pgsql: Avoid invalid alloc size error in shm_mq

Avoid invalid alloc size error in shm_mq

In shm_mq_receive(), a huge payload could trigger an unjustified
"invalid memory alloc request size" error due to the way the buffer
size is increased.

Add error checks (documenting the upper limit) and avoid the error by
limiting the allocation size to MaxAllocSize.

Author: Markus Wanner <markus.wanner@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/3bb363e7-ac04-0ac4-9fe8-db1148755bfa%402ndquadrant.com

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/d27c18d26f00fc13401931fe9deb5012610faf89

Modified Files
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src/backend/storage/ipc/shm_mq.c | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)