Do not always start a db txn on Postgres

changelog.d/14840.misc

@@ -0,0 +1 @@
+Prevent "WARNING:  there is already a transaction in progress" lines appearing in PostgreSQL's logs on some occasions.

synapse/storage/prepare_database.py

@@ -23,7 +23,7 @@
 
 from synapse.config.homeserver import HomeServerConfig
 from synapse.storage.database import LoggingDatabaseConnection
-from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine
+from synapse.storage.engines import BaseDatabaseEngine, PostgresEngine, Sqlite3Engine
 from synapse.storage.schema import SCHEMA_COMPAT_VERSION, SCHEMA_VERSION
 from synapse.storage.types import Cursor
 
@@ -108,9 +108,14 @@ def prepare_database(
         # so we start one before running anything. This ensures that any upgrades
         # are either applied completely, or not at all.
         #
-        # (psycopg2 automatically starts a transaction as soon as we run any statements
-        # at all, so this is redundant but harmless there.)
-        cur.execute("BEGIN TRANSACTION")
+        # psycopg2 does not automatically start transactions when in autocommit mode.
+        # While it is technically harmless to nest transactions in postgres, doing so
+        # results in a warning in Postgres' logs per query. And we'd rather like to
+        # avoid doing that.
+        if isinstance(database_engine, Sqlite3Engine) or (
+            isinstance(database_engine, PostgresEngine) and db_conn.autocommit
+        ):
+            cur.execute("BEGIN TRANSACTION")
 
         logger.info("%r: Checking existing schema version", databases)
         version_info = _get_or_create_schema_state(cur, database_engine)