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sd-event: always call epoll_ctl() on mask-updates if edge-triggered
A call to sd_event_source_set_io_events() skipps calling into the kernel if the new event-mask matches the old one. This is safe for level-triggered sources as the kernel moves them onto the ready-list automatically if events change. However, edge-triggered sources might not be on the ready-list even though events are present. A call to sd_event_source_set_io_events() with EPOLLET set might thus be used to just move the io-source onto the ready-list so the next poll will return it again. This is very useful to avoid starvation in priority-based event queues. Imagine a read() loop on an edge-triggered fd. If we cannot read data fast enough to drain the receive queue, we might decide to skip reading for now and schedule it for later. On edge-triggered io-sources we have to make sure it's put on the ready-list so the next dispatch-round will return it again if it's still the highest priority task. We could make sd-event handle edge-triggered sources directly and allow marking them ready again. However, it's much simpler to let the kernel do that for now via EPOLL_CTL_MOD.
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@ -1282,7 +1282,8 @@ _public_ int sd_event_source_set_io_events(sd_event_source *s, uint32_t events)
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assert_return(s->event->state != SD_EVENT_FINISHED, -ESTALE);
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assert_return(!event_pid_changed(s->event), -ECHILD);
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if (s->io.events == events)
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/* edge-triggered updates are never skipped, so we can reset edges */
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if (s->io.events == events && !(events & EPOLLET))
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return 0;
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if (s->enabled != SD_EVENT_OFF) {
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