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// 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]--
}
}
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