core: fix monster fork FD leak

_sockets only refers to the idle sockets list, it doesn't refer to every
socket currently in use by a Latch, for example, the 2*16 used by e.g.
Ansible's sleeping service pool.

mitogen/core.py

@@ -981,6 +981,7 @@ class Latch(object):
     closed = False
     _waking = 0
     _sockets = []
+    _allsockets = []
 
     def __init__(self):
         self._lock = threading.Lock()
@@ -989,10 +990,9 @@ class Latch(object):
 
     @classmethod
     def _on_fork(cls):
-        while cls._sockets:
-            rsock, wsock = cls._sockets.pop()
-            rsock.close()
-            wsock.close()
+        cls._sockets = []
+        while cls._allsockets:
+            cls._allsockets.pop().close()
 
     def close(self):
         self._lock.acquire()
@@ -1015,6 +1015,7 @@ class Latch(object):
             rsock, wsock = socket.socketpair()
             set_cloexec(rsock.fileno())
             set_cloexec(wsock.fileno())
+            self._allsockets.extend((rsock, wsock))
             return rsock, wsock
 
     def get(self, timeout=None, block=True):