PYTHON-2334: Fix gevent.Timeout race condition (#472)

If gevent raises a Timeout during self.lock acquisition, a _socket_semaphore count will be lost.
Using "with" will release the condition even on exception being raised.

Author: TylerWilley

pymongo/pool.py

@@ -1257,12 +1257,15 @@ def _get_socket(self, all_credentials):
         if not self._socket_semaphore.acquire(
                 True, self.opts.wait_queue_timeout):
             self._raise_wait_queue_timeout()
-        with self.lock:
-            self.active_sockets += 1
 
         # We've now acquired the semaphore and must release it on error.
         sock_info = None
+        incremented = False
         try:
+            with self.lock:
+                self.active_sockets += 1
+                incremented = True
+
             while sock_info is None:
                 try:
                     with self.lock:
@@ -1279,8 +1282,10 @@ def _get_socket(self, all_credentials):
                 # We checked out a socket but authentication failed.
                 sock_info.close_socket(ConnectionClosedReason.ERROR)
             self._socket_semaphore.release()
-            with self.lock:
-                self.active_sockets -= 1
+
+            if incremented:
+                with self.lock:
+                    self.active_sockets -= 1
 
             if self.enabled_for_cmap:
                 self.opts.event_listeners.publish_connection_check_out_failed(

pymongo/thread_util.py

@@ -40,22 +40,21 @@ def acquire(self, blocking=True, timeout=None):
             raise ValueError("can't specify timeout for non-blocking acquire")
         rc = False
         endtime = None
-        self._cond.acquire()
-        while self._value == 0:
-            if not blocking:
-                break
-            if timeout is not None:
-                if endtime is None:
-                    endtime = _time() + timeout
-                else:
-                    timeout = endtime - _time()
-                    if timeout <= 0:
-                        break
-            self._cond.wait(timeout)
-        else:
-            self._value = self._value - 1
-            rc = True
-        self._cond.release()
+        with self._cond:
+            while self._value == 0:
+                if not blocking:
+                    break
+                if timeout is not None:
+                    if endtime is None:
+                        endtime = _time() + timeout
+                    else:
+                        timeout = endtime - _time()
+                        if timeout <= 0:
+                            break
+                self._cond.wait(timeout)
+            else:
+                self._value = self._value - 1
+                rc = True
         return rc
 
     __enter__ = acquire