Go Goroutine Leaks

Goroutine Leak Prevention

Pattern

Every goroutine must have a way to exit. Use channels or context for shutdown signals.

CORRECT - Done channel

type Worker struct {
    done chan struct{}
}

func (w *Worker) Start() {
    w.done = make(chan struct{})
    go func() {
        for {
            select {
            case <-w.done:
                return
            case <-time.After(1 * time.Second):
                // do work
            }
        }
    }()
}

func (w *Worker) Stop() {
    close(w.done)
}

CORRECT - Context

func StartWorker(ctx context.Context) {
    go func() {
        ticker := time.NewTicker(1 * time.Second)
        defer ticker.Stop()

        for {
            select {
            case <-ctx.Done():
                return
            case <-ticker.C:
                // do work
            }
        }
    }()
}

WRONG - No exit mechanism

func StartWorker() {
    go func() {
        for {
            // Runs forever - goroutine leak!
            time.Sleep(1 * time.Second)
            // do work
        }
    }()
}

WRONG - Unbuffered channel send can block

func GetData() string {
    ch := make(chan string)
    go func() {
        ch <- fetchData() // Blocks forever if nobody reads
    }()

    // If timeout happens, goroutine leaks
    select {
    case result := <-ch:
        return result
    case <-time.After(1 * time.Second):
        return "timeout"
    }
}

Fix with buffered channel

func GetData() string {
    ch := make(chan string, 1) // Buffer size 1
    go func() {
        ch <- fetchData() // Won't block
    }()

    select {
    case result := <-ch:
        return result
    case <-time.After(1 * time.Second):
        return "timeout"
    }
}

Rules

1. Every go func() needs an exit condition
2. Use select with ctx.Done() or done channel
3. Buffered channels (size 1) for single sends
4. Close channels to signal completion
5. Test with runtime.NumGoroutine() to detect leaks

Detection

func TestNoLeaks(t *testing.T) {
    before := runtime.NumGoroutine()

    worker := NewWorker()
    worker.Start()
    worker.Stop()

    time.Sleep(100 * time.Millisecond) // Allow cleanup
    after := runtime.NumGoroutine()

    if after > before {
        t.Errorf("goroutine leak: before=%d after=%d", before, after)
    }
}

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