Use transactional isolation only if available

syncapi/storage/postgres/syncserver.go

@@ -80,6 +80,7 @@ func NewDatabase(dbProperties *config.DatabaseOptions) (*SyncServerDatasource, e
 	}
 	d.Database = shared.Database{
 		DB:                     d.db,
+		TransactionalIsolation: true,
 		Writer:                 d.writer,
 		Invites:                invites,
 		AccountData:            accountData,

syncapi/storage/shared/syncserver.go

@@ -37,6 +37,7 @@ import (
 // For now this contains the shared functions
 type Database struct {
 	DB                     *sql.DB
+	TransactionalIsolation bool
 	Writer                 sqlutil.Writer
 	Invites                tables.Invites
 	AccountData            tables.AccountData
@@ -49,6 +50,19 @@ type Database struct {
 	EDUCache               *cache.EDUCache
 }
 
+func (d *Database) getIsolatedTransactionIfAvailable(ctx context.Context, succeeded *bool) (txn *sql.Tx, done func(), err error) {
+	if d.TransactionalIsolation {
+		txn, err = d.DB.BeginTx(ctx, &txReadOnlySnapshot)
+		if err != nil {
+			return nil, nil, err
+		}
+		done = func() {
+			sqlutil.EndTransactionWithCheck(txn, succeeded, &err)
+		}
+	}
+	return
+}
+
 // Events lookups a list of event by their event ID.
 // Returns a list of events matching the requested IDs found in the database.
 // If an event is not found in the database then it will be omitted from the list.
@@ -422,17 +436,14 @@ func (d *Database) addPDUDeltaToResponse(
 	wantFullState bool,
 	res *types.Response,
 ) (joinedRoomIDs []string, err error) {
-	txn, err := d.DB.BeginTx(ctx, &txReadOnlySnapshot)
-	if err != nil {
-		return nil, err
-	}
 	succeeded := false
-	defer func() {
-		_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
-			sqlutil.EndTransactionWithCheck(txn, &succeeded, &err)
-			return nil
-		})
-	}()
+	txn, done, err := d.getIsolatedTransactionIfAvailable(ctx, &succeeded)
+	if err != nil {
+		return nil, fmt.Errorf("getIsolatedTransactionIfAvailable: %w", err)
+	}
+	if txn != nil {
+		defer done()
+	}
 
 	stateFilter := gomatrixserverlib.DefaultStateFilter() // TODO: use filter provided in request
 
@@ -620,17 +631,14 @@ func (d *Database) getResponseWithPDUsForCompleteSync(
 	// a consistent view of the database throughout. This includes extracting the sync position.
 	// This does have the unfortunate side-effect that all the matrixy logic resides in this function,
 	// but it's better to not hide the fact that this is being done in a transaction.
-	txn, err := d.DB.BeginTx(ctx, &txReadOnlySnapshot)
+	succeeded := false
+	txn, done, err := d.getIsolatedTransactionIfAvailable(ctx, &succeeded)
 	if err != nil {
 		return
 	}
-	succeeded := false
-	defer func() {
-		_ = d.Writer.Do(d.DB, txn, func(txn *sql.Tx) error {
-			sqlutil.EndTransactionWithCheck(txn, &succeeded, &err)
-			return nil
-		})
-	}()
+	if txn != nil {
+		defer done()
+	}
 
 	// Get the current sync position which we will base the sync response on.
 	toPos, err = d.syncPositionTx(ctx, txn)

syncapi/storage/sqlite3/syncserver.go

@@ -93,6 +93,7 @@ func (d *SyncServerDatasource) prepare() (err error) {
 	}
 	d.Database = shared.Database{
 		DB:                     d.db,
+		TransactionalIsolation: false,
 		Writer:                 d.writer,
 		Invites:                invites,
 		AccountData:            accountData,