Function WaitForParallelWorkersToFinish

src/backend/access/transam/parallel.c:810–914

* Wait for all workers to finish computing. * * Even if the parallel operation seems to have completed successfully, it's * important to call this function afterwards. We must not miss any errors * the workers may have thrown during the parallel operation, or any that they * may yet throw while shutting down. * * Also, we want to update our notion of XactLastRecEnd based on worker * feedb

 * feedback.
 */
void
WaitForParallelWorkersToFinish(ParallelContext *pcxt)
{
	for (;;)
	{
		bool		anyone_alive = false;
		int			nfinished = 0;
		int			i;

		/*
		 * This will process any parallel messages that are pending, which may
		 * change the outcome of the loop that follows.  It may also throw an
		 * error propagated from a worker.
		 */
		CHECK_FOR_INTERRUPTS();

		for (i = 0; i < pcxt->nworkers_launched; ++i)
		{
			/*
			 * If error_mqh is NULL, then the worker has already exited
			 * cleanly.  If we have received a message through error_mqh from
			 * the worker, we know it started up cleanly, and therefore we're
			 * certain to be notified when it exits.
			 */
			if (pcxt->worker[i].error_mqh == NULL)
				++nfinished;
			else if (pcxt->known_attached_workers[i])
			{
				anyone_alive = true;
				break;
			}
		}

		if (!anyone_alive)
		{
			/* If all workers are known to have finished, we're done. */
			if (nfinished >= pcxt->nworkers_launched)
			{
				Assert(nfinished == pcxt->nworkers_launched);
				break;
			}

			/*
			 * We didn't detect any living workers, but not all workers are
			 * known to have exited cleanly.  Either not all workers have
			 * launched yet, or maybe some of them failed to start or
			 * terminated abnormally.
			 */
			for (i = 0; i < pcxt->nworkers_launched; ++i)
			{
				pid_t		pid;
				shm_mq	   *mq;

				/*
				 * If the worker is BGWH_NOT_YET_STARTED or BGWH_STARTED, we
				 * should just keep waiting.  If it is BGWH_STOPPED, then
				 * further investigation is needed.
				 */