Copying a sync type
All primitive types in the sync package follow one hard rule, they should
never be copied.
- sync.Cond
- sync.Map
- sync.Mutex
- sync.RWMutex
- sync.Once
- sync.Pool
- sync.WaitGroup
Mistake
This is an example of a data race from copying a sync primitive.
type Counter struct {
mu sync.Mutex
counters map[string]int
}
func NewCounter() Counter {
return Counter{counters: map[string]int{}}
}
// The function receiver here is a value so it is copied instead of referenced
func (c Counter) Increment(name string){
c.mu.Lock()
defer c.mu.Unlock()
c.counters[name]++
}
Fix
Alternatively, we could make Counter.mu a pointer to a sync.Mutex
type Counter struct {
mu sync.Mutex
counters map[string]int
}
func NewCounter() Counter {
return Counter{counters: map[string]int{}}
}
func (c *Counter) Increment(name string){
c.mu.Lock()
defer c.mu.Unlock()
c.counters[name]++
}