Using GoMock with Concurrent Code Guide

Learn practical techniques for testing concurrent Go code using GoMock.

Discover how to handle race conditions, timeouts, and error scenarios with real-world examples and best practices.

Using GoMock with Concurrent Code: Techniques and Examples

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Testing concurrent code can be tricky, but with GoMock, we can make it manageable and reliable. In this post, we’ll explore practical techniques for testing concurrent Go code using GoMock, with real-world examples that you can apply to your projects.

Understanding the Challenge

When working with concurrent code, we often face challenges like race conditions, deadlocks, and timing issues. These challenges become even more complex when we try to test such code. GoMock helps us tackle these challenges by providing precise control over mock behaviors in concurrent scenarios.

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Setting Up GoMock for Concurrent Testing

First, let’s set up a simple example. Imagine we have a service that processes user data concurrently:

type DataProcessor interface {
    ProcessUserData(userData string) (string, error)
}

type UserService struct {
    processor DataProcessor
}

func (s *UserService) ProcessMultipleUsers(users []string) []string {
    results := make([]string, len(users))
    var wg sync.WaitGroup

    for i, user := range users {
        wg.Add(1)
        go func(index int, userData string) {
            defer wg.Done()
            result, _ := s.processor.ProcessUserData(userData)
            results[index] = result
        }(i, user)
    }

    wg.Wait()
    return results
}

Mocking Concurrent Calls

Here’s where GoMock shines. We can use it to verify concurrent calls and control their behavior:

func TestProcessMultipleUsers(t *testing.T) {
    ctrl := gomock.NewController(t)
    defer ctrl.Finish()

    mockProcessor := NewMockDataProcessor(ctrl)

    // Handle concurrent calls
    mockProcessor.EXPECT().
        ProcessUserData(gomock.Any()).
        Times(3).  // Expect 3 concurrent calls
        DoAndReturn(func(userData string) (string, error) {
            time.Sleep(10 * time.Millisecond) // Simulate work
            return "processed_" + userData, nil
        })

    service := &UserService{processor: mockProcessor}
    results := service.ProcessMultipleUsers([]string{"user1", "user2", "user3"})

    // Verify results
    for i, result := range results {
        if !strings.HasPrefix(result, "processed_user") {
            t.Errorf("Unexpected result for index %d: %s", i, result)
        }
    }
}

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Advanced Techniques

1. Testing Timeouts

We can test timeout scenarios by controlling mock response times:

mockProcessor.EXPECT().
    ProcessUserData(gomock.Any()).
    DoAndReturn(func(userData string) (string, error) {
        time.Sleep(2 * time.Second) // Simulate slow processing
        return "", errors.New("timeout")
    })

2. Testing Race Conditions

GoMock helps us test race conditions by controlling the order of concurrent operations:

var mu sync.Mutex
var callOrder []string

mockProcessor.EXPECT().
    ProcessUserData(gomock.Any()).
    AnyTimes().
    DoAndReturn(func(userData string) (string, error) {
        mu.Lock()
        callOrder = append(callOrder, userData)
        mu.Unlock()
        return "processed", nil
    })

3. Testing Error Scenarios

We can simulate various error scenarios in concurrent code:

mockProcessor.EXPECT().
    ProcessUserData(gomock.Any()).
    AnyTimes().
    Return("", errors.New("simulated error"))

Best Practices

Always use gomock.NewController(t) with proper cleanup
Be careful with .Times() expectations in concurrent code
Use DoAndReturn for complex mock behaviors
Don’t forget to handle timeouts in your tests
Consider using channels for synchronization in tests

Conclusion

Testing concurrent code doesn’t have to be intimidating. With GoMock, we can create reliable tests that verify our concurrent code behaves correctly under various conditions. Remember to always consider race conditions, timeouts, and error scenarios in your tests.

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