lib/posix-poll: Fix epoll missing events

[andreittr]

Description of changes

Previously epoll would miss events when configured for edge-polling and unable to return all captured events in a single call to epoll_wait, by erroneously marking the epoll fd as not ready, even though it may still have pending events.
This change fixes this oversight, restoring expected behavior to epoll_wait.

Prerequisite checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Ran the checkpatch.uk on your commit series before opening this PR;
• Updated relevant documentation.

Base target

• Architecture(s): N/A
• Platform(s): N/A
• Application(s): N/A

Additional configuration

Trigger/test snippet:

int main(void)
{
	struct epoll_event ev;
	int p[2], q[2];
	int r;

	r = pipe(p);
	assert(!r);
	r = pipe(q);
	assert(!r);

	int e = epoll_create(2);
	assert(e >= 0);

	ev.events = EPOLLIN|EPOLLET;
	ev.data.fd = p[0];
	r = epoll_ctl(e, EPOLL_CTL_ADD, p[0], &ev);
	assert(!r);

	ev.events = EPOLLIN|EPOLLET;
	ev.data.fd = q[0];
	r = epoll_ctl(e, EPOLL_CTL_ADD, q[0], &ev);
	assert(!r);

	write(p[1], "a", 1);
	write(q[1], "b", 1);

	do {
		char buf[2];
		r = epoll_wait(e, &ev, 1, 0);
		printf("%d\n", r);
		if (r) {
			read(ev.data.fd, buf, 1);
			buf[1] = 0;
			puts(buf);
		}
	} while (r);
	return 0;
}

Output should be

1
a
1
b
0

On both Linux and Unikraft; previously unikraft would miss one of the messages (most likely b).

[StefanJum]

All good, thanks.
Reviewed-by: Stefan Jumarea stefanjumarea02@gmail.com

[mogasergiu]

Doesn't this function return something? Is there a reasion we are not checking for return values?

[mogasergiu]

In fact I don't see its return values being checked at any of its callsite 🤔

[andreittr]

uk_file_event_* functions return the previous events, as a convenience due to the fact that event setting is done atomically so there's no other way to get that previous value. This is not needed often, but is needed sometimes, thus the need to return it.

In this case we don't care what the previous events were. I used to cast such invocations to void, but this turned out to be everywhere, visually polluting the code, not to mention increasing the pressure on keeping everything within 80 cols with size 8 tabs 😛 ; so I stopped that.

[mogasergiu]

Oh well I guess we also call printf while ignoring return value...

[mogasergiu]

Reviewed-by: Sergiu Moga sergiu@unikraft.io

[razvand]

Approved-by: Razvan Deaconescu razvand@unikraft.io