It's hard to use gomock when there are goroutines involved. gomock sometimes expects them to be called and sometimes it doesn't

fixed_main_test.go

// file: main_test.go
package main

import (
	"math/rand"
	"sync"
	"testing"
	"time"

	gomock "github.com/golang/mock/gomock"
)

/*
It's hard to use gomock when there are goroutines involved.
gomock sometimes expects them to be called and sometimes it doesn't(depending on when the goroutines are scheduled).

Run this tests as(make sure you run them a number of times.):

	go test ./... -count=1
*/
func TestFoo(t *testing.T) {
	rand.Seed(time.Now().UTC().UnixNano())

	// WaitGroup is needed so as to await the mocked func that is in a goroutine to finish.
	// see: https://jeffy-mathew.medium.com/unit-testing-and-mock-calls-inside-goroutines-7a19b853e084
	var wg sync.WaitGroup
	wg.Add(1)
	// defer wg.Wait()

	t.Run("sync", func(t *testing.T) {
		ctrl := gomock.NewController(t)
		defer ctrl.Finish()

		m := NewMockFoo(ctrl)
		m.EXPECT().
			Bar(gomock.Eq(99)).
			Return(101)

		helloSync(m)
	})

	// This test will fail sometime and succed at other times.
	t.Run("async", func(t *testing.T) {
		ctrl := gomock.NewController(t)
		defer ctrl.Finish()

		m := NewMockFoo(ctrl)
		m.EXPECT().
			Bar(gomock.Eq(99)).
			DoAndReturn(
				// signature of anonymous function must have the same number of input and output arguments as the mocked method.
				// ie, It must match the signature of `impl.Bar()`
				func(x int) int {
					defer wg.Done()
					return 101
				},
			)

		/*
		Another alternative is to do;
		m.EXPECT().
		  Bar(gomock.Eq(99)).
		  Return(101).
                  AnyTimes()
		*/
		
		helloAsync(m)

		if rand.Intn(10) > 4 {
			time.Sleep(1 * time.Second)
		}

		wg.Wait()
	})
}

type Foo interface {
	Bar(x int) int
}

type impl struct{}

func (i impl) Bar(x int) int {
	return 101
}

func helloSync(f Foo) {
	_ = f.Bar(99)
}

func helloAsync(f Foo) {
	go f.Bar(99)
}

main_gen.go

// file: main_gen.go
// Code generated by MockGen. DO NOT EDIT.
// Source: main.go

// Package mock_main is a generated GoMock package.
package main

import (
	reflect "reflect"

	gomock "github.com/golang/mock/gomock"
)

// MockFoo is a mock of Foo interface.
type MockFoo struct {
	ctrl     *gomock.Controller
	recorder *MockFooMockRecorder
}

// MockFooMockRecorder is the mock recorder for MockFoo.
type MockFooMockRecorder struct {
	mock *MockFoo
}

// NewMockFoo creates a new mock instance.
func NewMockFoo(ctrl *gomock.Controller) *MockFoo {
	mock := &MockFoo{ctrl: ctrl}
	mock.recorder = &MockFooMockRecorder{mock}
	return mock
}

// EXPECT returns an object that allows the caller to indicate expected use.
func (m *MockFoo) EXPECT() *MockFooMockRecorder {
	return m.recorder
}

// Bar mocks base method.
func (m *MockFoo) Bar(x int) int {
	m.ctrl.T.Helper()
	ret := m.ctrl.Call(m, "Bar", x)
	ret0, _ := ret[0].(int)
	return ret0
}

// Bar indicates an expected call of Bar.
func (mr *MockFooMockRecorder) Bar(x interface{}) *gomock.Call {
	mr.mock.ctrl.T.Helper()
	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Bar", reflect.TypeOf((*MockFoo)(nil).Bar), x)
}

main_test.go

// file: main_test.go
package main

import (
	"math/rand"
	"testing"
	"time"

	gomock "github.com/golang/mock/gomock"
)

/*
It's hard to use gomock when there are goroutines involved.
gomock sometimes expects them to be called and sometimes it doesn't(depending on when the goroutines are scheduled).

// See `fixed_main_test.go` for a way to fix this issue.

Run this tests as(make sure you run them a number of times.):

	go test ./... -count=1
*/
func TestFoo(t *testing.T) {
	rand.Seed(time.Now().UTC().UnixNano())

	t.Run("sync", func(t *testing.T) {
		ctrl := gomock.NewController(t)
		defer ctrl.Finish()

		m := NewMockFoo(ctrl)
		m.EXPECT().
			Bar(gomock.Eq(99)).
			Return(101)

		helloSync(m)
	})

	// This test will fail sometime and succed at other times.
	t.Run("async", func(t *testing.T) {
		ctrl := gomock.NewController(t)
		defer ctrl.Finish()

		m := NewMockFoo(ctrl)
		m.EXPECT().
			Bar(gomock.Eq(99)).
			Return(101)

		helloAsync(m)

		if rand.Intn(10) > 4 {
			time.Sleep(1 * time.Second)
		}
	})
}

type Foo interface {
	Bar(x int) int
}

type impl struct{}

func (i impl) Bar(x int) int {
	return 101
}

func helloSync(f Foo) {
	_ = f.Bar(99)
}

func helloAsync(f Foo) {
	go f.Bar(99)
}