Add an assert_receive to ExUnit that allows specifying which PID received the message

[Adam Lancaster]

It would be great if we could expose a function that allowed us to say something like this:

defmodule MyGen do
  use GenServer

  @impl true
  def init(data) do
    {:ok, data}
  end

end

{:ok, pid} = MyGen.start_link(MyGen, :hi)

assert_receive(pid, :hello)

I recently learned you can do something like this:

:erlang.trace(pid, true, [:receive])

So I wondered if that or :erlang.trace_delivered may be helpful here.

Best

Adam

[Aaron Ross]

`:erlang.trace/3` allows a process to trace _all_ other process message send/receives by passing `:processes` as the first argument. Depending on how much overhead that adds, this could be called when any test process that includes a `assert_pid_receive` is started. I'm not familiar with ExUnit's internals, but the implementation could look something like

```elixir

# somewhere during setup

Module.register_attribute(__MODULE__, :needs_trace, accumulate: true)

defmacro assert_pid_receive(pid, message, timeout \\ 5000) do

  quote do

    Module.put_attribute(__MODULE__, :needs_trace, __EX_UNIT_TEST_NAME__)

    receive do

      {:trace, unquote(pid), :receive, unquote(message)} ->

        :ok

    after

      unquote(timeout) ->

        {:error, :timeout}

end

# somewhere in test process bootstrap

if (__EX_UNIT_TEST_NAME__ in @needs_trace) do

  :erlang.trace(:processes, true, [:receive])

end

```

[Aaron Ross]

# somewhere during setup
Module.register_attribute(__MODULE__, :needs_trace, accumulate: true)

defmacro assert_pid_receive(pid, message, timeout \\ 5000) do
  quote do
    Module.put_attribute(__MODULE__, :needs_trace, __EX_UNIT_TEST_NAME__)
    receive do
      {:trace, unquote(pid), :receive, unquote(message)} ->
        :ok
    after
      unquote(timeout) ->
        {:error, :timeout}
  end
end

# somewhere in test process bootstrap
if (__EX_UNIT_TEST_NAME__ in @needs_trace) do
  :erlang.trace(:processes, true, [:receive])
end

fixed the formatting

On Tuesday, June 2, 2020 at 1:26:40 PM UTC-7, Adam Lancaster wrote: