Move pkg/kmutex to internal/kmutex

Signed-off-by: Derek McGowan <derek@mcg.dev>

internal/kmutex/kmutex.go

@@ -0,0 +1,105 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+// Package kmutex provides synchronization primitives to lock/unlock resource by unique key.
+package kmutex
+
+import (
+	"context"
+	"fmt"
+	"sync"
+
+	"golang.org/x/sync/semaphore"
+)
+
+// KeyedLocker is the interface for acquiring locks based on string.
+type KeyedLocker interface {
+	Lock(ctx context.Context, key string) error
+	Unlock(key string)
+}
+
+func New() KeyedLocker {
+	return newKeyMutex()
+}
+
+func newKeyMutex() *keyMutex {
+	return &keyMutex{
+		locks: make(map[string]*klock),
+	}
+}
+
+type keyMutex struct {
+	mu sync.Mutex
+
+	locks map[string]*klock
+}
+
+type klock struct {
+	*semaphore.Weighted
+	ref int
+}
+
+func (km *keyMutex) Lock(ctx context.Context, key string) error {
+	km.mu.Lock()
+
+	l, ok := km.locks[key]
+	if !ok {
+		km.locks[key] = &klock{
+			Weighted: semaphore.NewWeighted(1),
+		}
+		l = km.locks[key]
+	}
+	l.ref++
+	km.mu.Unlock()
+
+	if err := l.Acquire(ctx, 1); err != nil {
+		km.mu.Lock()
+		defer km.mu.Unlock()
+
+		l.ref--
+
+		if l.ref < 0 {
+			panic(fmt.Errorf("kmutex: release of unlocked key %v", key))
+		}
+
+		if l.ref == 0 {
+			delete(km.locks, key)
+		}
+		return err
+	}
+	return nil
+}
+
+func (km *keyMutex) Unlock(key string) {
+	km.mu.Lock()
+	defer km.mu.Unlock()
+
+	l, ok := km.locks[key]
+	if !ok {
+		panic(fmt.Errorf("kmutex: unlock of unlocked key %v", key))
+	}
+	l.Release(1)
+
+	l.ref--
+
+	if l.ref < 0 {
+		panic(fmt.Errorf("kmutex: released of unlocked key %v", key))
+	}
+
+	if l.ref == 0 {
+		delete(km.locks, key)
+	}
+}