Mox mocks not working for other processes

,

Background

I have a small app where I have a pool that creates several workers. The pool acts as a supervisor for said workers.

Both (the pool and the workers) have a dependency on another module, the ExRegistry. This module registers any process that provides a key.

Test

While using Mox I want to make sure the worker is doing its job correctly, I don’t really want to test the registry just yet. So I stub the RegistryMock with the real one:

stub(RegistryMock, :via_tuple, &ExRegistry.via_tuple/1)
{:ok, _pid} = Worker.start_link({1})

Problem

The problem here is that the pool fails to start:

 Could not start application mox_issue: MoxIssue.Application.start(:normal, []) returned an error: shutdown: failed to start child: MoxIssue.Pool
    ** (EXIT) an exception was raised:
        ** (Mox.UnexpectedCallError) no expectation defined for MoxIssue.RegistryMock.via_tuple/1 in process #PID<0.163.0>

I think this happens because the pool is another process, which also needs access to the RegistryMock but Mox fails to give it access. To test this I set the options on Mox for global so all processes can use the mocks, but it still fails with the same error.

Question

How does Mox behave with multiple processes requiring the same Mock?

1 Like

A quick scan of the source code suggests to me that expectations are registered by process id - if that is true the same expectation would have to registered by/for the pool process.

So, how would I make this test pass? It would be impossible unless I override the global_owner_pid and that’s impossible/not a good idea.

Perhaps my approach to testing this concept is wrong? How would you do it?

Mox supports a global mode.

This approach seems to work (when setup :set_mox_global doesn’t):

defmodule MockShare do
  import Mox

  def init(args) do
    MyApp.CalcMock
    |> expect(:add, fn x, y -> x + y end)
    |> expect(:mult, fn x, y -> x * y end)

    {:ok, args}
  end

  def handle_call({:share, pid}, _from, state) do
    Mox.allow(MyApp.CalcMock, self(), pid)
    {:reply, :ok, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def share(share_id, pid),
    do: GenServer.call(share_id, {:share, pid})
end

defmodule MyAppTest do
  use ExUnit.Case, async: true

  import Mox

  def share_mock(_context) do
    {:ok, share_pid} = start_supervised(MockShare)

    [share_pid: share_pid]
  end

  setup_all :share_mock
  setup :verify_on_exit!

  test "Use add expectation in task", context do
    MockShare.share(context.share_pid, self())

    task = Task.async(fn -> MyApp.CalcMock.add(2, 3) end)
    result = Task.await(task)
    assert result == 5
  end

  test "Use mult expectation in task", context do
    MockShare.share(context.share_pid, self())

    task = Task.async(fn -> MyApp.CalcMock.mult(2, 3) end)
    result = Task.await(task)
    assert result == 6
  end
end

Mox.allow/3


The only way I could get global mode to work:

defmodule MyAppTest do
  use ExUnit.Case

  import Mox

  # setup :set_mox_global - doesn't work

  setup_all do
    expect(MyApp.CalcMock, :add, fn x, y -> x + y end)
    expect(MyApp.CalcMock, :mult, fn x, y -> x * y end)
    set_mox_global()
    %{}
  end

  test "Use add expectation in task" do
    task = Task.async(fn -> MyApp.CalcMock.add(2, 3) end)
    result = Task.await(task)
    assert result == 5
  end

  test "Use mult expectation in task" do
    task = Task.async(fn -> MyApp.CalcMock.mult(2, 3) end)
    result = Task.await(task)
    assert result == 6
  end
end

The way I understand the documentation this should only work in global mode …

defmodule MyAppTest do
  use ExUnit.Case, async: true

  import Mox

  setup [:set_mox_private, :verify_on_exit!]

  def add,
    do: MyApp.CalcMock.add(2, 3)

  def mult,
    do: MyApp.CalcMock.mult(2, 3)

  test "Use add expectation in task" do
    expect(MyApp.CalcMock, :add, fn x, y -> x + y end)
    task = Task.async(&add/0)
    result = Task.await(task)
    assert result == 5
  end

  test "Use mult expectation in task" do
    expect(MyApp.CalcMock, :mult, fn x, y -> x * y end)
    task = Task.async(&mult/0)
    result = Task.await(task)
    assert result == 6
  end
end

and yet the task process has no problem using the mocked functions.

