Ignore already sent responses

[SteffenDE]

Closes #247.

Since Plug does not expose the full status list of what is considered already sent, rescuing is cleanest, I think.

[josevalim]

We should be able to check for the conn.state accordingly, no? We will also need a new release

[josevalim]

Wait, this is a bug in plug perhaps

[SteffenDE]

Plug uses

@unsent [:unset, :set, :set_chunked, :set_file]

to determine when to raise already sent, so yes we could check conn.state, but then we'd duplicate that logic.

For the websock upgrade, the state is :upgraded.

[josevalim]

Yeah, this is a bug in Plug. We added run_before_send(conn, :upgraded) but it is running after set. We should do run_before_send(conn, :set_upgrade) and then update it for upgraded.

[SteffenDE]

I also found phoenixframework/phoenix#6741, which might also be solved on the Plug side? I think conn.status should be 101 after upgrade, but right now it is nil.

[SteffenDE]

elixir-plug/plug#1320

[SteffenDE]

Closing in favor of upstream fix!