postgres_fdw: Fix connection leak.

In postgres_fdw, the cached connections to foreign servers will not be
closed until the local session exits if the user mappings or foreign servers
that those connections depend on are dropped. Those connections can be
leaked.

To fix that connection leak issue, after a change to a pg_foreign_server
or pg_user_mapping catalog entry, this commit makes postgres_fdw close
the connections depending on that entry immediately if current
transaction has not used those connections yet. Otherwise, mark those
connections as invalid and then close them at the end of current transaction,
since they cannot be closed in the midst of the transaction using them.
Closed connections will be remade at the next opportunity if necessary.

Back-patch to all supported branches.

Author: Bharath Rupireddy
Reviewed-by: Zhihong Yu, Zhijie Hou, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACVNcGH_6qLY-4_tXz8JLvA+4yeBThRfxMz7Oxbk1aHcpQ@mail.gmail.com

contrib/postgres_fdw/connection.c

@@ -833,12 +833,14 @@ pgfdw_xact_callback(XactEvent event, void *arg)
 		entry->xact_depth = 0;
 
 		/*
-		 * If the connection isn't in a good idle state, discard it to
-		 * recover. Next GetConnection will open a new connection.
+		 * If the connection isn't in a good idle state or it is marked as
+		 * invalid, then discard it to recover. Next GetConnection will open a
+		 * new connection.
 		 */
 		if (PQstatus(entry->conn) != CONNECTION_OK ||
 			PQtransactionStatus(entry->conn) != PQTRANS_IDLE ||
-			entry->changing_xact_state)
+			entry->changing_xact_state ||
+			entry->invalidated)
 		{
 			elog(DEBUG3, "discarding connection %p", entry->conn);
 			disconnect_pg_server(entry);
@@ -962,9 +964,12 @@ pgfdw_subxact_callback(SubXactEvent event, SubTransactionId mySubid,
  * Connection invalidation callback function
  *
  * After a change to a pg_foreign_server or pg_user_mapping catalog entry,
- * mark connections depending on that entry as needing to be remade.
- * We can't immediately destroy them, since they might be in the midst of
- * a transaction, but we'll remake them at the next opportunity.
+ * close connections depending on that entry immediately if current transaction
+ * has not used those connections yet. Otherwise, mark those connections as
+ * invalid and then make pgfdw_xact_callback() close them at the end of current
+ * transaction, since they cannot be closed in the midst of the transaction
+ * using them. Closed connections will be remade at the next opportunity if
+ * necessary.
  *
  * Although most cache invalidation callbacks blow away all the related stuff
  * regardless of the given hashvalue, connections are expensive enough that
@@ -995,7 +1000,21 @@ pgfdw_inval_callback(Datum arg, int cacheid, uint32 hashvalue)
 			 entry->server_hashvalue == hashvalue) ||
 			(cacheid == USERMAPPINGOID &&
 			 entry->mapping_hashvalue == hashvalue))
-			entry->invalidated = true;
+		{
+			/*
+			 * Close the connection immediately if it's not used yet in this
+			 * transaction. Otherwise mark it as invalid so that
+			 * pgfdw_xact_callback() can close it at the end of this
+			 * transaction.
+			 */
+			if (entry->xact_depth == 0)
+			{
+				elog(DEBUG3, "discarding connection %p", entry->conn);
+				disconnect_pg_server(entry);
+			}
+			else
+				entry->invalidated = true;
+		}
 	}
 }
 

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -3968,3 +3968,21 @@ QUERY:  CREATE FOREIGN TABLE t5 (
 OPTIONS (schema_name 'import_source', table_name 't5');
 CONTEXT:  importing foreign table "t5"
 ROLLBACK;
+-- ===================================================================
+-- test connection invalidation cases
+-- ===================================================================
+-- This test case is for closing the connection in pgfdw_xact_callback
+BEGIN;
+-- Connection xact depth becomes 1 i.e. the connection is in midst of the xact.
+SELECT 1 FROM ft1 LIMIT 1;
+ ?column? 
+----------
+        1
+(1 row)
+
+-- Connection is not closed at the end of the alter statement in
+-- pgfdw_inval_callback. That's because the connection is in midst of this
+-- xact, it is just marked as invalid.
+ALTER SERVER loopback OPTIONS (ADD use_remote_estimate 'off');
+-- The invalid connection gets closed in pgfdw_xact_callback during commit.
+COMMIT;

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -935,3 +935,17 @@ DROP TYPE "Colors" CASCADE;
 IMPORT FOREIGN SCHEMA import_source LIMIT TO (t5)
   FROM SERVER loopback INTO import_dest5;  -- ERROR
 ROLLBACK;
+
+-- ===================================================================
+-- test connection invalidation cases
+-- ===================================================================
+-- This test case is for closing the connection in pgfdw_xact_callback
+BEGIN;
+-- Connection xact depth becomes 1 i.e. the connection is in midst of the xact.
+SELECT 1 FROM ft1 LIMIT 1;
+-- Connection is not closed at the end of the alter statement in
+-- pgfdw_inval_callback. That's because the connection is in midst of this
+-- xact, it is just marked as invalid.
+ALTER SERVER loopback OPTIONS (ADD use_remote_estimate 'off');
+-- The invalid connection gets closed in pgfdw_xact_callback during commit.
+COMMIT;