Ecto.Adapters.SQL.Sandbox and GenServer restarting with new PID

An approach I’ve used is passing the test process’s PID along when starting the child in DynamicSupervisor.

The GenServer then calls Ecto.Adapters.SQL.Sandbox.allow(YourRepo, parent_pid, self())

I don’t know if there’s an “official” way to ask the repo “are you sandbox”; there wasn’t when I wrote the code using this feature, so I did it by checking Application.fetch_env!(:your_app_name, YourRepo)[:pool] - if it’s the atom Ecto.Adapters.SQL.Sandbox, then the sandbox is enabled.

While your GenServer is doing that, another useful thing to add is Process.monitor(parent_pid). That allows avoiding two race-for-the-flag scenarios that spam the logs and crash unexpectedly:

• if the test process exits but the GenServer is still running, subsequent Repo interactions will raise an error. Once you’re monitoring parent_pid, the GenServer can take appropriate action when the :DOWN message arrives. Usually that action is to return :stop, but you may also have cleanup to do.
• if the GenServer crashes at just the wrong time, the DynamicSupervisor will restart it after the test process has already exited. Again, this will cause any Repo interaction in the GenServer to fail.

One other note: make sure you’ve tuned the restart parameters of DynamicSupervisor correctly, especially when running fast tests in parallel. Nothing more frustrating than trying to figure out why your tests fail randomly, but only when all run together, and only on a fast machine…

Make your genserver set :$callers key in process dictionary and Ecto sandbox should be able to figure ownership. More info on caller tracking: Task — Elixir v1.20.2

basically, you’ll have something like this:

defmodule MyGenserver do
  use GenServer

  def start_link(arg) do
    callers = Process.get(:"$callers", [])
    GenServer.start_link(__MODULE__, {arg, callers})
  end

  def init({arg, callers}) do
    Process.put(:"$callers", [self() | callers])
    ...
  end
end

This is my preferred approach as well, and it works for tests because :"$callers" is already set in the test process

that’s a really elegant solution, thanks!