build HTTP server device

[warner]

For interaction with UIs and CLI-based tools, we need a way to send messages into a machine. Not everything is a swingset machine, so we should enable some sort of lower-tech protocol.

I'm thinking the device node should use Node's built-in HTTP server, and the root object should provide a registerHandler function. Each time an HTTP request arrives, the handler will receive a message that includes the full contents of the request as arguments, plus a "response index" or handle of some sort. Calling a second function on the root object with the response index causes the HTTP request to be answered. We can then wrap this in a Vat which does some minimal dispatch/routing, and presents a promise-based interface (so the real target of the message can trigger a response by just fulfilling their response promise to the HTTP response data).

It might also be interesting to add an HTTP client device, but that's lower priority for now.

[warner]

0cfc866 adds the "command" device, which is the part that belongs in SwingSet. It gives a way to send commands into a swingset machine, returning a promise for the result. Internally, each command is given a number, and the recipient of the command is responsible for calling D(devices.command).sendResponse(number, value) when that promise ought to be resolved. There's also a broadcast channel which we need for the REPL frontend.

So I think I can close this now. For an example of how the command device should be used to provide an HTTP-based invocation tool, look in cosmic-swingset at lib/ag-solo/web.js and start.js.

[warner]

in the old repo. this was SwingSet issue 46