Fix race condition of Tesla.Mock.mock_global/1

[sircinek]

Problem:
Intermittent test failures when using Tesla.Mock.mock_global/1

      ** (exit) exited in: GenServer.call(#PID<0.16741.0>, {:update, #Function<2.71048325/1 in Tesla.Mock.agent_set/1>}, 5000)
          ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
      code: Tesla.Mock.mock_global(fn
      stacktrace:
        (elixir 1.10.4) lib/gen_server.ex:1023: GenServer.call/3

Current implementation:
agent_set/1 is not atomic, and there's no guarantee pid returned by Process.whereis/1 will be alive when making Agent.update/2 call. That's because Agent process is linked to the test process, but the exit(:shutdown) signal is asynchronous, thus the same process can "leak" between test cases in certain edge cases.

Proposed implementation:
Instead of using plain Agent.start_link/2 use ExUnit.Callbacks.start_supervised!/1, that puts the Tesla.Mock agent under the test supervision tree and guarantees the process is terminated synchronously, thus prevents the leak between test cases.

[teamon]

Thanks!!

[ZeLarpMaster]

Hey there, I was using mock_global in my test_helper.exs because I had to mock a call done by my application on startup (and before tests are executed), but this change broke my tests because start_supervised/2 can only be invoked from the test process and because test_helper.exs isn't a test process. Maybe what I was doing wasn't supported or a good idea, but I was wondering about what's your opinion on that and if you have suggestions on how to solve this issue? Afaik there isn't another mock_global which could be used globally outside tests. Maybe a different mock_global scoped to tests would be better than changing mock_global to be scoped to tests? Not sure on what's your opinion on this.

If you need access, this is the project where I do that and I noticed the issue because of this PR by renovate

[teamon]

@ZeLarpMaster As a quick answer, you can work around this by replacing:

Tesla.Mock.mock_global(myfun)

with

{:ok, _} = Agent.start_link(myfun, name: Tesla.Mock)