GIN reindex: Fix syntax errors, improve exception handling

synapse/storage/search.py

@@ -157,29 +157,43 @@ class SearchStore(BackgroundUpdateStore):
 
     @defer.inlineCallbacks
     def _background_reindex_gin_search(self, progress, batch_size):
-        '''This handles old synapses which used GIST indexes, if any;
+        """This handles old synapses which used GIST indexes, if any;
         converting them back to be GIN as per the actual schema.
-        '''
+        """
 
         def create_index(conn):
-            try:
             conn.rollback()
+
+            # we have to set autocommit, because postgres refuses to
+            # CREATE INDEX CONCURRENTLY without it.
             conn.set_session(autocommit=True)
+
+            try:
                 c = conn.cursor()
 
+                # if we skipped the conversion to GIST, we may already/still
+                # have an event_search_fts_idx; unfortunately postgres 9.4
+                # doesn't support CREATE INDEX IF EXISTS so we just catch the
+                # exception and ignore it.
+                import psycopg2
+                try:
                     c.execute(
                         "CREATE INDEX CONCURRENTLY event_search_fts_idx"
                         " ON event_search USING GIN (vector)"
                     )
-
+                except psycopg2.ProgrammingError as e:
-                c.execute("DROP INDEX event_search_fts_idx_gist")
-
-                conn.set_session(autocommit=False)
-            except e:
                     logger.warn(
-                    "Ignoring error %s when trying to switch from GIST to GIN" % (e,)
+                        "Ignoring error %r when trying to switch from GIST to GIN",
+                        e
                     )
 
+                # we should now be able to delete the GIST index.
+                c.execute(
+                    "DROP INDEX IF EXISTS event_search_fts_idx_gist"
+                )
+            finally:
+                conn.set_session(autocommit=False)
+
         if isinstance(self.database_engine, PostgresEngine):
             yield self.runWithConnection(create_index)