This happens when the shm_mq does not have an associated bgworker handle registered because the other end is not known at mq creation time or is a normal backend not a bgworker. So a BGW handle cannot be passed.
shm_mq_wait_internal() will CHECK_FOR_INTERRUPTS() when its latch wait is interrupted by a SIGTERM. But it doesn't actually respond to SIGTERM in any way; it just merrily resets its latch and keeps looping.
It will bail out correctly on SIGQUIT.
If the proc waiting to attach was known at queue creation time and was a bgworker, we'd pass a bgworker handle and the mq would notice it failed to start and stop waiting. There's only a problem if no bgworker handle can be supplied.
The underlying problem is that CHECK_FOR_INTERRUPTS() doesn't care about SIGTERM or have any way to test for it. And we don't have any global management of SIGTERM like we do SIGQUIT so the shm_mq_wait_internal loop can't test for it.
The only ways I can see to fix this are:
* Generalize SIGTERM handling across postgres, so there's a global "got_SIGTERM" global that shm_mq_wait_internal can test to break out of its loop, and every backend's signal handler must set it. Lots of churn.
* In a proc's signal handler, use globals set before entry and after exit from shm_mq operations to detect if we're currently in shm_mq and promote SIGTERM to SIGQUIT by sending a new signal to ourselves. Or set up state so CHECK_FOR_INTERRUPTS() will notice when the handler returns.
* Allow passing of a *bool that tests for SIGTERM, or a function pointer called on each iteration to test whether looping should continue, to be passed to shm_mq_attach. So if you can't supply a bgw handle, you supply that instead. Provide a shm_mq_set_handle equivalent for it too.
Any objections to the last approach?