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

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 <katokazu@amazon.com>

content/local/locks.go

@@ -18,6 +18,7 @@ package local
 
 import (
 	"sync"
+	"time"
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/pkg/errors"
@@ -25,9 +26,13 @@ import (
 
 // 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
 )
 
@@ -35,11 +40,11 @@ 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
 }