Fix RBF root record cache build

This commit fixes an issue where the root record cache is only
built when a write transaction successfully commits. However, if
no write transactions are occurring then the the cache is never
built and saved so it is recomputed on every read tx.

rbf/db.go

@@ -496,6 +496,17 @@ func (db *DB) Begin(writable bool) (_ *Tx, err error) {
 	// Track transaction with the DB.
 	db.txs[tx] = struct{}{}
 
+	// If no root records are cached, build the cache the first time.
+	// Normally the cache is updated by successful write transactions but
+	// this avoids recomputing the cache if there are no write txs for a while.
+	if db.rootRecords == nil {
+		if db.rootRecords, err = tx.RootRecords(); err != nil {
+			db.mu.Unlock()
+			tx.Rollback()
+			return nil, err
+		}
+	}
+
 	db.mu.Unlock()
 	return tx, nil
 }

rbf/tx.go

@@ -1217,7 +1217,9 @@ func (tx *Tx) Count(name string) (uint64, error) {
 	}
 	defer c.Close()
 
-	if err := c.First(); err != nil {
+	if err := c.First(); err == io.EOF {
+		return 0, nil
+	} else if err != nil {
 		return 0, err
 	}
 

rbf/tx_test.go

@@ -95,6 +95,30 @@ func TestTx_CommitRollback(t *testing.T) {
 		}
 	})
 
+	t.Run("ReopenReadOnly", func(t *testing.T) {
+		db := MustOpenDB(t)
+		defer func() { MustCloseDB(t, db) }()
+
+		// Create bitmap in transaction and commit.
+		if tx, err := db.Begin(true); err != nil {
+			t.Fatal(err)
+		} else if err := tx.CreateBitmap("x"); err != nil {
+			t.Fatal(err)
+		} else if err := tx.Commit(); err != nil {
+			t.Fatal(err)
+		}
+		db = MustReopenDB(t, db)
+
+		// Create bitmap again but it should fail as it already exists.
+		if tx, err := db.Begin(false); err != nil {
+			t.Fatal(err)
+		} else if _, err := tx.Count("x"); err != nil {
+			t.Fatal(err)
+		} else if err := tx.Commit(); err != nil {
+			t.Fatal(err)
+		}
+	})
+
 	t.Run("SingleWriter", func(t *testing.T) {
 		db := MustOpenDB(t)
 		defer MustCloseDB(t, db)