select(sock) indicates not-ready, but sock.recv does not block

[Nikolaus Rath]

Nikolaus Rath <Nikolaus at rath.org> writes:
> Hello,
>
> I have a problem with using select. I can reliably reproduce a situation
> where select.select((sock.fileno(),), (), (), 0) returns ((),(),())
> (i.e., no data ready for reading), but an immediately following
> sock.recv() returns data without blocking.
[...]

Turns out that I fell into the well-known (except to me) ssl-socket
trap: http://docs.python.org/3/library/ssl.html#notes-on-non-blocking-sockets

TL;DR: Relying on select() on an SSL socket is not a good idea because
some internal buffering is done. Better put the socket in non-blocking
mode and try to read something, catching the ssl.Want* exception if
nothing is ready.

Best,
-Nikolaus

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