[issue19876] selectors (and asyncio?): document behaviour on closed files/sockets

[Charles-François Natali]

Charles-François Natali added the comment:

> STINNER Victor added the comment:
>
> I don't like generic "except OSError: pass". Here is a first patch for epoll() to use "except FileNotFoundError: pass" instead. Kqueue selector should also be patched.

Except that it can fail with ENOENT, but also EBADF, and EPERM if the
FD has been reused by a FD which doesn't support epoll.
So if we want to go this way, we should at least catach ENOENT, EBADF
and EPERM. Same for kqueue: we should at least catch ENOENT and EBADF.

> I tested to close epoll FD (os.close(epoll.fileno())): on Linux 3.11, epoll.unregister(fd) and epoll.close() don't raise an error. Strange. (The C code looks correct).

unregister() ignores EBADF.

> (About the commit: I don't like "_fileobj_lookup" method name, we loose the information (compared to "_fileobj_to_fd" name) that the method returns a file dscriptor. I would prefer "_get_fd" or "_get_fileobj_fd".)

Well, Guido likes it, I like it, and this is really nit-picking
(especially since it's a private method).

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