Confusing error message when test finishes before expectation is called

[axelson]

In the case where:

• You set the expectation in the test process
• A task (with proper $callers) is started from the test process
• The test process finishes before the task

A confusing error is raised. The error says that "no expectation defined for <mock> in process (or its callers)", but the given expectation was set in that processes callers. Here's an example error output for this case:

> mix test
set expectation in #PID<0.209.0> (mox_repro_test.exs:21): WeatherBehaviourMock
Inside task (#PID<0.210.0>)
test finished
pids (mox_repro_test.exs:10): [#PID<0.210.0>]
calling mock
calling get_weather!

18:47:17.203 [error] Task #PID<0.210.0> started from #PID<0.209.0> terminating
** (Mox.UnexpectedCallError) no expectation defined for WeatherBehaviourMock.get_weather/1 in process #PID<0.210.0> (or in its callers [#PID<0.209.0>]) with args ["Chicago"]
    (mox 1.1.0) lib/mox.ex:820: Mox.__dispatch__/4
    (elixir 1.15.7) lib/task/supervised.ex:101: Task.Supervised.invoke_mfa/2
Function: #Function<0.87561592/0 in Bound.start_weather_task/1>
    Args: []

  1) test fails (MoxReproTest)
     test/mox_repro_test.exs:22
     Assertion with in failed
     code:  assert reason in [:normal, :noproc]
     left:  {%Mox.UnexpectedCallError{message: "no expectation defined for WeatherBehaviourMock.get_weather/1 in process #PID<0.210.0> (or in its callers [#PID<0.209.0>]) with args [\"Chicago\"]"}, [{Mox, :__dispatch__, 4, [file: ~c"lib/mox.ex", line: 820]}, {Task.Supervised, :invoke_mfa, 2, [file: ~c"lib/task/supervised.ex", line: 101]}]}
     right: [:normal, :noproc]
     stacktrace:
       (elixir 1.15.7) lib/enum.ex:984: Enum."-each/2-lists^foreach/1-0-"/2
       test/mox_repro_test.exs:11: anonymous fn/0 in MoxReproTest.__ex_unit_setup_0/1
       (ex_unit 1.15.7) lib/ex_unit/on_exit_handler.ex:143: ExUnit.OnExitHandler.exec_callback/1
       (ex_unit 1.15.7) lib/ex_unit/on_exit_handler.ex:129: ExUnit.OnExitHandler.on_exit_runner_loop/0

Finished in 0.5 seconds (0.00s async, 0.5s sync)
1 test, 1 failure

Here's a git repo that reproduces the error: https://github.com/axelson/mox_repro

I have a suspicion that what is happening is that Mox is seeing that the test process has finished and clearing out the expectations.

Instead my ideal behavior would be to either keep the expectation valid or modify the error message to say that expectations against now dead processes are not valid.

[josevalim]

Because the process is dead, we really can't know if it is because there are no expectations defined or because its callers crashed. So the best we can do is to add more information to the error message saying that all of its callers are dead. And perhaps say that "no expectations found" instead of "no expectations defined". WDYT @whatyouhide?

[whatyouhide]

Yeah there's not much we can do here, agreed. When the test process dies, we need to clear its expectations, so the task won't find any in its callers. I think the best we can do is to (maybe significantly) improve the error message. @axelson wanna make a PR? 🙃

[axelson]

I am interested in creating a PR for this, I've started one but haven't finished it yet.

[whatyouhide]

@axelson fantastic! Let us know if we can help. Would love a PR absolutely 🙃