Fix subxact release crash

src/backend/distributed/connection/remote_commands.c

@@ -1063,9 +1063,16 @@ bool
 SendCancelationRequest(MultiConnection *connection)
 {
 	char errorBuffer[ERROR_BUFFER_SIZE] = { 0 };
-	PGcancel *cancelObject = PQgetCancel(connection->pgConn);
+	bool cancelSent = false;
 
-	bool cancelSent = PQcancel(cancelObject, errorBuffer, sizeof(errorBuffer));
+	PGcancel *cancelObject = PQgetCancel(connection->pgConn);
+	if (cancelObject == NULL)
+	{
+		/* this can happen if connection is invalid */
+		return false;
+	}
+
+	cancelSent = PQcancel(cancelObject, errorBuffer, sizeof(errorBuffer));
 	if (!cancelSent)
 	{
 		ereport(WARNING, (errmsg("could not issue cancel request"),

src/backend/distributed/transaction/transaction_management.c

@@ -396,6 +396,16 @@ CoordinatedSubTransactionCallback(SubXactEvent event, SubTransactionId subId,
 {
 	switch (event)
 	{
+		/*
+		 * Our subtransaction stack should be consistent with postgres' internal
+		 * transaction stack. In case of subxact begin, postgres calls our
+		 * callback after it has pushed the transaction into stack, so we have to
+		 * do the same even if worker commands fail, so we PushSubXact() first.
+		 * In case of subxact commit, callback is called before pushing subxact to
+		 * the postgres transaction stack, so we call PopSubXact() after making sure
+		 * worker commands didn't fail. Otherwise, Postgres would roll back that
+		 * would cause us to call PopSubXact again.
+		 */
 		case SUBXACT_EVENT_START_SUB:
 		{
 			PushSubXact(subId);
@@ -408,21 +418,21 @@ CoordinatedSubTransactionCallback(SubXactEvent event, SubTransactionId subId,
 
 		case SUBXACT_EVENT_COMMIT_SUB:
 		{
-			PopSubXact(subId);
 			if (InCoordinatedTransaction())
 			{
 				CoordinatedRemoteTransactionsSavepointRelease(subId);
 			}
+			PopSubXact(subId);
 			break;
 		}
 
 		case SUBXACT_EVENT_ABORT_SUB:
 		{
-			PopSubXact(subId);
 			if (InCoordinatedTransaction())
 			{
 				CoordinatedRemoteTransactionsSavepointRollback(subId);
 			}
+			PopSubXact(subId);
 
 			UnsetCitusNoticeLevel();
 			break;

src/test/regress/bin/normalize.sed

@@ -15,3 +15,6 @@ s/shard [0-9]+/shard xxxxx/g
 # the generated plan
 s/"(fkey_ref_|referenced_table_|referencing_table_)[0-9]+"/"\1xxxxxxx"/g
 s/\(id\)=\([0-9]+\)/(id)=(X)/g
+
+# Savepoint error messages changed between postgres 10 and 11.
+s/savepoint ".*" does not exist/no such savepoint/g

src/test/regress/bin/normalized_tests.lst

@@ -2,3 +2,4 @@
 multi_alter_table_add_constraints
 multi_alter_table_statements
 foreign_key_to_reference_table
+failure_savepoints

src/test/regress/expected/failure_savepoints.out

@@ -38,8 +38,6 @@ CONTEXT:  while executing command on localhost:9060
 WARNING:  connection not open
 CONTEXT:  while executing command on localhost:9060
 DELETE FROM artists WHERE id=4;
-WARNING:  could not issue cancel request
-DETAIL:  Client error: PQcancel() -- no cancel object supplied
 WARNING:  connection not open
 CONTEXT:  while executing command on localhost:9060
 WARNING:  connection error: localhost:9060
@@ -66,16 +64,24 @@ SELECT citus.mitmproxy('conn.onQuery(query="^RELEASE").kill()');
 (1 row)
 
 BEGIN;
-INSERT INTO artists VALUES (5, 'Asher Lev');
+UPDATE artists SET name='a';
 SAVEPOINT s1;
 DELETE FROM artists WHERE id=4;
 RELEASE SAVEPOINT s1;
+WARNING:  AbortSubTransaction while in COMMIT state
+WARNING:  connection not open
+CONTEXT:  while executing command on localhost:9060
+WARNING:  connection error: localhost:9060
+DETAIL:  connection not open
 WARNING:  connection not open
 CONTEXT:  while executing command on localhost:9060
 WARNING:  connection not open
 CONTEXT:  while executing command on localhost:9060
-COMMIT;
-ERROR:  could not make changes to shard 100950 on any node
+WARNING:  savepoint "savepoint_2" does not exist
+CONTEXT:  while executing command on localhost:57637
+ERROR:  connection not open
+CONTEXT:  while executing command on localhost:9060
+ROLLBACK;
 SELECT * FROM artists WHERE id IN (4, 5);
  id |      name       
 ----+-----------------
@@ -197,7 +203,6 @@ INSERT INTO artists VALUES (7, 'Emily Carr');
 ROLLBACK TO SAVEPOINT s1;
 WARNING:  connection not open
 WARNING:  connection not open
-WARNING:  could not issue cancel request
 COMMIT;
 ERROR:  could not make changes to shard 100950 on any node
 SELECT * FROM artists WHERE id=6;
@@ -233,7 +238,6 @@ WARNING:  connection not open
 WARNING:  connection not open
 INSERT INTO researchers VALUES (8, 4, 'Alonzo Church');
 ROLLBACK TO s1;
-WARNING:  could not issue cancel request
 WARNING:  connection not open
 WARNING:  connection error: localhost:9060
 WARNING:  connection not open

src/test/regress/sql/failure_savepoints.sql

@@ -33,11 +33,11 @@ SELECT * FROM artists WHERE id IN (4, 5);
 SELECT citus.mitmproxy('conn.onQuery(query="^RELEASE").kill()');
 
 BEGIN;
-INSERT INTO artists VALUES (5, 'Asher Lev');
+UPDATE artists SET name='a';
 SAVEPOINT s1;
 DELETE FROM artists WHERE id=4;
 RELEASE SAVEPOINT s1;
-COMMIT;
+ROLLBACK;
 
 SELECT * FROM artists WHERE id IN (4, 5);