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(dot)wanner(at)2ndquadrant(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/3bb363e7-ac04-0ac4-9fe8-db1148755bfa%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/26ec6b5948a73d0e07ed9435ee4554594acdf34f

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