Move content/local to plugins/content/local

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

plugins/content/local/locks.go

@@ -0,0 +1,62 @@
+/*
+   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 local
+
+import (
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/containerd/containerd/v2/errdefs"
+)
+
+// Handles locking references
+
+type lock struct {
+	since time.Time
+}
+
+var (
+	// locks lets us lock in process
+	locks   = make(map[string]*lock)
+	locksMu sync.Mutex
+)
+
+func tryLock(ref string) error {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	if v, ok := locks[ref]; ok {
+		// Returning the duration may help developers distinguish dead locks (long duration) from
+		// lock contentions (short duration).
+		now := time.Now()
+		return fmt.Errorf(
+			"ref %s locked for %s (since %s): %w", ref, now.Sub(v.since), v.since,
+			errdefs.ErrUnavailable,
+		)
+	}
+
+	locks[ref] = &lock{time.Now()}
+	return nil
+}
+
+func unlock(ref string) {
+	locksMu.Lock()
+	defer locksMu.Unlock()
+
+	delete(locks, ref)
+}