Add rbf.DB.TxN() function and test check

rbf/db.go

@@ -100,6 +100,13 @@ func CreateDirIfNotExist(path string) {
 	}
 }
 
+// TxN returns the number of active transactions.
+func (db *DB) TxN() int {
+	db.mu.RLock()
+	defer db.mu.RUnlock()
+	return len(db.txs)
+}
+
 // Open opens a database with the file specified in Path.
 // Creates a new file if one does not already exist.
 func (db *DB) Open() (err error) {

rbf/rbf_test.go

@@ -86,6 +86,8 @@ func MustCloseDB(tb testing.TB, db *rbf.DB) {
 	tb.Helper()
 	if err := db.Check(); err != nil && err != rbf.ErrClosed {
 		tb.Fatal(err)
+	} else if n := db.TxN(); n != 0 {
+		tb.Fatalf("db still has %d active transactions; must closed before closing db", n)
 	} else if err := db.Close(); err != nil && err != rbf.ErrClosed {
 		tb.Fatal(err)
 	} else if err := os.RemoveAll(db.Path); err != nil {