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/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
index a3e34c69800..6858c91e8c2 100644
--- a/src/backend/executor/nodeGatherMerge.c
+++ b/src/backend/executor/nodeGatherMerge.c
@@ -710,7 +710,14 @@ gm_readnext_tuple(GatherMergeState *gm_state, int nreader, bool nowait,
 	/* Check for async events, particularly messages from workers. */
 	CHECK_FOR_INTERRUPTS();
 
-	/* Attempt to read a tuple. */
+	/*
+	 * Attempt to read a tuple.
+	 *
+	 * 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.
+	 */
 	reader = gm_state->reader[nreader - 1];
 	tup = TupleQueueReaderNext(reader, nowait, done);