How to verify if a specific function is called

Question

I'm trying my hand at writing TDD in Go. I am however stuck at the following.

The test to write:

func TestFeatureStart(t *testing.T) {}

Implementation to test:

func (f *Feature) Start() error {
  cmd := exec.Command(f.Cmd)
  cmd.Start()
}

How would one test this simple bit? I figured I only wanted to verify that the exec library is spoken to correctly. That's the way I would do it in Java using Mockito. Can anyone help me write this test? From what I've read the usage of interfaces is suggested.

The Feature-struct only contains a string Cmd.

Answer 1

You can fake the whole deal with interfaces, but you could also use fakeable functions. In the code:

var cmdStart = (*exec.Cmd).Start
func (f *Feature) Start() error {
    cmd := exec.Command(f.Cmd)
    return cmdStart(cmd)
}

In the tests:

called := false
cmdStart = func(*exec.Cmd) error { called = true; return nil }
f.Start()
if !called {
    t.Errorf("command didn't start")
}

See also: Andrew Gerrand's Testing Techniques talk.