How to test if a goroutine has been called while unit testing in Golang?

Question

suppose that we have a method like this:

func method(intr MyInterface) {
    go intr.exec()
} 

In unit testing method, we want to assert that inter.exec has been called once and only once; so we can mock it with another mock struct in tests, which will give us functionality to check if it has been called or not:

type mockInterface struct{
    CallCount int
}

func (m *mockInterface) exec() {
    m.CallCount += 1
}

And in unit tests:

func TestMethod(t *testing.T) {
    var mock mockInterface{}
    method(mock)
    if mock.CallCount != 1 {
        t.Errorf("Expected exec to be called only once but it ran %d times", mock.CallCount)
    }
}

Now, the problem is that since intr.exec is being called with go keyword, we can't be sure that when we reach our assertion in tests, it has been called or not.

Possible Solution 1:

Adding a channel to arguments of intr.exec may solve this: we could wait on receiving any object from it in tests, and after receiving an object from it then we could continue to assert it being called. This channel will be completely unused in production (non-test) codes. This will work but it adds unnecessary complexity to non-test codes, and may make large codebases incomprehensible.

Possible Solution 2:

Adding a relatively small sleep to tests before assertion may give us some assurance that the goroutine will be called before sleep is finished:

func TestMethod(t *testing.T) {
    var mock mockInterface{}
    method(mock)

    time.sleep(100 * time.Millisecond)

    if mock.CallCount != 1 {
        t.Errorf("Expected exec to be called only once but it ran %d times", mock.CallCount)
    }
}

This will leave non-test codes as they are now.
The problem is that it will make tests slower, and will make them flaky, since they may break in some random circumstances.

Possible Solution 3:

Creating a utility function like this:

var Go = func(function func()) {
    go function()
} 

And rewrite method like this:

func method(intr MyInterface) {
    Go(intr.exec())
} 

In tests, we could change Go to this:

var Go = func(function func()) {
    function()
} 

So, when we're running tests, intr.exec will be called synchronously, and we can be sure that our mock method is called before assertion.
The only problem of this solution is that it's overriding a fundamental structure of golang, which is not right thing to do.

---

These are solutions that I could find, but non are satisfactory as far as I can see. What is best solution?

Answer 1

Use a sync.WaitGroup inside the mock

You can extend mockInterface to allow it to wait for the other goroutine to finish

type mockInterface struct{
    wg sync.WaitGroup // create a wait group, this will allow you to block later
    CallCount int
}

func (m *mockInterface) exec() {
    m.wg.Done() // record the fact that you've got a call to exec
    m.CallCount += 1
}

func (m *mockInterface) currentCount() int {
    m.wg.Wait() // wait for all the call to happen. This will block until wg.Done() is called.
    return m.CallCount
}

In the tests you can do:

mock := &mockInterface{}
mock.wg.Add(1) // set up the fact that you want it to block until Done is called once.

method(mock)

if mock.currentCount() != 1 {  // this line with block
    // trimmed
}

Answer 2

This test won't hang forever as with sync.WaitGroup solution proposed above. It will hang for a second (in this particular example) in case when there is no call to mock.exec:

package main

import (
    "testing"
    "time"
)

type mockInterface struct {
    closeCh chan struct{}
}

func (m *mockInterface) exec() {
    close(closeCh)
}

func TestMethod(t *testing.T) {
    mock := mockInterface{
        closeCh: make(chan struct{}),
    }

    method(mock)

    select {
    case <-closeCh:
    case <-time.After(time.Second):
        t.Fatalf("expected call to mock.exec method")
    }
}

This is basically what mc.Wait(time.Second) in my answer above.