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/*
Package locker provides a mechanism for creating finer-grained locking to help
free up more global locks to handle other tasks.
The implementation looks close to a sync.Mutex, however the user must provide a
reference to use to refer to the underlying lock when locking and unlocking,
and unlock may generate an error.
If a lock with a given name does not exist when `Lock` is called, one is
created.
Lock references are automatically cleaned up on `Unlock` if nothing else is
waiting for the lock.
*/
package locker
import (
"errors"
"sync"
"sync/atomic"
)
// ErrNoSuchLock is returned when the requested lock does not exist
var ErrNoSuchLock = errors.New("no such lock")
// Locker provides a locking mechanism based on the passed in reference name
type Locker struct {
mu sync.Mutex
locks map[string]*lockCtr
}
// lockCtr is used by Locker to represent a lock with a given name.
type lockCtr struct {
mu sync.Mutex
// waiters is the number of waiters waiting to acquire the lock
// this is int32 instead of uint32 so we can add `-1` in `dec()`
waiters int32
}
// inc increments the number of waiters waiting for the lock
func (l *lockCtr) inc() {
atomic.AddInt32(&l.waiters, 1)
}
// dec decrements the number of waiters waiting on the lock
func (l *lockCtr) dec() {
atomic.AddInt32(&l.waiters, -1)
}
// count gets the current number of waiters
func (l *lockCtr) count() int32 {
return atomic.LoadInt32(&l.waiters)
}
// Lock locks the mutex
func (l *lockCtr) Lock() {
l.mu.Lock()
}
// Unlock unlocks the mutex
func (l *lockCtr) Unlock() {
l.mu.Unlock()
}
// New creates a new Locker
func New() *Locker {
return &Locker{
locks: make(map[string]*lockCtr),
}
}
// Lock locks a mutex with the given name. If it doesn't exist, one is created
func (l *Locker) Lock(name string) {
l.mu.Lock()
if l.locks == nil {
l.locks = make(map[string]*lockCtr)
}
nameLock, exists := l.locks[name]
if !exists {
nameLock = &lockCtr{}
l.locks[name] = nameLock
}
// increment the nameLock waiters while inside the main mutex
// this makes sure that the lock isn't deleted if `Lock` and `Unlock` are called concurrently
nameLock.inc()
l.mu.Unlock()
// Lock the nameLock outside the main mutex so we don't block other operations
// once locked then we can decrement the number of waiters for this lock
nameLock.Lock()
nameLock.dec()
}
// Unlock unlocks the mutex with the given name
// If the given lock is not being waited on by any other callers, it is deleted
func (l *Locker) Unlock(name string) error {
l.mu.Lock()
nameLock, exists := l.locks[name]
if !exists {
l.mu.Unlock()
return ErrNoSuchLock
}
if nameLock.count() == 0 {
delete(l.locks, name)
}
nameLock.Unlock()
l.mu.Unlock()
return nil
}
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