Assert one process gets message from another

Erlang has a very useful-for-testing function :erlang.trace/3, that can serve as a window into all sorts of behaviour.

In this case I want to test that one process sent a message to another. While it's always best to test outputs rather than implementation, when everything is asynchronous and paralleized you might need some extra techniques to verify your code works right.

pid_a =
  spawn(fn ->
    receive do
      :red -> IO.puts("got red")
      :blue -> IO.puts("got blue")
    end
  end)

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

spawn(fn ->
  send(pid_a, :blue)
end)

assert_receive({:trace, captured_pid, :receive, captured_message})

assert captured_pid == pid_a
assert :blue == captured_message

In the above example we setup a trace on receive for the first process:

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

Now, the process that called trace will receive a message whenever traced process receives a message. That message will look like this:

{
  :trace,
  pid_that_received_the_message,
  :receive, # the action being traced
  message_that_was_received
}

This in combination with assert_receive allows you to test that the test process receives the trace message.