[issue16105] Pass read only FD to signal.set_wakeup_fd

[Charles-François Natali]

Charles-François Natali added the comment:

> I agree with Felipe that issues here can be difficult to diagnose. For example the fd could get mistakingly closed, but the write() EBADF would then be ignored and the expected signal wakeups would be lost.

Yeah, but it's also completely possible that the saved fd now points
to another file descriptor, and you end up corrupting a file, without
getting any error at all, and not getting signal wakeups. This API is
really fragile and dangerous, and warrants care anyway...

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<http://bugs.python.org/issue16105>
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