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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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6e03a8a304a2f31ffafb5f229794747c01af6936

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