---

name: go-sync-primitives description: sync.WaitGroup and sync.Mutex patterns

Sync Primitives

sync.WaitGroup - Wait for Goroutines

CORRECT

Copyfunc processBatch(items []string) {
    var wg sync.WaitGroup

    for _, item := range items {
        wg.Add(1) // BEFORE launching goroutine
        go func(item string) {
            defer wg.Done()
            process(item)
        }(item)
    }

    wg.Wait() // Block until all done
}

WRONG - Add inside goroutine

Copyfunc processBatch(items []string) {
    var wg sync.WaitGroup

    for _, item := range items {
        go func(item string) {
            wg.Add(1) // WRONG: race condition
            defer wg.Done()
            process(item)
        }(item)
    }

    wg.Wait() // May return early
}

WRONG - Missing variable capture

Copyfunc processBatch(items []string) {
    var wg sync.WaitGroup

    for _, item := range items {
        wg.Add(1)
        go func() {
            defer wg.Done()
            process(item) // WRONG: captures loop variable
        }()
    }

    wg.Wait()
}

sync.Mutex - Protect Shared State

CORRECT

Copytype Counter struct {
    mu    sync.Mutex
    value int
}

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

func (c *Counter) Value() int {
    c.mu.Lock()
    defer c.mu.Unlock()
    return c.value
}

WRONG - Unlocked access

Copytype Counter struct {
    mu    sync.Mutex
    value int
}

func (c *Counter) Increment() {
    c.mu.Lock()
    c.value++ // What if panic happens?
    c.mu.Unlock()
}

func (c *Counter) Value() int {
    return c.value // WRONG: race condition
}

sync.RWMutex - Multiple Readers

Copytype Cache struct {
    mu   sync.RWMutex
    data map[string]string
}

func (c *Cache) Get(key string) (string, bool) {
    c.mu.RLock() // Multiple readers OK
    defer c.mu.RUnlock()
    val, ok := c.data[key]
    return val, ok
}

func (c *Cache) Set(key, value string) {
    c.mu.Lock() // Exclusive writer
    defer c.mu.Unlock()
    c.data[key] = value
}

Rules

WaitGroup

1. Call Add() before go statement
2. Always use defer wg.Done()
3. Pass loop variables as function parameters
4. One Add(n) can count multiple goroutines

Mutex

1. Always use defer mu.Unlock()
2. Keep critical sections small
3. Don't hold locks during I/O or slow operations
4. Use RWMutex for read-heavy workloads
5. Never copy a mutex (pass by pointer)

sync.Once - Run Exactly Once

Copyvar (
    instance *Singleton
    once     sync.Once
)

func GetInstance() *Singleton {
    once.Do(func() {
        instance = &Singleton{}
    })
    return instance
}

Race Detection

Copygo test -race ./...
go run -race main.go