[5pt,9pt] HTTP block

[RalphSteinhagen]

Implement a GR 4.0 Block, that can provide its values over a HTTP REST API.

For network calls, the native client will use httplib, the emscripten build will use emscripten’s own Fetch API.

The idea is that the Block will post a GET request to some server, which will then return a response (in the first stage, only raw messages and mime-type export are implemented (i.e. mime-type application/octet-stream, later on text/yaml and other serialized formats can follow). The Block implementation will then provide the decoded values on its output port(s). For the raw message mime-type, this means one output port OutPort<pmt> with keys mime-type and raw-data.

There are a few different ways how to tell the Block when to do a GET request. For example, one idea is to send a GET request every time activity happens on an input port (for now, use InPort<uint8_t>). If the Block is supposed to act in an interrupt-style way, long-polling can be used. An input port can be used to signal to the Block whether the subscription should be kept active. The long-polling should work like the already existing OpenCMW schema and should have transparent caching proxy capabilities so that clients subscribing to the same feed can be served efficiently.

The URI of the remote server should be changeable at runtime. This requires implementing the settings_changed(...), start() and stop() methods.

Since HTTP calls may be unreliable (the server may stop responding altogether), a circuit breaker pattern could be used to avoid flooding the server with too many calls in such a scenario and to automatically start allowing calls after some time when the server is up again.