There’s a note about this below the example - on Elixir 1.8+ Mox can use $callers to detect when a parent process has an expectation set. (see also the similar machinery in DBConnection)

That’s not going to help for your case, though, since the test process and the registry aren’t related in a parent->child sense.

2 Likes

It seems in global mode the expectations have to come from a single process:

defmodule Mocker do
  import Mox

  def init(args) do
    set_mox_global()

    MyApp.CalcMock
    |> expect(:add, fn x, y -> x + y end)
    |> expect(:mult, fn x, y -> x * y end)

    {:ok, args}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])
end

defmodule Adder do
  def init(args),
    do: {:ok, args}

  def handle_call({:add, term1, term2}, _from, state) do
    sum = MyApp.CalcMock.add(term1, term2)
    {:reply, {:ok, sum}, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def add(adder_id, arg1, arg2),
    do: GenServer.call(adder_id, {:add, arg1, arg2})
end

defmodule Multiplier do
  def init(args),
    do: {:ok, args}

  def handle_call({:multiply, factor1, factor2}, _from, state) do
    product = MyApp.CalcMock.mult(factor1, factor2)
    {:reply, {:ok, product}, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def multiply(multiplier_id, arg1, arg2),
    do: GenServer.call(multiplier_id, {:multiply, arg1, arg2})
end

defmodule MyAppTest do
  use ExUnit.Case

  setup_all do
    {:ok, _mocker_id} = start_supervised(Mocker)
    {:ok, adder_id} = start_supervised(Adder)
    {:ok, multiplier_id} = start_supervised(Multiplier)

    %{
      adder_id: adder_id,
      multiplier_id: multiplier_id
    }
  end

  test "Use add expectation", context do
    assert Adder.add(context.adder_id, 2, 3) == {:ok, 5}
  end

  test "Use mult expectation", context do
    assert Multiplier.multiply(context.multiplier_id, 2, 3) == {:ok, 6}
  end
end

Managing multiple sets of mocks with explicit allowances is a bit more work:

defmodule MockShare do
  import Mox

  def init(args),
    do: {:ok, args}

  def handle_call({:allow, module, pid}, _from, state) do
    Mox.allow(module, self(), pid)
    {:reply, :ok, state}
  end

  def handle_call({:expect, expect_fun}, _from, state) do
    expect_fun.()
    {:reply, :ok, state}
  end

  # ---

  def child_spec(_args) do
    %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }
  end

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def allow(share_id, module, pid),
    do: GenServer.call(share_id, {:allow, module, pid})

  def expect(share_id, expect_fun),
    do: GenServer.call(share_id, {:expect, expect_fun})
end

defmodule Adder do
  def init(args),
    do: {:ok, args}

  def handle_call({:add, term1, term2}, _from, state) do
    sum = MyApp.CalcMock.add(term1, term2)
    {:reply, {:ok, sum}, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def add(adder_id, arg1, arg2),
    do: GenServer.call(adder_id, {:add, arg1, arg2})
end

defmodule Multiplier do
  def init(args),
    do: {:ok, args}

  def handle_call({:multiply, factor1, factor2}, _from, state) do
    product = MyApp.CalcMock.mult(factor1, factor2)
    {:reply, {:ok, product}, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def multiply(multiplier_id, arg1, arg2),
    do: GenServer.call(multiplier_id, {:multiply, arg1, arg2})
end

defmodule MyAppTest do
  use ExUnit.Case

  import Mox, only: [expect: 3]

  setup_all do
    {:ok, add_share_id} = start_supervised(MockShare, id: :add_share)

    MockShare.expect(
      add_share_id,
      fn ->
        expect(MyApp.CalcMock, :add, fn x, y -> x + y end)
      end
    )

    {:ok, mult_share_id} = start_supervised(MockShare, id: :mult_share)

    MockShare.expect(
      mult_share_id,
      fn ->
        expect(MyApp.CalcMock, :mult, fn x, y -> x * y end)
      end
    )

    {:ok, adder_id} = start_supervised(Adder)
    MockShare.allow(add_share_id, MyApp.CalcMock, adder_id)

    {:ok, multiplier_id} = start_supervised(Multiplier)
    MockShare.allow(mult_share_id, MyApp.CalcMock, multiplier_id)

