Function ModifyWaitEvent

src/backend/storage/ipc/latch.c:1025–1094

* Change the event mask and, in the WL_LATCH_SET case, the latch associated * with the WaitEvent. The latch may be changed to NULL to disable the latch * temporarily, and then set back to a latch later. * * 'pos' is the id returned by AddWaitEventToSet. */

 * 'pos' is the id returned by AddWaitEventToSet.
 */
void
ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch)
{
	WaitEvent  *event;
#if defined(WAIT_USE_KQUEUE)
	int			old_events;
#endif

	Assert(pos < set->nevents);

	event = &set->events[pos];
#if defined(WAIT_USE_KQUEUE)
	old_events = event->events;
#endif

	/*
	 * If neither the event mask nor the associated latch changes, return
	 * early. That's an important optimization for some sockets, where
	 * ModifyWaitEvent is frequently used to switch from waiting for reads to
	 * waiting on writes.
	 */
	if (events == event->events &&
		(!(event->events & WL_LATCH_SET) || set->latch == latch))
		return;

	if (event->events & WL_LATCH_SET &&
		events != event->events)
	{
		elog(ERROR, "cannot modify latch event");
	}

	if (event->events & WL_POSTMASTER_DEATH)
	{
		elog(ERROR, "cannot modify postmaster death event");
	}

	/* FIXME: validate event mask */
	event->events = events;

	if (events == WL_LATCH_SET)
	{
		if (latch && latch->owner_pid != MyProcPid)
			elog(ERROR, "cannot wait on a latch owned by another process");
		set->latch = latch;

		/*
		 * On Unix, we don't need to modify the kernel object because the
		 * underlying pipe (if there is one) is the same for all latches so we
		 * can return immediately.  On Windows, we need to update our array of
		 * handles, but we leave the old one in place and tolerate spurious
		 * wakeups if the latch is disabled.
		 */
#if defined(WAIT_USE_WIN32)
		if (!latch)
			return;
#else
		return;
#endif