Use ssl module locking when available

Thread locking callbacks into the interpreter impose considerable overhead.
The standard library's ssl module registers its own thread locking and id
callback functions. Registration of the latter can be detected by a call to
the OpenSSL library. In this case, do not set the Python callback function
and therefore keep using the more efficient ssl module functions.

dtls/openssl.py

@@ -486,6 +486,7 @@ map(lambda x: _make_function(*x), (
     ("SSLeay_version", libcrypto, ((c_char_p, "ret"), (c_int, "t"))),
     ("CRYPTO_set_locking_callback", libcrypto,
      ((None, "ret"), (c_void_p, "func")), False),
+    ("CRYPTO_get_id_callback", libcrypto, ((c_void_p, "ret"),), True, None),
     ("CRYPTO_num_locks", libcrypto, ((c_int, "ret"),)),
     ("DTLSv1_server_method", libssl, ((DTLSv1Method, "ret"),)),
     ("DTLSv1_client_method", libssl, ((DTLSv1Method, "ret"),)),

dtls/tlock.py

@@ -21,6 +21,10 @@ DO_DEBUG_LOG = False
 def tlock_init():
     if not globals().has_key("threading"):
         return  # nothing to configure
+    # The standard library ssl module's lock implementation is more efficient;
+    # do not override it if it has been established
+    if CRYPTO_get_id_callback():
+        return
     global _locks
     num_locks = CRYPTO_num_locks()
     _locks = tuple(threading.Lock() for _ in range(num_locks))