content: include the staleness of the lock when tryLock() fails

Kazuyoshi Kato · Kazuyoshi Kato · commit c95d71cf6080 · 2020-12-02T14:08:02.000-08:00
When multiple clients are pulling the same image, we may have this
lock error. Short-lived locks are probably fine, but long-lived locks
may indicate that containerd has some issues.

Signed-off-by: Kazuyoshi Kato &lt;katokazu@amazon.com&gt;
diff --git a/content/local/locks.go b/content/local/locks.go
@@ -18,28 +18,33 @@ package local
 
 import (
 	"sync"
+	"time"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/pkg/errors"
 )
 
 // Handles locking references
 
+type lock struct {
+	since time.Time
+}
+
 var (
 	// locks lets us lock in process
-	locks   = map[string]struct{}{}
+	locks   = make(map[string]*lock)
 	locksMu sync.Mutex
 )
 
 func tryLock(ref string) error {
 	locksMu.Lock()
 	defer locksMu.Unlock()
 
-	if _, ok := locks[ref]; ok {
-		return errors.Wrapf(errdefs.ErrUnavailable, "ref %s locked", ref)
+	if v, ok := locks[ref]; ok {
+		return errors.Wrapf(errdefs.ErrUnavailable, "ref %s locked since %s", ref, v.since)
 	}
 
-	locks[ref] = struct{}{}
+	locks[ref] = &lock{time.Now()}
 	return nil
 }
 
diff --git a/content/local/locks_test.go b/content/local/locks_test.go
@@ -0,0 +1,32 @@
+/*
+   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 (
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestTryLock(t *testing.T) {
+	err := tryLock("testref")
+	assert.NilError(t, err)
+	defer unlock("testref")
+
+	err = tryLock("testref")
+	assert.ErrorContains(t, err, "ref testref locked since ")
+}
