// Anonymous goroutine with defer go func() { defer cleanup() doWork() }()


### Goroutine Leaks - How to Avoid

```go
// ❌ BAD - goroutine leak
func process() {
    ch := make(chan int)
    go func() {
        for val := range ch {  // Never exits if channel closes
            fmt.Println(val)
        }
    }()
    // If we return here, goroutine leaks forever
}

// ✅ GOOD - explicit cancellation
func process(ctx context.Context) error {
    ch := make(chan int)
    done := make(chan struct{})

    go func() {
        defer close(done)
        for {
            select {
            case val, ok := <-ch:
                if !ok {
                    return
                }
                fmt.Println(val)
            case <-ctx.Done():
                return
            }
        }
    }()

    // ... use ch
    close(ch)
    <-done
    return nil
}

Channel Patterns

Buffered vs Unbuffered

// Unbuffered - synchronous
unbuf := make(chan int)
// Sender blocks until receiver ready

// Buffered - async capacity
buf := make(chan int, 100)
// Sender blocks only when buffer full

Fan-in Pattern

// Merge multiple channels into one
func fanIn[T any](ctx context.Context, channels ...<-chan T) <-chan T {
    out := make(chan T)

    var wg sync.WaitGroup
    for _, ch := range channels {
        wg.Add(1)
        go func(c <-chan T) {
            defer wg.Done()
            for {
                select {
                case v, ok := <-c:
                    if !ok {
                        return
                    }
                    select {
                    case out <- v:
                    case <-ctx.Done():
                        return
                    }
                case <-ctx.Done():
                    return
                }
            }
        }(ch)
    }

    go func() {
        wg.Wait()
        close(out)
    }()

    return out
}

Fan-out Pattern

// Distribute work to multiple workers
func fanOut[T any, R any](ctx context.Context, workers int, input <-chan T, work func(T) R) <-chan R {
    out := make(chan R)

    var wg sync.WaitGroup
    for i := 0; i < workers; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            for item := range input {
                result := work(item)
                select {
                case out <- result:
                case <-ctx.Done():
                    return
                }
            }
        }()
    }

    go func() {
        wg.Wait()
        close(out)
    }()

    return out
}

Pipeline Pattern

// Stage 1: Generate
func generate(ctx context.Context, nums ...int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for _, n := range nums {
            select {
            case out <- n:
            case <-ctx.Done():
                return
            }
        }
    }()
    return out
}

// Stage 2: Transform
func square(ctx context.Context, in <-chan int) <-chan int {
    out := make(chan int)
    go func() {
        defer close(out)
        for n := range in {
            result := n * n
            select {
            case out <- result:
            case <-ctx.Done():
                return
            }
        }
    }()
    return out
}

// Stage 3: Consume
func consume(ctx context.Context, in <-chan int) []int {
    var results []int
    for n := range in {
        results = append(results, n)
    }
    return results
}

// Pipeline
func pipeline(ctx context.Context) []int {
    nums := generate(ctx, 1, 2, 3, 4, 5)
    squared := square(ctx, nums)
    return consume(ctx, squared)
}

Worker Pool Pattern

type Job struct {
    ID   int
    Data string
}

type Result struct {
    JobID int
    Value string
    Err   error
}

func workerPool(ctx context.Context, jobs <-chan Job, workers int) <-chan Result {
    results := make(chan Result)

    var wg sync.WaitGroup
    for i := 0; i < workers; i++ {
        wg.Add(1)
        go func(workerID int) {
            defer wg.Done()
            for job := range jobs {
                result := processJob(job)
                select {
                case results <- result:
                case <-ctx.Done():
                    return
                }
            }
        }(i)
    }

    go func() {
        wg.Wait()
        close(results)
    }()

    return results
}

func processJob(job Job) Result {
    // Simulate work
