Is Python SSL API thread-safe?

Grant Edwards grant.b.edwards at
Sun Jan 22 20:01:59 EST 2017

On 2017-01-22, Christian Heimes <christian at> wrote:
> On 2017-01-22 21:18, Grant Edwards wrote:
>> Is the Python SSL API thread-safe with respect to recv() and send()?
>> IOW, can I have one thread doing blocking recv() calls on an SSL
>> connection object while "simultaneously" a second thread is calling
>> send() on that same connection object?
>> I assumed that was allowed, but I can't find anything in the
>> documentation that actually says it is.
> OpenSSL and Python's ssl module are thread-safe. However IO is not safe
> concerning reentrancy. You cannot safely share a SSLSocket between
> threads without a mutex. Certain aspects of the TLS protocol can cause
> interesting side effects. A recv() call can send data across a wire and
> a send() call can receive data from the wire, e.g. during re-keying.
> In order to archive reentrancy, you have to do all IO yourself by
> operating the SSL connection in non-blocking mode or with a Memorio-BIO

IOW, what I'm doing is not safe.  Rats.


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