issue #533: update routing to account for DEL_ROUTE propagation race

docs/changelog.rst

@@ -198,6 +198,18 @@ Core Library
   `closed` flag, preventing historical bugs where a double close could destroy
   descriptors belonging to unrelated streams.
 
+* `#533 <https://github.com/dw/mitogen/issues/533>`_: routing accounts for
+  a race between a parent sending a message to a child via an intermediary,
+  where the child had recently disconnected, and ``DEL_ROUTE`` propagating from
+  the intermediary to the parent, informing it that the child no longer exists.
+  This condition is detected at the intermediary and a dead message is returned
+  to the parent.
+
+  Previously since the intermediary had already removed its route for the
+  child, the *route messages upwards* rule would be triggered, causing the
+  message (with a privileged ``src_id``/``auth_id``) to be sent upstream,
+  resulting in a ``bad auth_id`` log message and a hang.
+
 * `#586 <https://github.com/dw/mitogen/issues/586>`_: fix import of
   :mod:`__main__` on later versions of Python 3 when running from the
   interactive console.

mitogen/core.py

@@ -3272,34 +3272,48 @@ class Router(object):
             ))
             return
 
-        # Perform source verification.
+        parent_stream = self._stream_by_id.get(mitogen.parent_id)
+        src_stream = self._stream_by_id.get(msg.src_id, parent_stream)
+
+        # When the ingress stream is known, verify the message was received on
+        # the same as the stream we would expect to receive messages from the
+        # src_id and auth_id. This is like Reverse Path Filtering in IP, and
+        # ensures messages from a privileged context cannot be spoofed by a
+        # child.
         if in_stream:
-            parent = self._stream_by_id.get(mitogen.parent_id)
+            auth_stream = self._stream_by_id.get(msg.auth_id, parent_stream)
-            expect = self._stream_by_id.get(msg.auth_id, parent)
+            if in_stream != auth_stream:
-            if in_stream != expect:
                 LOG.error('%r: bad auth_id: got %r via %r, not %r: %r',
-                          self, msg.auth_id, in_stream, expect, msg)
+                          self, msg.auth_id, in_stream, auth_stream, msg)
                 return
 
-            if msg.src_id != msg.auth_id:
+            if msg.src_id != msg.auth_id and in_stream != src_stream:
-                expect = self._stream_by_id.get(msg.src_id, parent)
-                if in_stream != expect:
                 LOG.error('%r: bad src_id: got %r via %r, not %r: %r',
-                              self, msg.src_id, in_stream, expect, msg)
+                          self, msg.src_id, in_stream, src_stream, msg)
                 return
 
+            # If the stream's MitogenProtocol has auth_id set, copy it to the
+            # message. This allows subtrees to become privileged by stamping a
+            # parent's context ID. It is used by mitogen.unix to mark client
+            # streams (like Ansible WorkerProcess) as having the same rights as
+            # the parent.
             if in_stream.protocol.auth_id is not None:
                 msg.auth_id = in_stream.protocol.auth_id
 
-            # Maintain a set of IDs the source ever communicated with.
+            # Record the IDs the source ever communicated with.
             in_stream.protocol.egress_ids.add(msg.dst_id)
 
         if msg.dst_id == mitogen.context_id:
             return self._invoke(msg, in_stream)
 
         out_stream = self._stream_by_id.get(msg.dst_id)
-        if out_stream is None:
+        if (not out_stream) and (parent_stream != src_stream or not in_stream):
-            out_stream = self._stream_by_id.get(mitogen.parent_id)
+            # No downstream route exists. The message could be from a child or
+            # ourselves for a parent, in which case we must forward it
+            # upstream, or it could be from a parent for a dead child, in which
+            # case its src_id/auth_id would fail verification if returned to
+            # the parent, so in that case reply with a dead message instead.
+            out_stream = parent_stream
 
         if out_stream is None:
             self._maybe_send_dead(True, msg, self.no_route_msg,

tests/router_test.py

@@ -76,6 +76,34 @@ class SourceVerifyTest(testlib.RouterMixin, testlib.TestCase):
         expect = 'bad auth_id: got %r via' % (self.child2_msg.auth_id,)
         self.assertTrue(expect in log.stop())
 
+    def test_parent_unaware_of_disconnect(self):
+        # Parent -> Child A -> Child B. B disconnects concurrent to Parent
+        # sending message. Parent does not yet know B has disconnected, A
+        # receives message from Parent with Parent's auth_id, for a stream that
+        # no longer exists.
+        c1 = self.router.local()
+        strm = self.router.stream_by_id(c1.context_id)
+        recv = mitogen.core.Receiver(self.router)
+
+        self.broker.defer(lambda:
+            strm.protocol._send(
+                mitogen.core.Message(
+                    dst_id=1234,  # nonexistent child
+                    handle=1234,
+                    src_id=mitogen.context_id,
+                    reply_to=recv.handle,
+                )
+            )
+        )
+
+        e = self.assertRaises(mitogen.core.ChannelError,
+            lambda: recv.get().unpickle()
+        )
+        self.assertEquals(e.args[0], self.router.no_route_msg % (
+            1234,
+            c1.context_id,
+        ))
+
     def test_bad_src_id(self):
         # Deliver a message locally from child2 with the correct auth_id, but
         # the wrong src_id.