Fix savepoint rollback after multi-shard update failure.

src/backend/distributed/connection/connection_management.c

@@ -464,7 +464,7 @@ ShutdownConnection(MultiConnection *connection)
 	if (PQstatus(connection->pgConn) == CONNECTION_OK &&
 		PQtransactionStatus(connection->pgConn) == PQTRANS_ACTIVE)
 	{
-		char errorMessage[256] = { 0 };
+		char errorMessage[ERROR_BUFFER_SIZE] = { 0 };
 		PGcancel *cancel = PQgetCancel(connection->pgConn);
 
 		if (!PQcancel(cancel, errorMessage, sizeof(errorMessage)))

src/backend/distributed/connection/remote_commands.c

@@ -1008,3 +1008,26 @@ BuildWaitEventSet(MultiConnection **allConnections, int totalConnectionCount,
 
 	return waitEventSet;
 }
+
+
+/*
+ * MultiClientCancel sends a cancelation request on the given connection. Return
+ * value indicates whether the cancelation request was sent successfully.
+ */
+bool
+SendCancelationRequest(MultiConnection *connection)
+{
+	char errorBuffer[ERROR_BUFFER_SIZE] = { 0 };
+	PGcancel *cancelObject = PQgetCancel(connection->pgConn);
+
+	bool cancelSent = PQcancel(cancelObject, errorBuffer, sizeof(errorBuffer));
+	if (!cancelSent)
+	{
+		ereport(WARNING, (errmsg("could not issue cancel request"),
+						  errdetail("Client error: %s", errorBuffer)));
+	}
+
+	PQfreeCancel(cancelObject);
+
+	return cancelSent;
+}

src/backend/distributed/executor/multi_client_executor.c

@@ -443,27 +443,13 @@ bool
 MultiClientCancel(int32 connectionId)
 {
 	MultiConnection *connection = NULL;
-	PGcancel *cancelObject = NULL;
-	int cancelSent = 0;
 	bool canceled = true;
-	char errorBuffer[STRING_BUFFER_SIZE];
 
 	Assert(connectionId != INVALID_CONNECTION_ID);
 	connection = ClientConnectionArray[connectionId];
 	Assert(connection != NULL);
 
-	cancelObject = PQgetCancel(connection->pgConn);
+	canceled = SendCancelationRequest(connection);
-
-	cancelSent = PQcancel(cancelObject, errorBuffer, sizeof(errorBuffer));
-	if (cancelSent == 0)
-	{
-		ereport(WARNING, (errmsg("could not issue cancel request"),
-						  errdetail("Client error: %s", errorBuffer)));
-
-		canceled = false;
-	}
-
-	PQfreeCancel(cancelObject);
 
 	return canceled;
 }

src/backend/distributed/executor/multi_router_executor.c

@@ -1574,6 +1574,7 @@ ExecuteModifyTasks(List *taskList, bool expectResults, ParamListInfo paramListIn
 			queryOK = SendQueryInSingleRowMode(connection, queryString, paramListInfo);
 			if (!queryOK)
 			{
+				UnclaimAllShardConnections(shardConnectionHash);
 				ReportConnectionError(connection, ERROR);
 			}
 		}
@@ -1616,6 +1617,8 @@ ExecuteModifyTasks(List *taskList, bool expectResults, ParamListInfo paramListIn
 				SetCitusNoticeLevel(INFO);
 			}
 
+			PG_TRY();
+			{
 				/*
 				 * If caller is interested, store query results the first time
 				 * through. The output of the query's execution on other shards is
@@ -1634,10 +1637,18 @@ ExecuteModifyTasks(List *taskList, bool expectResults, ParamListInfo paramListIn
 					queryOK = ConsumeQueryResult(connection, alwaysThrowErrorOnFailure,
 												 &currentAffectedTupleCount);
 				}
+			}
+			PG_CATCH();
+			{
+				UnclaimAllShardConnections(shardConnectionHash);
+				PG_RE_THROW();
+			}
+			PG_END_TRY();
 
 			/* We error out if the worker fails to return a result for the query. */
 			if (!queryOK)
 			{
+				UnclaimAllShardConnections(shardConnectionHash);
 				ReportConnectionError(connection, ERROR);
 			}
 

src/backend/distributed/transaction/remote_transaction.c

@@ -1068,6 +1068,12 @@ CoordinatedRemoteTransactionsSavepointRollback(SubTransactionId subId)
 		MultiConnection *connection = dlist_container(MultiConnection, transactionNode,
 													  iter.cur);
 		RemoteTransaction *transaction = &connection->remoteTransaction;
+
+		/* cancel any ongoing queries before issuing rollback */
+		ClearResultsIfReady(connection);
+		SendCancelationRequest(connection);
+		ForgetResults(connection);
+
 		if (transaction->transactionFailed)
 		{
 			if (transaction->lastSuccessfulSubXact <= subId)

src/include/distributed/connection_management.h

@@ -21,6 +21,9 @@
 /* maximum (textual) lengths of hostname and port */
 #define MAX_NODE_LENGTH 255 /* includes 0 byte */
 
