issue #155: core: refactor Latch to avoid TLS use

TLS destructors are not called after fork, therefore we must explicitly
track a global list of free file descriptors, and arrange for that list
to explicitly be destroyed from fork.py.

mitogen/core.py

@@ -899,12 +899,20 @@ def _unpickle_context(router, context_id, name):
 class Latch(object):
     closed = False
     _waking = 0
+    _sockets = []
 
     def __init__(self):
         self._lock = threading.Lock()
         self._queue = []
         self._sleeping = []
 
+    @classmethod
+    def _on_fork(cls):
+        while cls._sockets:
+            rsock, wsock = cls._sockets.pop()
+            rsock.close()
+            wsock.close()
+
     def close(self):
         self._lock.acquire()
         try:
@@ -919,10 +927,12 @@ class Latch(object):
         return len(self._queue) == 0
 
     def _tls_init(self):
-        if not hasattr(_tls, 'rsock'):
-            _tls.rsock, _tls.wsock = socket.socketpair()
-            set_cloexec(_tls.rsock.fileno())
-            set_cloexec(_tls.wsock.fileno())
+        if self._sockets:
+            return self._sockets.pop()
+        rsock, wsock = socket.socketpair()
+        set_cloexec(rsock.fileno())
+        set_cloexec(wsock.fileno())
+        return rsock, wsock
 
     def get(self, timeout=None, block=True):
         _vv and IOLOG.debug('%r.get(timeout=%r, block=%r)',
@@ -938,30 +948,31 @@ class Latch(object):
                 return self._queue.pop(i)
             if not block:
                 raise TimeoutError()
-            self._tls_init()
-            self._sleeping.append(_tls.wsock)
+            rsock, wsock = self._tls_init()
+            self._sleeping.append(wsock)
         finally:
             self._lock.release()
 
-        return self._get_sleep(timeout, block)
+        return self._get_sleep(timeout, block, rsock, wsock)
 
-    def _get_sleep(self, timeout, block):
+    def _get_sleep(self, timeout, block, rsock, wsock):
         _vv and IOLOG.debug('%r._get_sleep(timeout=%r, block=%r)',
                             self, timeout, block)
         e = None
         try:
-            io_op(select.select, [_tls.rsock], [], [], timeout)
+            io_op(select.select, [rsock], [], [], timeout)
         except Exception, e:
             pass
 
         self._lock.acquire()
         try:
-            i = self._sleeping.index(_tls.wsock)
+            i = self._sleeping.index(wsock)
             del self._sleeping[i]
+            self._sockets.append((rsock, wsock))
             if i >= self._waking:
                 raise TimeoutError()
             self._waking -= 1
-            if _tls.rsock.recv(2) != '\x7f':
+            if rsock.recv(2) != '\x7f':
                 raise LatchError('internal error: received >1 wakeups')
             if e:
                 raise e