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

diff --git a/contrib/postgres_fdw/connection.c b/contrib/postgres_fdw/connection.c
index 1f49402d1f4fce0830283593a2965fa536265459..512bdfb07c2961206d5941aaf33fecd0cb744e8d 100644 (file)
--- a/contrib/postgres_fdw/connection.c
+++ b/contrib/postgres_fdw/connection.c
@@ -868,12 +868,14 @@ 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);
@@ -997,9 +999,12 @@ 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
@@ -1030,7 +1035,21 @@ 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;
+       }
    }
 }
 

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 90db550b92140ad9216b1f662bd288e699975574..747ab40d2f52c5fe716802eac061ab6b2feff127 100644 (file)
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -8974,3 +8974,21 @@ PREPARE TRANSACTION 'fdw_tpc';
 ERROR:  cannot PREPARE a transaction that has operated on postgres_fdw foreign tables
 ROLLBACK;
 WARNING:  there is no transaction in progress
+-- ===================================================================
+-- 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;

diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 83971665e3519c39e1bd2126b1e4b7f948d7f0c9..29c61e7ff45b602677098aa67aa9a4d805e62322 100644 (file)
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -2634,3 +2634,17 @@ SELECT count(*) FROM ft1;
 -- error here
 PREPARE TRANSACTION 'fdw_tpc';
 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;