Create an infrastructure for parallel computation in PostgreSQL.

This does four basic things.  First, it provides convenience routines
to coordinate the startup and shutdown of parallel workers.  Second,
it synchronizes various pieces of state (e.g. GUCs, combo CID
mappings, transaction snapshot) from the parallel group leader to the
worker processes.  Third, it prohibits various operations that would
result in unsafe changes to that state while parallelism is active.
Finally, it propagates events that would result in an ErrorResponse,
NoticeResponse, or NotifyResponse message being sent to the client
from the parallel workers back to the master, from which they can then
be sent on to the client.

Robert Haas, Amit Kapila, Noah Misch, Rushabh Lathia, Jeevan Chalke.
Suggestions and review from Andres Freund, Heikki Linnakangas, Noah
Misch, Simon Riggs, Euler Taveira, and Jim Nasby.

src/backend/postmaster/bgworker.c

@@ -995,6 +995,56 @@ WaitForBackgroundWorkerStartup(BackgroundWorkerHandle *handle, pid_t *pidp)
 	return status;
 }
 
+/*
+ * Wait for a background worker to stop.
+ *
+ * If the worker hasn't yet started, or is running, we wait for it to stop
+ * and then return BGWH_STOPPED.  However, if the postmaster has died, we give
+ * up and return BGWH_POSTMASTER_DIED, because it's the postmaster that
+ * notifies us when a worker's state changes.
+ */
+BgwHandleStatus
+WaitForBackgroundWorkerShutdown(BackgroundWorkerHandle *handle)
+{
+	BgwHandleStatus status;
+	int			rc;
+	bool		save_set_latch_on_sigusr1;
+
+	save_set_latch_on_sigusr1 = set_latch_on_sigusr1;
+	set_latch_on_sigusr1 = true;
+
+	PG_TRY();
+	{
+		for (;;)
+		{
+			pid_t		pid;
+
+			CHECK_FOR_INTERRUPTS();
+
+			status = GetBackgroundWorkerPid(handle, &pid);
+			if (status == BGWH_STOPPED)
+				return status;
+
+			rc = WaitLatch(&MyProc->procLatch,
+						   WL_LATCH_SET | WL_POSTMASTER_DEATH, 0);
+
+			if (rc & WL_POSTMASTER_DEATH)
+				return BGWH_POSTMASTER_DIED;
+
+			ResetLatch(&MyProc->procLatch);
+		}
+	}
+	PG_CATCH();
+	{
+		set_latch_on_sigusr1 = save_set_latch_on_sigusr1;
+		PG_RE_THROW();
+	}
+	PG_END_TRY();
+
+	set_latch_on_sigusr1 = save_set_latch_on_sigusr1;
+	return status;
+}
+
 /*
  * Instruct the postmaster to terminate a background worker.
  *