Add new function WaitForParallelWorkersToAttach.

Once this function has been called, we know that all workers have
started and attached to their error queues -- so if any of them
subsequently exit uncleanly, we'll be sure to throw an ERROR promptly.
Otherwise, users of the ParallelContext machinery must be careful not
to wait forever for a worker that has failed to start.  Parallel query
manages to work without needing this for reasons explained in new
comments added by this patch, but it's a useful primitive for other
parallel operations, such as the pending patch to make creating a
btree index run in parallel.

Amit Kapila, revised by me.  Additional review by Peter Geoghegan.

Discussion: http://postgr.es/m/CAA4eK1+e2MzyouF5bg=OtyhDSX+=Ao=3htN=T-r_6s3gCtKFiw@mail.gmail.com

1 files changed, 8 insertions, 1 deletions

diff --git a/src/backend/executor/nodeGather.c b/src/backend/executor/nodeGather.c
index 89266b53712..58eadd45b83 100644
--- a/src/backend/executor/nodeGather.c
+++ b/src/backend/executor/nodeGather.c
@@ -312,7 +312,14 @@ gather_readnext(GatherState *gatherstate)
 		/* Check for async events, particularly messages from workers. */
 		CHECK_FOR_INTERRUPTS();
 
-		/* Attempt to read a tuple, but don't block if none is available. */
+		/*
+		 * Attempt to read a tuple, but don't block if none is available.
+		 *
+		 * Note that TupleQueueReaderNext will just return NULL for a worker
+		 * which fails to initialize.  We'll treat that worker as having
+		 * produced no tuples; WaitForParallelWorkersToFinish will error out
+		 * when we get there.
+		 */
 		Assert(gatherstate->nextreader < gatherstate->nreaders);
 		reader = gatherstate->reader[gatherstate->nextreader];
 		tup = TupleQueueReaderNext(reader, true, &readerdone);