The Marionette protocol is designed to facilitate concurrent,
asynchronous messages. Each request message includes a message
ID, and the corresponding response includes the same message ID. This
allows several requests to be in flight at once. In order for this to
be useful, the client needs to maintain a record of each request it has
sent so that it knows how to handle responses, even if they arrive out
of order.
To implement this functionality in *mqttmarionette*, the
`MarionetteConnection` structure spawns a Tokio task to handle all
incoming messages from the server. When a message arrives, its ID is
looked up in a registry that maps message IDs to Tokio "oneshot"
channels. If a channel is found in the map, the response is sent back
to the caller through the channel.
In order to handle incoming messages in a separate task, the TCP stream
has to be split into its read and write parts. The receiver task cannot
be spawned, though, until after the first unsolicited message is read
from the socket, since a) there is no caller to send the message back to
and b) it does not follow the same encoding scheme as the rest of the
Marionette messages. As such, I've refactored the
`MarionetteConnection` structure to handle the initial message in the
`connect` function and dropped the `handshake` method. A new
`start_session` method is responsible for initiating the Marionette
session.
Dustin