+/* used for libpq commands that get an error buffer. Postgres docs recommend 256. */
+#define ERROR_BUFFER_SIZE 256
+
 /* default notice level */
 #define DEFAULT_CITUS_NOTICE_LEVEL DEBUG1
 

src/include/distributed/remote_commands.h

@@ -57,5 +57,6 @@ extern bool PutRemoteCopyEnd(MultiConnection *connection, const char *errormsg);
 /* waiting for multiple command results */
 extern void WaitForAllConnections(List *connectionList, bool raiseInterrupts);
 
+extern bool SendCancelationRequest(MultiConnection *connection);
 
 #endif /* REMOTE_COMMAND_H */

src/test/regress/expected/failure_savepoints.out

@@ -38,6 +38,10 @@ 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 not open
 CONTEXT:  while executing command on localhost:9060
 WARNING:  connection error: localhost:9060
@@ -195,6 +199,8 @@ 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
+WARNING:  connection not open
 COMMIT;
 ERROR:  could not make changes to shard 100950 on any node
 SELECT * FROM artists WHERE id=6;
@@ -230,6 +236,8 @@ 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 not open
 WARNING:  connection error: localhost:9060
 WARNING:  connection not open

src/test/regress/expected/multi_subtransactions.out

@@ -129,6 +129,28 @@ ERROR:  current transaction is aborted, commands ignored until end of transactio
 ROLLBACK TO SAVEPOINT s3;
 ERROR:  savepoint "s3" does not exist
 COMMIT;
+-- Recover from multi-shard modify errors
+BEGIN;
+INSERT INTO artists VALUES (8, 'Uncle Yaakov');
+SAVEPOINT s1;
+UPDATE artists SET name = NULL;
+ERROR:  null value in column "name" violates not-null constraint
+ROLLBACK TO s1;
+INSERT INTO artists VALUES (9, 'Anna Schaeffer');
+COMMIT;
+SELECT * FROM artists ORDER BY id;
+ id |       name       
+----+------------------
+  1 | Pablo Picasso
+  2 | Vincent van Gogh
+  3 | Claude Monet
+  4 | William Kurelek
+  5 | Jacob Kahn
+  6 | Emily Carr
+  8 | Uncle Yaakov
+  9 | Anna Schaeffer
+(8 rows)
+
 -- ===================================================================
 -- Tests for replication factor > 1
 -- ===================================================================

src/test/regress/expected/multi_subtransactions_0.out

@@ -129,6 +129,28 @@ ERROR:  current transaction is aborted, commands ignored until end of transactio
 ROLLBACK TO SAVEPOINT s3;
 ERROR:  no such savepoint
 COMMIT;
+-- Recover from multi-shard modify errors
+BEGIN;
+INSERT INTO artists VALUES (8, 'Uncle Yaakov');
+SAVEPOINT s1;
+UPDATE artists SET name = NULL;
+ERROR:  null value in column "name" violates not-null constraint
+ROLLBACK TO s1;
+INSERT INTO artists VALUES (9, 'Anna Schaeffer');
+COMMIT;
+SELECT * FROM artists ORDER BY id;
+ id |       name       
+----+------------------
+  1 | Pablo Picasso
+  2 | Vincent van Gogh
+  3 | Claude Monet
+  4 | William Kurelek
+  5 | Jacob Kahn
+  6 | Emily Carr
+  8 | Uncle Yaakov
+  9 | Anna Schaeffer
+(8 rows)
+
 -- ===================================================================
 -- Tests for replication factor > 1
 -- ===================================================================

src/test/regress/sql/multi_subtransactions.sql

@@ -105,6 +105,17 @@ SAVEPOINT s3;
 ROLLBACK TO SAVEPOINT s3;
 COMMIT;
 
+-- Recover from multi-shard modify errors
+BEGIN;
+INSERT INTO artists VALUES (8, 'Uncle Yaakov');
+SAVEPOINT s1;
+UPDATE artists SET name = NULL;
+ROLLBACK TO s1;
+INSERT INTO artists VALUES (9, 'Anna Schaeffer');
+COMMIT;
+
+SELECT * FROM artists ORDER BY id;
+
 -- ===================================================================
 -- Tests for replication factor > 1
 -- ===================================================================
@@ -216,3 +227,4 @@ SELECT * FROM researchers WHERE lab_id=10;
 -- Clean-up
 DROP TABLE artists;
 DROP TABLE researchers;
+