modules/sync: move sync objects to independent module

modules/sync/single_instance_pool.go

@@ -0,0 +1,67 @@
+// Copyright 2016 The Gogs Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+import (
+	"sync"
+)
+
+// SingleInstancePool 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 exist
+// at the same time.
+//
+// This pool is particularly useful for performing tasks on same resource
+// on the file system in different goroutines.
+type SingleInstancePool 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.
+	count map[string]int
+}
+
+// NewSingleInstancePool initializes and returns a new SingleInstancePool object.
+func NewSingleInstancePool() *SingleInstancePool {
+	return &SingleInstancePool{
+		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 indentity is using the lock.
+func (p *SingleInstancePool) 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 *SingleInstancePool) 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]--
+	}
+}