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shm_mq: Fix detach race condition.

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Commit 34db06ef9a1d7f36391c64293bf1e0ce44a33915 adopted a lock-free
design for shm_mq.c, but it introduced a race condition that could
lose messages.  When shm_mq_receive_bytes() detects that the other end
has detached, it must make sure that it has seen the final version of
mq_bytes_written, or it might miss a message sent before detaching.

Thomas Munro

Discussion: https://postgr.es/m/CAEepm%3D2myZ4qxpt1a%3DC%2BwEv3o188K13K3UvD-44FK0SdAzHy%2Bw%40mail.gmail.com
This commit is contained in:
Robert Haas 2018-03-05 15:12:49 -05:00
parent 2f3e2340cd
commit 42d7074ebb
1 changed files with 12 additions and 0 deletions
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12
src/backend/storage/ipc/shm_mq.c
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@ -1058,7 +1058,19 @@ shm_mq_receive_bytes(shm_mq_handle *mqh, Size bytes_needed, bool nowait,
* detached.
*/
if (mq->mq_detached)
{
/*
* If the writer advanced mq_bytes_written and then set
* mq_detached, we might not have read the final value of
* mq_bytes_written above. Insert a read barrier and then check
* again if mq_bytes_written has advanced.
*/
pg_read_barrier();
if (written != pg_atomic_read_u64(&mq->mq_bytes_written))
continue;
return SHM_MQ_DETACHED;
}
/*
* We didn't get enough data to satisfy the request, so mark any data