content: return the error with its timestamp

The current timestamp is probably included in production logs, but
won't be in testing environments such as GitHub Actions.

Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>

content/local/locks.go

@@ -41,7 +41,13 @@ func tryLock(ref string) error {
 	defer locksMu.Unlock()
 
 	if v, ok := locks[ref]; ok {
-		return errors.Wrapf(errdefs.ErrUnavailable, "ref %s locked since %s", ref, v.since)
+		// Returning the duration may help developers distinguish dead locks (long duration) from
+		// lock contentions (short duration).
+		now := time.Now()
+		return errors.Wrapf(
+			errdefs.ErrUnavailable,
+			"ref %s locked for %s (since %s)", ref, now.Sub(v.since), v.since,
+		)
 	}
 
 	locks[ref] = &lock{time.Now()}