Fix snapshot generation of deleted database

cmd/litefs/mount_test.go

@@ -447,6 +447,45 @@ func TestSingleNode_BackupClient(t *testing.T) {
 		waitForBackupSync(t, cmd0)
 	})
 
+	// Ensure LiteFS can send a deleted database to backup.
+	t.Run("InitiateSnapshot/Empty", func(t *testing.T) {
+		cmd0 := newMountCommand(t, t.TempDir(), nil)
+		cmd0.Config.Data.Retention = 1 * time.Microsecond
+		cmd0.Config.Backup = main.BackupConfig{Type: "file", Path: t.TempDir(), Delay: 1 * time.Millisecond}
+		runMountCommand(t, cmd0)
+
+		// Create a simple table and delete the database.
+		db := testingutil.OpenSQLDB(t, filepath.Join(cmd0.Config.FUSE.Dir, "db"))
+		if _, err := db.Exec(`CREATE TABLE t (x)`); err != nil {
+			t.Fatal(err)
+		} else if err := db.Close(); err != nil {
+			t.Fatal(err)
+		} else if err := os.Remove(filepath.Join(cmd0.Config.FUSE.Dir, "db")); err != nil {
+			t.Fatal(err)
+		}
+
+		// Enforce retention to remove initial LTX file.
+		time.Sleep(cmd0.Config.Data.Retention)
+		if err := cmd0.Store.EnforceRetention(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		// Sync to backup.
+		if err := cmd0.Store.SyncBackup(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+
+		// Recreate database
+		db = testingutil.OpenSQLDB(t, filepath.Join(cmd0.Config.FUSE.Dir, "db"))
+		if _, err := db.Exec(`CREATE TABLE v (y)`); err != nil {
+			t.Fatal(err)
+		}
+		if err := cmd0.Store.SyncBackup(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+		waitForBackupSync(t, cmd0)
+	})
+
 	// Ensure LiteFS can restore a database from backup if it doesn't exist locally.
 	t.Run("RestoreFromBackup/NoLocalDatabase", func(t *testing.T) {
 		backupDir := t.TempDir()

db.go

@@ -3380,12 +3380,13 @@ func (db *DB) WriteSnapshotTo(ctx context.Context, dst io.Writer) (header ltx.He
 	// Log transaction ID for the snapshot.
 	log.Printf("writing snapshot %q @ %s", db.name, pos.TXID.String())
 
-	// Open database file.
-	dbFile, err := db.os.Open("WRITESNAPSHOT:DB", db.DatabasePath())
-	if err != nil {
+	// Open database file. File may not exist if the database has been deleted.
+	var dbFile *os.File
+	if dbFile, err = db.os.Open("WRITESNAPSHOT:DB", db.DatabasePath()); err != nil && !os.IsNotExist(err) {
 		return header, trailer, fmt.Errorf("open database file: %w", err)
+	} else if err == nil {
+		defer func() { _ = dbFile.Close() }()
 	}
-	defer func() { _ = dbFile.Close() }()
 
 	// Open WAL file if we have overriding WAL frames.
 	var walFile *os.File