core: throw error on duplicate add_handler(); closes #447.

docs/changelog.rst

@@ -304,6 +304,10 @@ Core Library
   have dead messages sent in reply to them, preventing peer contexts from
   hanging due to a forgotten buffered message.
 
+* `#447 <https://github.com/dw/mitogen/issues/447>`_: duplicate attempts to
+  invoke :meth:`mitogen.core.Router.add_handler` cause an error to be raised,
+  ensuring accidental re-registration of service pools are reported correctly.
+
 * `#453 <https://github.com/dw/mitogen/issues/453>`_: the loggers used in
   children for standard IO redirection have propagation disabled, preventing
   accidental reconfiguration of the :mod:`logging` package in a child from

mitogen/core.py

@@ -2321,7 +2321,8 @@ class Router(object):
             self._handles_by_respondent[respondent].discard(handle)
 
     def add_handler(self, fn, handle=None, persist=True,
-                    policy=None, respondent=None):
+                    policy=None, respondent=None,
+                    overwrite=False):
         """
         Invoke `fn(msg)` on the :class:`Broker` thread for each Message sent to
         `handle` from this context. Unregister after one invocation if
@@ -2367,12 +2368,19 @@ class Router(object):
             nonzero, a :class:`mitogen.core.CallError` is delivered to the
             sender indicating refusal occurred.
 
+        :param bool overwrite:
+            If :data:`True`, allow existing handles to be silently overwritten.
+
         :return:
             `handle`, or if `handle` was :data:`None`, the newly allocated
             handle.
+        :raises Error:
+            Attemp to register handle that was already registered.
         """
         handle = handle or next(self._last_handle)
         _vv and IOLOG.debug('%r.add_handler(%r, %r, %r)', self, fn, handle, persist)
+        if handle in self._handle_map and not overwrite:
+            raise Error(self.duplicate_handle_msg)
 
         self._handle_map[handle] = persist, fn, policy, respondent
         if respondent:
@@ -2384,6 +2392,7 @@ class Router(object):
 
         return handle
 
+    duplicate_handle_msg = 'cannot register a handle that is already exists'
     refused_msg = 'refused by policy'
     invalid_handle_msg = 'invalid handle'
     too_large_msg = 'message too large (max %d bytes)'

mitogen/parent.py

@@ -1646,12 +1646,14 @@ class RouteMonitor(object):
             handle=mitogen.core.ADD_ROUTE,
             persist=True,
             policy=is_immediate_child,
+            overwrite=True,
         )
         self.router.add_handler(
             fn=self._on_del_route,
             handle=mitogen.core.DEL_ROUTE,
             persist=True,
             policy=is_immediate_child,
+            overwrite=True,
         )
         #: Mapping of Stream instance to integer context IDs reachable via the
         #: stream; used to cleanup routes during disconnection.

tests/router_test.py

@@ -180,12 +180,36 @@ class CrashTest(testlib.BrokerMixin, testlib.TestCase):
 class AddHandlerTest(testlib.TestCase):
     klass = mitogen.master.Router
 
-    def test_invoked_at_shutdown(self):
+    def test_dead_message_sent_at_shutdown(self):
         router = self.klass()
         queue = Queue.Queue()
         handle = router.add_handler(queue.put)
         router.broker.shutdown()
         self.assertTrue(queue.get(timeout=5).is_dead)
+        router.broker.join()
+
+    def test_cannot_double_register(self):
+        router = self.klass()
+        try:
+            router.add_handler((lambda: None), handle=1234)
+            e = self.assertRaises(mitogen.core.Error,
+                lambda: router.add_handler((lambda: None), handle=1234))
+            self.assertEquals(router.duplicate_handle_msg, e.args[0])
+            router.del_handler(1234)
+        finally:
+            router.broker.shutdown()
+            router.broker.join()
+
+    def test_can_reregister(self):
+        router = self.klass()
+        try:
+            router.add_handler((lambda: None), handle=1234)
+            router.del_handler(1234)
+            router.add_handler((lambda: None), handle=1234)
+            router.del_handler(1234)
+        finally:
+            router.broker.shutdown()
+            router.broker.join()
 
 
 class MessageSizeTest(testlib.BrokerMixin, testlib.TestCase):