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Fixing linux poller.
In short: epoll royally sucks. The longer version: epoll doesn't allow 2 separate events to be maintained per fd. If we register an fd for read, and then decide to register it for write too, the later will overwrite the original registered read event unless we register READ|WRITE. It seems harmless but it's not. When we register for READ|WRITE and only a READ event arrives, we first receive the READ event, and in the next epoll_wait if a READ + WRITE event occurs we'll receive READ|WRITE because the original registered event is READ|WRITE. What this means is that we received READ twice here, once as READ and the second time when a WRITE & READ occured, READ|WRITE. We were not expecting the second READ event and w'ell have to ignore it. This behavior happens because we are not passing EPOLLONESHOT. The reason we don't want to do this is that the EPOLLONESHOT will get applied to both events (read + write) when we only need it for one. Ex: If we register for a READ event, followed by registering WRITE event READ|WRITE|EPOLLONESHOT this will mean if either of the READ or WRITE event occurs it will be EPOLLONESHOT and the second event will need to be registered. I certainly don't want to track such events and reregister them. Hence I am going with the first solution by not use EPOLLONESHOT and ignore events I am not expecting. If only epoll supports fd+event as a key rather than fd only.
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