76d07a2a06
Previously, GetBackgroundWorkerPid() would return BGWH_NOT_YET_STARTED if the slot used for the worker registration had not been reused by unrelated activity, and BGWH_STOPPED if it had. Either way, a process that had requested notification when the state of one of its background workers changed did not receive such notifications. Fix things so that GetBackgroundWorkerPid() always returns BGWH_STOPPED in this situation, so that we do not erroneously give waiters the impression that the worker will eventually be started; and send notifications just as we would if the process terminated after having been started, so that it's possible to wait for the postmaster to process a worker termination request without polling. Discovered by Amit Kapila during testing of parallel sequential scan. Analysis and fix by me. Back-patch to 9.4; there may not be anyone relying on this interface yet, but if anyone is, the new behavior is a clear improvement.
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