctx context.Context, db *sql.DB, roots []string,
 ) (scanStats, error) {
 	s := &scanState{db: db}
 
+	// Types are checked before pruning so that an operand under a
+	// dropped one is still scanned, not dropped as lying under it.
 	roots = pruneRoots(s.walkableRoots(roots))
 
 	err := s.loadIndex(ctx, roots)