Testing time with Golang

We can all agree that TDD is the bees knees, right? Something that can trip newbies up with golang is writing tests against a function that has a timeout.

Often when waiting for a result over a channel, it can make sense to add a timeout so we don't wait forever. Consider the following code

func doSomething(done chan struct{}) {
  // simulate an operation that takes too long
  time.Sleep(time.Second * 5)
  done <- struct{}
}

// We need to test this
func MyCode() {
  c := make(chan struct{})

  select {
  case <- done:
    // hurray!
  case <- time.After(time.Second * 5)
    // oh no!
}

Writing a unit test that exercises our timeout logic and doesn't take 5 seconds to run can seem difficult at first, but it is actually quite simple. We can use something called remote function overriding to declare a variable that we can override in our test.

var after = time.After

And our switch statement changes like so

select {
  case <- done:
    // hurray!
  case <- after(time.Second * 5)
    // oh no!
}

Now, in our tests, we can easily change the definition of after, to make it happen instantly

func TestSomething(t *testing.T) {
  after = func(d time.Duration) <-chan time.Time {
    return time.After(0)
  }

  // your test code here
}