Keeping self() straight in on_exit callbacks

An on_exit callback looks like it belongs to the test that registered it, but self() inside the callback is not the test PID:

test_pid = self()

on_exit(fn ->
  IO.inspect(self() == test_pid)
end)

#=> false

ExUnit runs exit callbacks in a separate process after the test process exits. It reminds me of debugging JavaScript’s this: the name is familiar, but you still need to check which execution context you’re actually in.

Capture the test PID before registering identity-sensitive cleanup:

test_pid = self()

on_exit(fn ->
  MyRegistry.release(test_pid)
end)

Capturing the PID does not keep the test process alive. Use it only when the cleanup API needs the original PID as an ownership key; you cannot send work back to the finished test. ExUnit also stops processes started with start_supervised/2 before it runs on_exit callbacks.

https://hexdocs.pm/ex_unit/ExUnit.Callbacks.html#on_exit/2