pgsql: Block interrupts during HandleParallelMessages().

Block interrupts during HandleParallelMessages().

As noted by Alvaro, there are CHECK_FOR_INTERRUPTS() calls in the shm_mq.c
functions called by HandleParallelMessages(). I believe they're all
unreachable since we always pass nowait = true, but it doesn't seem like
a great idea to assume that no such call will ever be reachable from
HandleParallelMessages(). If that did happen, there would be a risk of a
recursive call to HandleParallelMessages(), which it does not appear to be
designed for --- for example, there's nothing that would prevent
out-of-order processing of received messages. And certainly such cases
cannot easily be tested. So let's prevent it by holding off interrupts for
the duration of the function. Back-patch to 9.5 which contains identical
code.

Discussion: <14869(dot)1470083848(at)sss(dot)pgh(dot)pa(dot)us>

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/75c452a755a0c1e1500362b9bd7976a0be2588d2

Modified Files
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src/backend/access/transam/parallel.c | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)