pgsql: Allow DSM allocation to be interrupted.

Allow DSM allocation to be interrupted.

Chris Travers reported that the startup process can repeatedly try to
cancel a backend that is in a posix_fallocate()/EINTR loop and cause it
to loop forever. Teach the retry loop to give up if an interrupt is
pending. Don't actually check for interrupts in that loop though,
because a non-local exit would skip some clean-up code in the caller.

Back-patch to 9.4 where DSM was added (and posix_fallocate() was later
back-patched).

Author: Chris Travers
Reviewed-by: Ildar Musin, Murat Kabilov, Oleksii Kliukin
Tested-by: Oleksii Kliukin
Discussion: https://postgr.es/m/CAN-RpxB-oeZve_J3SM_6%3DHXPmvEG%3DHX%2B9V9pi8g2YR7YW0rBBg%40mail.gmail.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/f547035a0e1bfb338cd2113976d7f384854a64e3

Modified Files
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src/backend/storage/ipc/dsm_impl.c | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)