// Copyright 2016 The Gogs Authors. All rights reserved.// SPDX-License-Identifier: MIT package sync import (	"sync") // ExclusivePool is a pool of non-identical instances// that only one instance with same identity is in the pool at a time.// In other words, only instances with different identities can be in// the pool the same time. If another instance with same identity tries// to get into the pool, it hangs until previous instance left the pool.//// This pool is particularly useful for performing tasks on same resource// on the file system in different goroutines.type ExclusivePool struct {	lock sync.Mutex 	// pool maintains locks for each instance in the pool.	pool map[string]*sync.Mutex 	// count maintains the number of times an instance with same identity checks in	// to the pool, and should be reduced to 0 (removed from map) by checking out	// with same number of times.	// The purpose of count is to delete lock when count down to 0 and recycle memory	// from map object.	count map[string]int} // NewExclusivePool initializes and returns a new ExclusivePool object.func NewExclusivePool() *ExclusivePool {	return &ExclusivePool{		pool:  make(map[string]*sync.Mutex),		count: make(map[string]int),	}} // CheckIn checks in an instance to the pool and hangs while instance// with same identity is using the lock.func (p *ExclusivePool) CheckIn(identity string) {	p.lock.Lock() 	lock, has := p.pool[identity]	if !has {		lock = &sync.Mutex{}		p.pool[identity] = lock	}	p.count[identity]++ 	p.lock.Unlock()	lock.Lock()} // CheckOut checks out an instance from the pool and releases the lock// to let other instances with same identity to grab the lock.func (p *ExclusivePool) CheckOut(identity string) {	p.lock.Lock()	defer p.lock.Unlock() 	p.pool[identity].Unlock()	if p.count[identity] == 1 {		delete(p.pool, identity)		delete(p.count, identity)	} else {		p.count[identity]--	}}