fixup: make postgres behaviour consistent with sqlite

synapse/storage/engines/_base.py

@@ -133,9 +133,9 @@ def executescript(cursor: CursorType, script: str) -> None:
 
         This is not provided by DBAPI2, and so needs engine-specific support.
 
-        Some database engines may automatically COMMIT the ongoing transaction before
-        executing the script in its own transaction. The script transaction is left
-        open and it is the responsibility of the caller to commit it.
+        Any ongoing transaction is committed before executing the script in its own
+        transaction. The script transaction is left open and it is the responsibility of
+        the caller to commit it.
         """
         ...
 

synapse/storage/engines/postgres.py

@@ -220,5 +220,9 @@ def executescript(cursor: psycopg2.extensions.cursor, script: str) -> None:
         """Execute a chunk of SQL containing multiple semicolon-delimited statements.
 
         Psycopg2 seems happy to do this in DBAPI2's `execute()` function.
+
+        For consistency with SQLite, any ongoing transaction is committed before
+        executing the script in its own transaction. The script transaction is
+        left open and it is the responsibility of the caller to commit it.
         """
-        cursor.execute(script)
+        cursor.execute(f"COMMIT; BEGIN TRANSACTION; {script}")