Non-fatal exception raised during automatic code reload

[YellowApple]

During the request immediately following a source code change:

08:21:18.469 [info]  GET /

08:21:18.470 [error] #PID<0.1025.0> running Foo.Router terminated
Server: localhost:4000 (http)
Request: GET /
** (exit) an exception was raised:
    ** (Plug.Conn.AlreadySentError) the response was already sent
        (plug) lib/plug/conn.ex:864: Plug.Conn.register_before_send/2
        (splor) lib/foo/router.ex:1: Foo.Router.do_call/2
        (splor) lib/plug/debugger.ex:93: Foo.Router.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

08:21:18.499 [info]  GET /

08:21:18.499 [info]  Sent 200 in 26µs

Router resembles the following:

defmodule Foo.Router do
  use Sugar.Router
  plug Sugar.Plugs.HotCodeReload
  plug Plug.Logger  # Not necessary to reproduce, but logs the requests

  # ...

  get "/", Foo.Controllers.Main, :index
end

Controller resembles the following:

defmodule Foo.Controllers.Main do
  use Sugar.Controller

  def index(conn, _) do
    conn |> render  # or something; this doesn't matter
  end
end

---

My hypothesis is that the exception is coming from a stale connection (namely, the one that initiated the code reload). Totally harmless, but would be worth checking out nonetheless if it's a sign of some optimization issue (it's possible that we might be able to shave a bit more off that rather slow 26-microsecond response time ;) ).

[CharlesOkwuagwu]

I guess its more than that. I should mention, it happens even when my source has not changed, so I don't know it is the re-load feature thats responsible

[YellowApple]

That's interesting. In your case, does the connection still go through (i.e. the page still loads)? Does it happen on every page load? Or is it intermittent.

The issue documented here has been a rather longstanding one. Any new wrinkles might help identify the actual cause.

[CharlesOkwuagwu]

Most pages. And yes, the page still goes through

[dottorblaster]

I can confirm this. Anyone figured it out?

[YellowApple]

@dottorblaster The error should be non-fatal and harmless (page should still load fine) per above. Is this still the case in your situation?

[dottorblaster]

Yeah absolutely harmless but it polls my log 😬

[xFluffyke420]

https://www.techempower.com/benchmarks/#hw=cl&test=plaintext&section=data-r21