BUG #17441: shm_mq receive less data than the sender sends

The following bug has been logged on the website:

Bug reference: 17441
Logged by: ytf
Email address: fft_665665(at)163(dot)com
PostgreSQL version: 10.17
Operating system: centos
Description:

Dears,

I launch one dynamic worker as the sender, use shm_mq send data to the
SELECT, which I register a copy hook to receive data from sender.
Each data length is 800.

When I set shm_mq SIZE to 16, there is no problem. Only speed is very
slow.

However, when I set shm_mq Size to 8192 or larger, I meet a problem.

I first receive 800 length data , and after a few times, I only receive 732
length data.

I registered the shm_mq like following :
#define QUEUE_SIZE 8192
mq = shm_mq_create(shm_toc_allocate(pcxt->toc, QUEUE_SIZE), QUEUE_SIZE);
shm_toc_insert(pcxt->toc, COS_KEY_QUEUE, mq);
shm_mq_set_receiver(mq, MyProc);