This change blocks off 2 common scenarios where a race condition is
upgraded to a hang, when the library could internally do better.
* Since we don't know whether the receiver of a `reply_to` is expecting
a raw or pickled message, and since in the case of a raw reply, there
is no way to signal "dead" to the receiver, override the reply_to
field to explicitly mark a message as dead using a special handle.
This replaces the serialized _DEAD sentinel value with a slightly
neater interface, in the form of the reserved IS_DEAD handle, and
enables an important subsequent change: when a context cannot route a
message, it can send a generic 'dead' reply back towards the message
source, ensuring any sleeping thread is woken with ChannelError.
The use of this field could potentially be extended later on if
additional flags are needed, but for now this seems to suffice.
* Teach Router._invoke() to reply with a dead message when it receives a
message for an invalid local handle.
* Teach Router._async_route() to reply with a dead message when it
receives an unroutable message.
e.g. assert x == y -> self.assertEqual(x, y);
self.assertTrue(isinstance(x, y)) -> self.assertIsInstance(x, y)
These specific methods give more useful errors in the case of a test
failure.
Now there is a separate SHUTDOWN message that relies only on being
received by the broker thread, the main thread can be hung horribly and
the process will still eventually receive a SIGTERM.
* Support passing Context() objects in function calls and return values.
Now the fakessh demo from the documentation index would work
correctly.
* Since slaves can communicate with each other now, they should also use
the same approach to unpickling as the master already used. Collapse
away all the unpickle extension crap and hard-wire just the 3 types
that support unpickling.
David Wilson