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If a WaitEventSetWait() caller asks for multiple events, an already set latch would previously prevent other events from being reported at the same time. Now, we'll also poll the kernel for other events that would fit in the caller's output buffer with a zero wait time. This policy change doesn't affect callers that ask for only one event. The main caller affected is the postmaster. If its latch is set extremely frequently by backends launching workers and workers exiting, we don't want it to handle only those jobs and ignore incoming client connections. Back-patch to 16 where the postmaster began using the API. The fast-return policy changed here is older than that, but doesn't cause any known problems in earlier releases. Reported-by: Nathan Bossart <nathandbossart@gmail.com> Reviewed-by: Nathan Bossart <nathandbossart@gmail.com> Discussion: https://postgr.es/m/Z1n5UpAiGDmFcMmd%40nathan