Bring back c3d8a92e which was reverted by accident

src/backend/cdb/dispatcher/cdbdisp_async.c

@@ -418,6 +418,7 @@ checkDispatchResult(CdbDispatcherState *ds,
 		int			sock;
 		int			n;
 		int			nfds = 0;
+		PGconn		*conn;
 
 		/*
 		 * bail-out if we are dying. Once QD dies, QE will recognize it
@@ -441,6 +442,7 @@ checkDispatchResult(CdbDispatcherState *ds,
 		{
 			dispatchResult = pParms->dispatchResultPtrArray[i];
 			segdbDesc = dispatchResult->segdbDesc;
+			conn = segdbDesc->conn;
 
 			/*
 			 * Already finished with this QE?
@@ -450,10 +452,26 @@ checkDispatchResult(CdbDispatcherState *ds,
 
 			Assert(!cdbconn_isBadConnection(segdbDesc));
 
+			/*
+			 * Flush out buffer in case some commands are not fully
+			 * dispatched to QEs, this can prevent QD from polling
+			 * on such QEs forever.
+			 */
+			if (conn->outCount > 0)
+			{
+				/*
+				 * Don't error out here, let following poll() routine to
+				 * handle it.
+				 */
+				if (pqFlush(conn) < 0)
+					elog(LOG, "Failed flushing outbound data to %s:%s",
+						 segdbDesc->whoami, PQerrorMessage(conn));
+			}
+
 			/*
 			 * Add socket to fd_set if still connected.
 			 */
-			sock = PQsocket(segdbDesc->conn);
+			sock = PQsocket(conn);
 			Assert(sock >= 0);
 			fds[nfds].fd = sock;
 			fds[nfds].events = POLLIN;

src/test/regress/input/dispatch.source

@@ -308,3 +308,13 @@ select 1 from gp_dist_random('gp_id') limit 1;
 -- if previous gang is not destroyed, snapshot collision would happen
 select 1 from gp_dist_random('gp_id') limit 1;
 select gp_inject_fault('gang_created', 'reset', 1);
+
+--
+-- Test that an error happens after a big command is dispatched.
+--
+select gp_inject_fault('after_one_slice_dispatched', 'error', 1);
+select * from gp_dist_random('gp_id')
+	where gpname > (select * from repeat('sssss', 10000000));
+select gp_inject_fault('after_one_slice_dispatched', 'reset', 1);
+select * from gp_dist_random('gp_id')
+	where gpname > (select * from repeat('sssss', 10000000));

src/test/regress/output/dispatch.source

@@ -554,3 +554,29 @@ NOTICE:  Success:
  t
 (1 row)
 
+--
+-- Test that an error happens after a big command is dispatched.
+--
+select gp_inject_fault('after_one_slice_dispatched', 'error', 1);
+NOTICE:  Success:
+ gp_inject_fault 
+-----------------
+ t
+(1 row)
+
+select * from gp_dist_random('gp_id')
+	where gpname > (select * from repeat('sssss', 10000000));
+ERROR:  fault triggered, fault name:'after_one_slice_dispatched' fault type:'error'
+select gp_inject_fault('after_one_slice_dispatched', 'reset', 1);
+NOTICE:  Success:
+ gp_inject_fault 
+-----------------
+ t
+(1 row)
+
+select * from gp_dist_random('gp_id')
+	where gpname > (select * from repeat('sssss', 10000000));
+ gpname | numsegments | dbid | content 
+--------+-------------+------+---------
+(0 rows)
+