1 files changed, 69 insertions, 0 deletions
diff --git a/modules/sync/exclusive_pool.go b/modules/sync/exclusive_pool.go
new file mode 100644
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+++ b/modules/sync/exclusive_pool.go
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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]--
+	}
+}