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<H2><A NAME="SECTION000520000000000000000"> </A>
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There are a number of problems with callbacks. First of all, OpenSSL is written
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as a C library, it's not meant to have Python callbacks, so a way around that
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is needed. Another problem is thread support. A lot of the OpenSSL I/O
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functions can block if the socket is in blocking mode, and then you want other
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Python threads to be able to do other things. The real trouble is if you've
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released the global CPython interpreter lock to do a potentially blocking
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operation, and the operation calls a callback. Then we must take the GIL back,
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since calling Python APIs without holding it is not allowed.
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There are two solutions to the first problem, both of which are necessary. The
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first solution to use is if the C callback allows ''userdata'' to be passed to
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it (an arbitrary pointer normally). This is great! We can set our Python
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function object as the real userdata and emulate userdata for the Python
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function in another way. The other solution can be used if an object with an
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''app_data'' system always is passed to the callback. For example, the SSL
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object in OpenSSL has app_data functions and in e.g. the verification
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callbacks, you can retrieve the related SSL object. What we do is to set our
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wrapper <tt class="class">Connection</tt> object as app_data for the SSL object, and we can
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easily find the Python callback.
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The other problem is solved using thread local variables. Whenever the GIL is
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released before calling into an OpenSSL API, the PyThreadState pointer returned
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by <tt class="cfunction">PyEval_SaveState</tt> is stored in a global thread local variable
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(using Python's own TLS API, <tt class="cfunction">PyThread_set_key_value</tt>). When it is
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necessary to re-acquire the GIL, either after the OpenSSL API returns or in a C
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callback invoked by that OpenSSL API, the value of the thread local variable is
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retrieved (<tt class="cfunction">PyThread_get_key_value</tt>) and used to re-acquire the GIL.
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This allows Python threads to execute while OpenSSL APIs are running and allows
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use of any particular pyOpenSSL object from any Python thread, since there is
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no per-thread state associated with any of these objects and since OpenSSL is
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threadsafe (as long as properly initialized, as pyOpenSSL initializes it).
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<span class="release-info">Release 0.13.</span>
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