    %{
      adder_id: adder_id,
      multiplier_id: multiplier_id
    }
  end

  test "Use add expectation", context do
    assert Adder.add(context.adder_id, 2, 3) == {:ok, 5}
  end

  test "Use mult expectation", context do
    assert Multiplier.multiply(context.multiplier_id, 2, 3) == {:ok, 6}
  end
end

Getting setup :set_mox_global to work:

defmodule Adder do
  def init(args),
    do: {:ok, args}

  def handle_call({:add, term1, term2}, _from, state) do
    sum = MyApp.CalcMock.add(term1, term2)
    {:reply, {:ok, sum}, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def add(adder_id, arg1, arg2),
    do: GenServer.call(adder_id, {:add, arg1, arg2})
end

defmodule Multiplier do
  def init(args),
    do: {:ok, args}

  def handle_call({:multiply, factor1, factor2}, _from, state) do
    product = MyApp.CalcMock.mult(factor1, factor2)
    {:reply, {:ok, product}, state}
  end

  # ---

  def child_spec(_args),
    do: %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, []},
      restart: :permanent,
      shutdown: 5000,
      type: :worker
    }

  def start_link(),
    do: GenServer.start_link(__MODULE__, [])

  def multiply(multiplier_id, arg1, arg2),
    do: GenServer.call(multiplier_id, {:multiply, arg1, arg2})
end

defmodule MyAppTest do
  use ExUnit.Case

  import Mox

  setup_all do
    {:ok, adder_id} = start_supervised(Adder)
    {:ok, multiplier_id} = start_supervised(Multiplier)

    %{
      adder_id: adder_id,
      multiplier_id: multiplier_id
    }
  end

  setup :set_mox_global

  test "Use add expectation", context do
    expect(MyApp.CalcMock, :add, fn x, y -> x + y end)
    assert Adder.add(context.adder_id, 2, 3) == {:ok, 5}
  end

  test "Use mult expectation", context do
    expect(MyApp.CalcMock, :mult, fn x, y -> x * y end)
    assert Multiplier.multiply(context.multiplier_id, 2, 3) == {:ok, 6}
  end
end

Seems what is happening is that setup :set_mox_global makes the individual test the global owner for the duration of the test - consequently that testing process would have to register all the expectations that are to be shared for the scope of that particular test.

Been there, done that :stuck_out_tongue:

@peerreynders That’s quite a lot to go through, I am still digesting it!

@peerreynders After reading all of your replies, I do feel like it’s totally overkill to define a GenServer module for each function of the interface I want to test.

Surely, @josevalim had a better idea in mind when he created Mox, I am fairly certain this is not the way Mox should be used and that I am missing something here.

Your replies have been invaluable though, I have learned a lot!
I will however look into other solutions for this issue and if I fail, I guess I will simply ditch Mox, as I don’t think the library is worth all the extra complexity I am paying - unless I am the only one here who thinks that creating a GenServer for each function of each interface is overkill, in which case I will just politely disagree :smiley:

I don’t know if you should be using it like that in first place. You are attempting to mock the piece of infrastructure you are in control, and mocking/stubbing in my mind should be reserved to dealing with elements of infrastructure you don’t have control over: like remote services/endpoints, systems talking to external systems etc.

If you don’t want to touch ExRegistry in your tests, you could write a testing module that does something similar that makes sense in your test and inject it in it’s place for testing purposes, or isolate the worker further and test it in isolation.

2 Likes

My sense is that a typical use case is something like:

defmodule MyAppTest do
  use ExUnit.Case

  import Mox

  setup :set_mox_global

  test "Use add expectation" do
    {:ok, adder_id} = start_supervised(Adder)

    expect(MyApp.CalcMock, :add, fn x, y -> x + y end)
    assert Adder.add(adder_id, 2, 3) == {:ok, 5}

    stop_supervised(adder_id)
  end

  test "Use mult expectation" do
    {:ok, multiplier_id} = start_supervised(Multiplier)

    expect(MyApp.CalcMock, :mult, fn x, y -> x * y end)
    assert Multiplier.multiply(multiplier_id, 2, 3) == {:ok, 6}

    stop_supervised(multiplier_id)
  end
end

where Adder and Multiplier are pieces of the SUT - setting up just enough environment to support the test.

I don’t think it was meant to be used with a whole and running SUT.

1 Like