    time.Sleep(100 * time.Millisecond)
    return Result{
        JobID: job.ID,
        Value: "processed: " + job.Data,
    }
}

// Usage
func main() {
    ctx := context.Background()
    jobs := make(chan Job, 100)

    // Feed jobs
    go func() {
        for i := 0; i < 50; i++ {
            jobs <- Job{ID: i, Data: fmt.Sprintf("item-%d", i)}
        }
        close(jobs)
    }()

    // Start workers
    results := workerPool(ctx, jobs, 5)

    // Collect results
    for result := range results {
        fmt.Printf("Job %d: %s\n", result.JobID, result.Value)
    }
}

Context for Cancellation and Deadlines

Context Hierarchy

// Create context with timeout
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()

// Create derived context with deadline
d := time.Now().Add(10 * time.Second)
ctx, cancel := context.WithDeadline(context.Background(), d)
defer cancel()

// Create cancelable context
ctx, cancel := context.WithCancel(context.Background())
defer cancel()  // Always call cancel

Propagation Pattern

func (s *Service) ProcessOrder(ctx context.Context, orderID string) error {
    // Pass context to all downstream calls
    order, err := s.repo.GetOrder(ctx, orderID)
    if err != nil {
        return fmt.Errorf("get order: %w", err)
    }

    // Create derived context with timeout for external call
    apiCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
    defer cancel()

    result, err := s.externalAPI.Validate(apiCtx, order)
    if err != nil {
        return fmt.Errorf("validate: %w", err)
    }

    return nil
}

Graceful Shutdown Pattern

type Server struct {
    http     *http.Server
    grpc     *grpc.Server
    shutdown chan struct{}
}

func (s *Server) Start(ctx context.Context) error {
    // Start servers
    go func() {
        if err := s.http.ListenAndServe(); err != nil && err != http.ErrServerClosed {
            log.Printf("HTTP server error: %v", err)
        }
    }()

    go func() {
        if err := s.grpc.Serve(s.lis); err != nil {
            log.Printf("gRPC server error: %v", err)
        }
    }()

    // Wait for shutdown signal
    <-ctx.Done()
    return s.shutdown()
}

func (s *Server) shutdown() error {
    ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
    defer cancel()

    var wg sync.WaitGroup
    errs := make(chan error, 2)

    // Shutdown HTTP
    wg.Add(1)
    go func() {
        defer wg.Done()
        errs <- s.http.Shutdown(ctx)
    }()

    // Shutdown gRPC
    wg.Add(1)
    go func() {
        defer wg.Done()
        s.grpc.GracefulStop()
        errs <- nil
    }()

    wg.Wait()
    close(errs)

    for err := range errs {
        if err != nil {
            return err
        }
    }
    return nil
}

Select for Multiplexing

// Select pattern for multiple channels
func multiplex(ctx context.Context, ch1, ch2 <-chan int) {
    for {
        select {
        case v, ok := <-ch1:
            if !ok {
                fmt.Println("ch1 closed")
                return
            }
            fmt.Printf("ch1: %d\n", v)

        case v, ok := <-ch2:
            if !ok {
                fmt.Println("ch2 closed")
                return
            }
            fmt.Printf("ch2: %d\n", v)

        case <-ctx.Done():
            fmt.Println("context cancelled")
            return

        case <-time.After(100 * time.Millisecond):
            fmt.Println("timeout - no data")
        }
    }
}

Synchronization Primitives

Mutex for State Protection

type SafeCounter struct {
    mu    sync.RWMutex
    value int
}

func (c *SafeCounter) Increment() {
    c.mu.Lock()
    defer c.mu.Unlock()
    c.value++
}

func (c *SafeCounter) Value() int {
    c.mu.RLock()
    defer c.mu.RUnlock()
    return c.value
}

Once for Initialization

var (
    instance *Database
    once     sync.Once
)

func GetDatabase() *Database {
    once.Do(func() {
        instance = &Database{conn: connect()}
    })
    return instance
}

WaitGroup for Coordination

func processItems(items []Item) {
    var wg sync.WaitGroup
    semaphore := make(chan struct{}, 10) // Limit concurrency

    for _, item := range items {
        wg.Add(1)
        go func(i Item) {
            defer wg.Done()
            semaphore <- struct{}{}        // Acquire
            defer func() { <-semaphore }() // Release
            process(i)
        }(item)
