Hash a file again when its mtime moved back, not only forward (closes #176) (#182)

internal/sfdupes/scan_test.go

@@ -1384,6 +1384,47 @@ func TestSyncScanMtimeBump(t *testing.T) {
 	}
 }
 
+// TestSyncScanMtimeMovedBack replaces a file with other content of the
+// same size and an older mtime, as cp -p or a restore from backup does.
+// The next scan must hash the file again.
+func TestSyncScanMtimeMovedBack(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	db := openTestDB(t)
+	a := writeFile(t, dir, "a.bin", pattern(1, 500))
+
+	recorded := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+
+	err := os.Chtimes(a, recorded, recorded)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	syncTree(t, db, dir)
+
+	replaced := pattern(3, 500)
+	writeFile(t, dir, "a.bin", replaced)
+
+	older := recorded.Add(-time.Hour)
+
+	err = os.Chtimes(a, older, older)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	st := syncTree(t, db, dir)
+	if st != (scanStats{walked: 1, updated: 1}) {
+		t.Fatalf("rescan stats = %+v, want 1 updated", st)
+	}
+
+	assertWholeFileHashed(t, db, a, replaced)
+
+	if r := recordByPath(t, dbRecords(t, db), a); !r.mtime.Equal(older) {
+		t.Fatalf("mtime = %v, want %v", r.mtime, older)
+	}
+}
+
 // assertWholeFileHashed fails unless the record for path holds the
 // whole-file hash of data as its head, tail, and content.
 func assertWholeFileHashed(t *testing.T, db *sql.DB, path string,