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This coding pattern creates a race condition, because if an interesting interrupt happens after we've checked InterruptPending but before we reset our latch, the latch-setting done by the signal handler would get lost, and then we might block at WaitLatch in the next iteration without ever noticing the interrupt condition. You can put the CHECK_FOR_INTERRUPTS before WaitLatch or after ResetLatch, but not between them. Aside from fixing the bugs, add some explanatory comments to latch.h to perhaps forestall the next person from making the same mistake. In HEAD, also replace gather_readnext's direct call of HandleParallelMessages with CHECK_FOR_INTERRUPTS. It does not seem clean or useful for this one caller to bypass ProcessInterrupts and go straight to HandleParallelMessages; not least because that fails to consider the InterruptPending flag, resulting in useless work both here (if InterruptPending isn't set) and in the next CHECK_FOR_INTERRUPTS call (if it is). This thinko seems to have been introduced in the initial coding of storage/ipc/shm_mq.c (commit ec9037df2), and then blindly copied into all the subsequent parallel-query support logic. Back-patch relevant hunks to 9.4 to extirpate the error everywhere. Discussion: <1661.1469996911@sss.pgh.pa.us>
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