    }

    wg.Wait()
}

Cond for Waiting on Conditions

type Queue struct {
    mu     sync.Mutex
    cond   *sync.Cond
    items  []Item
}

func NewQueue() *Queue {
    q := &Queue{
        items: make([]Item, 0),
    }
    q.cond = sync.NewCond(&q.mu)
    return q
}

func (q *Queue) Push(item Item) {
    q.mu.Lock()
    defer q.mu.Unlock()
    q.items = append(q.items, item)
    q.cond.Signal() // Wake one waiter
}

func (q *Queue) Pop() Item {
    q.mu.Lock()
    defer q.mu.Unlock()

    for len(q.items) == 0 {
        q.cond.Wait() // Wait for signal
    }

    item := q.items[0]
    q.items = q.items[1:]
    return item
}

Atomic for Simple Counters

type Metrics struct {
    requests atomic.Int64
    errors   atomic.Int64
}

func (m *Metrics) RecordRequest() {
    m.requests.Add(1)
}

func (m *Metrics) RecordError() {
    m.errors.Add(1)
}

func (m *Metrics) GetStats() (int64, int64) {
    return m.requests.Load(), m.errors.Load()
}

ErrorGroup Pattern

import "golang.org/x/sync/errgroup"

func processItems(ctx context.Context, items []Item) error {
    g, gctx := errgroup.WithContext(ctx)

    results := make(chan Result, len(items))

    for _, item := range items {
        item := item // Capture loop variable
        g.Go(func() error {
            result, err := processItem(gctx, item)
            if err != nil {
                return err
            }
            results <- result
            return nil
        })
    }

    // Wait for all goroutines
    if err := g.Wait(); err != nil {
        return err
    }

    close(results)
    return nil
}

Rate Limiting Pattern

import "golang.org/x/time/rate"

type RateLimiter struct {
    limiter *rate.Limiter
}

func NewRateLimiter(rps int) *RateLimiter {
    return &RateLimiter{
        limiter: rate.NewLimiter(rate.Limit(rps), rps),
    }
}

func (rl *RateLimiter) Allow() bool {
    return rl.limiter.Allow()
}

func (rl *RateLimiter) Wait(ctx context.Context) error {
    return rl.limiter.Wait(ctx)
}

// Usage in middleware
func (rl *RateLimiter) Middleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if !rl.Allow() {
            http.Error(w, "rate limit exceeded", http.StatusTooManyRequests)
            return
        }
        next.ServeHTTP(w, r)
    })
}

Backpressure Handling

// Producer-consumer with backpressure
func backpressureProducer(ctx context.Context, out chan<- Item, rate int) <-chan Item {
    results := make(chan Item)

    go func() {
        defer close(results)
        ticker := time.NewTicker(time.Second / time.Duration(rate))
        defer ticker.Stop()

        for {
            select {
            case <-ctx.Done():
                return
            case <-ticker.C:
                item := produce()
                select {
                case out <- item:
                case <-ctx.Done():
                    return
                }
            }
        }
    }()

    return results
}

Detecting Race Conditions

# Run tests with race detector
go test -race ./...

# Build with race detector
go run -race main.go

# Common race patterns
# - Shared state without mutex
# - Publishing reference during initialization
# - Data races on sync.Pool

// ❌ RACE - shared state
var counter int

func increment() {
    counter++  // Data race!
}

// ✅ SAFE - protected state
var (
    counter int
    mu      sync.Mutex
)

func increment() {
    mu.Lock()
    counter++
    mu.Unlock()
}

Performance Considerations

// Goroutine overhead is small but not zero
// ~2KB stack, grows as needed
// ~1.5µs to spawn

// Channel operations cost
// Buffered channel send: ~50ns
// Unbuffered channel send: ~90ns (requires context switch)

// Mutex vs Channel
// Use mutex for state protection
// Use channels for orchestration and communication

Best Practices Summary

Always cancel contexts - Use defer cancel()
Never block goroutines indefinitely - Use context or timeouts
Prefer channels over mutex for orchestration
Prefer mutex over channels for state protection
Limit concurrency with worker pools or semaphores
Always check for race conditions in tests
Use errgroup for coordinated goroutines
Close channels from the sender side only
Never close a channel from the receiver
Use select for multiplexing and timeouts

go-concurrency

Go Concurrency Patterns

Goroutine Lifecycle

Basic Goroutine