Mock is crashing process in umbella project

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Mock depends on :meck, which does swap out the complete module
within the runtime – as in make the VM unload the existing module and
load the module with the mock code. (…) The best improvement to get
would be better errors or disclaimers.

Source: Mock is crashing process in umbella project - #4 by LostKobrakai - Questions / Help - Elixir Programming Language Forum

At the time of this writing, I have tested all major mocking libraries (Mock, Mox and Mimic).
Both Mock and Mimic rely on :meck underneath. Rather surprisingly, I get the exact same issue with Mimic as I got with Mock, i.e., the process crashes without warning.

This leaves me to the only logical conclusion, which is that the problem I am facing is related to the underlying system that servers as a base for both libraries: :meck. This issue does not happen using Mox.

I have therefore decided not to use Mock (nor Mimic) for the application and I am instead injecting the dependencies directly into the functions that need them. This last approach is very lightweight and does allow for async: true which gives a noticeable speed increase when running mix test.

Because I am only using a very small portion of the external system’s API (2 - 3 functions) this fits well with my needs. However, If I were to use all functions from said API (let’s say 20) this solution would be rather difficult to manage.

I don’t expect the affected modules to evolve in such a direction, so for the time being, I am rather happy.

Here is a sample test for those searching for inspiration (using File as an example):

setup do
  %{
    paths: [products: ["products.json"]]
  }
end

test "returns list of available products from given syndicate", %{paths: paths} = deps do
  read_fn = fn filename ->
    assert filename == Path.join(paths[:products])

    {:ok, "[{\"name\": \"utc\"}]"}
  end

  deps = Map.put(deps, :io, %{read: read_fn})
  syndicate = Syndicate.new(name: "UTC", id: :utc)

  assert FileSystem.list_products(syndicate, deps) ==
           {:ok, [Product.new(%{"name" => "Bananas"})]}
end
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