Go Mutex and RWMutex
External
Internal
Overview
Mutex
A Mutex is a mutual exclusion lock, implemented as a binary semaphore. Only one goroutine can enter the critical section at a time. Not until the call to the Unlock() function issued can another goroutine enter the critical section.
The usage pattern follows, and the curly braces are not necessary, but can be used for clarity:
var mutex sync.Mutex
...
mutex.Lock()
// do something in a mutual exclusion mode
mutex.Unlock()
Methods
Lock()
The goroutine should call Lock() before it is about to use the shared data. If nobody called Lock() before, the call will proceed and the calling goroutine will acquire the lock. If somebody else has the lock, the call will block. Multiple threads may potentially block on the same lock.
Lock() ensures that only one thread can be in the mutually exclusion region.
Unlock()
The goroutine that is done using the shared data should call Unlock(). If other threads are waiting on the lock, one of the threads will be unblocked and will be able to continue.
A mutex that was not locked raises a panic on an attempt to unlock:
fatal error: sync: unlock of unlocked mutex
RWMutex
A RWMutex is a mutual exclusion lock that allows differentiated access to readers and writers.
Patterns
Guarding the Internal State of a struct
Guarding the internal state of a struct is is one of the not very numerous patterns where a sync primitive is actually recommended. This pattern defines the scope of atomicity for the SomeType type. Calls to something() can be considered atomic. By using memory access synchronization primitives, you can hide the implementation details of locking of your critical section from the callers.
type SomeType struct {
name string
...
sync.RWMutex
}
func (s * SomeType) something() {
s.Lock()
defer s.Unlock()
}