[#585] fstree: Add optional file counter

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/util/sync/key_locker.go

@@ -0,0 +1,56 @@
+package sync
+
+import "sync"
+
+type locker struct {
+	mtx     sync.Mutex
+	waiters int // not protected by mtx, must used outer mutex to update concurrently
+}
+
+type KeyLocker[K comparable] struct {
+	lockers    map[K]*locker
+	lockersMtx sync.Mutex
+}
+
+func NewKeyLocker[K comparable]() *KeyLocker[K] {
+	return &KeyLocker[K]{
+		lockers: make(map[K]*locker),
+	}
+}
+
+func (l *KeyLocker[K]) Lock(key K) {
+	l.lockersMtx.Lock()
+
+	if locker, found := l.lockers[key]; found {
+		locker.waiters++
+		l.lockersMtx.Unlock()
+
+		locker.mtx.Lock()
+		return
+	}
+
+	locker := &locker{
+		waiters: 1,
+	}
+	locker.mtx.Lock()
+
+	l.lockers[key] = locker
+	l.lockersMtx.Unlock()
+}
+
+func (l *KeyLocker[K]) Unlock(key K) {
+	l.lockersMtx.Lock()
+	defer l.lockersMtx.Unlock()
+
+	locker, found := l.lockers[key]
+	if !found {
+		return
+	}
+
+	if locker.waiters == 1 {
+		delete(l.lockers, key)
+	}
+	locker.waiters--
+
+	locker.mtx.Unlock()
+}