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up vote 7 down vote favorite
3

I want to test a function which is using Task.async

In order to make my test pass, I need to make it sleep for 100ms before assertions, otherwise the test process is killed before the async task is executed.

Is there a better way?

EDITED, adding code samples :

The code I would like to test (roughly) :

def search(params) do
  RateLimiter.rate_limit(fn ->
    parsed_params = ExTwitter.Parser.parse_request_params(params)
    json = ExTwitter.API.Base.request(:get, "1.1/search/tweets.json", parsed_params)
    Task.async(fn -> process_search_output(json) end)
    new_max_id(json)
  end)
end

And the test I already wrote (working only with the call to sleep)

test "processes and store tweets" do
  with_mock ExTwitter.API.Base, [request: fn(_,_,_) -> json_fixture end] do
    with_mock TwitterRateLimiter, [rate_limit: fn(fun) -> fun.() end] do
      TSearch.search([q: "my query"])
      :timer.sleep(100)
      # assertions 
      assert called TStore.store("some tweet from my fixtures")
      assert called TStore.store("another one")
    end
  end
end
share|improve this question
1  
Can you show us a minimal failing example that illustrates the specific type of assertion you want to make? – Patrick Oscity Feb 26 '15 at 14:40
    
Example code would help immensely in giving you a good answer. – Onorio Catenacci Feb 26 '15 at 15:49
    
Ok just added code samples – Chris Feb 26 '15 at 17:32
    
Any other comment on my code are OK :) (especially the mocking part) – Chris Feb 26 '15 at 17:40
1  
If you are not going to use the result of the task, don't use Task.async/1, you can use Task.start_link/1 directly. – José Valim Feb 26 '15 at 19:58
up vote 17 down vote accepted

Since the question is a bit vague, I will give the general answer here. The usual technique is to monitor the process and wait for the down message. Something like this:

task = Task.async(fn -> "foo" end)
ref  = Process.monitor(task.pid)
assert_receive {:DOWN, ^ref, :process, _, :normal}, 500

Some important things:

  • The fifth element of the tuple is the exit reason. I am asserting the Task exit is :normal. Change that accordingly if you are expecting another exit.

  • The second value in assert_receive is the timeout. 500 miliseconds sounds like a reasonable amount given you currently have a 100 ms sleep.

share|improve this answer
    
Thanks you José! Just added some code samples to my question. I guess I need to make my search function to return a tuple containing the Task pid ? (I don't like to change my code only for test purposes :/ ) – Chris Feb 26 '15 at 17:34
4  
Let me argue in favor of that. Your test is a consumer of your code as any other part of your application. Ensuring you are providing a proper result for tests is generally a good indicator other parts of your application will use that code properly too. For example, looking at the tests, I have no idea what the return results are. What happens if I reach the API limit? What happens if I don't? It seems the function is about side-effects and returning a task would indicate: hey, I will finish this later, watch this task if you care about it. – José Valim Feb 26 '15 at 19:57
    
Got it, thanks! – Chris Feb 26 '15 at 21:20
up vote 2 down vote

When I cannot use José's approach involving assert_receive, I use a small helper to repeatedly do assertion / sleep, until the assertion pass or finally times out.

Here is the helper module

defmodule TimeHelper do

  def wait_until(fun), do: wait_until(500, fun)

  def wait_until(0, fun), do: fun.()

  def wait_until(timeout, fun) defo
    try do
      fun.()
    rescue
      ExUnit.AssertionError ->
        :timer.sleep(10)
        wait_until(max(0, timeout - 10), fun)
    end
  end

end

It can be used like this in former example:

TSearch.search([q: "my query"])
wait_until fn ->
  assert called TStore.store("some tweet from my fixtures")
  assert called TStore.store("another one")
end
share|improve this answer

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