Let ExUnit shut down your test processes

Start test-owned processes with start_supervised!/1 so ExUnit waits for them to shut down before running your cleanup callbacks.

test "stops the worker before cleanup" do
  pid = start_supervised!({Agent, fn -> %{} end})

  on_exit(fn ->
    refute Process.alive?(pid)
  end)

  assert Agent.get(pid, & &1) == %{}
end

ExUnit stops the test supervisor and its children before running on_exit. The assertion inside the callback demonstrates that ordering; you do not need to add it to every test.

Starting a worker directly with start_link gives you a link, but shutdown through that link is asynchronous. The test process can exit before the worker finishes stopping. Cleanup that removes files or releases resources the worker still uses can race with it.

Use the test supervisor to make that boundary explicit. It accepts a child specification or a {Module, arguments} tuple, and start_supervised! returns the child PID.

This guarantee covers children managed by the test supervisor. A process spawned independently elsewhere still needs its own lifecycle management.

ExUnit.Callbacks.start_supervised/2