Updating WebSocket headers on every connection attempt

As brought up in #autobahn, currently a WebSocket connection gets its headers from the Factory (via. self.factory.headers). However, a Factory may make many connections and it is desirable to have a hook to change the headers for each connection (for example, when doing re-connections).

One way would be to mutate the .headers attribute in the Factory. This will “probably” work as there is likely only one “active” protocol instance created per-factory. However, it’s not very explicit.

@exarkun suggested: "maybe something like having WebSocketClientProtocol.onConnect allowed to return a ConnectionRequest (or similar) that can specify headers?

[meejah]

That would be a neat way to make backpressure work, yeah. I just mean that oberstet’s point is good: we’d want to profile it well first to make sure that doing that doesn’t slow down “most code which is using onMessage synchronously”

[meejah]

Should onConnecting handle async / future-returning overrides? (If so, it would need to delay starting the handshake until any future resolved). For consistency, I think it probably should (i.e. in Python3 then everything can be async def on*() for all the various “user callbacks”).