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Add new function WaitForParallelWorkersToAttach.

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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
This commit is contained in:
Robert Haas
2018-02-02 09:00:59 -05:00
parent a2a2205761
commit 9222c0d9ed
4 changed files with 163 additions and 11 deletions
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9
src/backend/executor/nodeGatherMerge.c
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@ -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);