Background

I have an umbrella project, where I run mix test from the root.
In one of the apps, I am mocking the File module using the Mock library.

Problem

The issue here is that when I run mix test the process dies, with no error message to show:

Manager.Impl.Store.ReaderTest [test/unit/store/reader_test.exs]
  * test list_syndicates/1 returns the list of all known syndicates [L#496]** (EXIT from #PID<0.98.0>) killed

Code

The code of the test is as follows:

defmodule Manager.Impl.Store.ReaderTest do
  use ExUnit.Case, async: false

  alias Manager.Impl.Store.Reader

  import Mock

  setup do
    %{
      file_io: File,
      paths: [syndicates: ["syndicates.json"]]
    }
  end

  describe "lists syndicates" do
    test_with_mock "returns the list of all known syndicates", %{paths: paths} = deps, File, [],
      read: fn _filename -> {:ok, "[\"utc\"]"} end do
      # Act
      actual = Reader.list_syndicates(deps)

      expected = {:ok, [%Syndicate{name: "UTC", id: :utc, catalog: []}]}

      expected_path = Path.join(paths[:syndicates])

      # Assert
      assert actual == expected
      assert_called(File.read(expected_path))
    end
  end
end

In comparison, the follow test (which does not use mock) works just fine:

defmodule Manager.Impl.Store.ReaderTest do
  @moduledoc false

  use ExUnit.Case, async: false

  alias Manager.Impl.Store.Reader

  import Mock

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

  describe "list_syndicates/1" do
    defmodule FileMockListSyndicates do
      @moduledoc false

      def read(path) do
        assert path == "syndicates.json"
        {:ok, "[\"utc\"]"}
      end
    end

    setup do
      %{
        file_io: Manager.Impl.Store.ReaderTest.FileMockListSyndicates
      }
    end

    test "returns the list of all known syndicates",
         %{paths: paths} = deps do
      # Act
      actual = FileSystem.list_syndicates(deps)
      expected = {:ok, [%Syndicate{name: "UTC", id: :utc, catalog: []}]}

      # Assert
      assert actual == expected
    end
  end
end

To me this is rather surprising. One alternative crashes the process with no error message, while the other makes everything work.

To me, this indicates one of three problems:

1. A problem with the library Mock
2. A problem with my setup of the library
3. A problem with the test that causes the process to crash

I believe the second and third options to be the most probable, but without any information about the error, I can’t be sure. The process simply dies.

Question

Why is my process dying, and how can I fix it?