Fix potential deadlock during process fork (#817)

* Fix a deadlock

close_socket acquires _socket_lock, and calling it with the lock held
deadlocks.

On top of this the warning did not prove very useful, as it would also
trigger when the application is sending metrics from another thread
when the process forks, in which case the warning is a bit misleading.

Without the warning there is no reason to check for socket being open,
so just silently call close_socket() after the fork to avoid sharing
the file descriptor between processes if another thread re-opened it
between pre_fork() and fork itself.

* Add test case for post_fork

Author: vickenty

datadog/dogstatsd/base.py

@@ -1370,10 +1370,7 @@ def post_fork(self):
 
         log.debug("[%d] post_fork for %s", os.getpid(), self)
 
-        with self._socket_lock:
-            if self.socket or self.telemetry_socket:
-                log.warning("Open socket detected after fork. Call pre_fork() before os.fork().")
-                self.close_socket()
+        self.close_socket()
 
         self._forking = False
 

tests/unit/dogstatsd/test_statsd.py

@@ -1963,3 +1963,16 @@ def test_max_payload_size(self):
         self.assertEqual(statsd._max_payload_size, UDP_OPTIMAL_PAYLOAD_LENGTH)
         statsd.socket_path = "/foo"
         self.assertEqual(statsd._max_payload_size, UDS_OPTIMAL_PAYLOAD_LENGTH)
+
+    def test_post_fork_locks(self):
+        def inner():
+            statsd = DogStatsd(socket_path=None, port=8125)
+            # Statsd should survive this sequence of events
+            statsd.pre_fork()
+            statsd.get_socket()
+            statsd.post_fork()
+        t = Thread(target=inner)
+        t.daemon = True
+        t.start()
+        t.join(timeout=5)
+        self.assertFalse(t.is_alive())