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 +108,12 @@ 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 automatically start transactions, and while this is technically
+        # harmless to do there as well, doing so results in nested transactions. Those
+        # will generate warnings in Postgres' logs per query, and we'd rather like to
+        # avoid doing that.
+        if isinstance(database_engine, Sqlite3Engine):
+            cur.execute("BEGIN TRANSACTION")
 
         logger.info("%r: Checking existing schema version", databases)
         version_info = _get_or_create_schema_state(cur, database_engine)