add concurrency limits for tag lookup and untag

Harbor is using the distribution for it's (harbor-registry) registry component.
The harbor GC will call into the registry to delete the manifest, which in turn
then does a lookup for all tags that reference the deleted manifest.
To find the tag references, the registry will iterate every tag in the repository
and read it's link file to check if it matches the deleted manifest (i.e. to see
if uses the same sha256 digest). So, the more tags in repository, the worse the
performance will be (as there will be more s3 API calls occurring for the tag
directory lookups and tag file reads).

Therefore, we can use concurrent lookup and untag to optimize performance as described in https://github.com/goharbor/harbor/issues/12948.

P.S. This optimization was originally contributed by @Antiarchitect, now I would like to take it over.
Thanks @Antiarchitect's efforts with PR https://github.com/distribution/distribution/pull/3890.

Signed-off-by: Liang Zheng <zhengliang0901@gmail.com>

registry/handlers/app.go

@@ -188,6 +188,21 @@ func NewApp(ctx context.Context, config *configuration.Configuration) *App {
 		}
 	}
 
+	// configure tag lookup concurrency limit
+	if p := config.Storage.TagParameters(); p != nil {
+		l, ok := p["concurrencylimit"]
+		if ok {
+			limit, ok := l.(int)
+			if !ok {
+				panic("tag lookup concurrency limit config key must have a integer value")
+			}
+			if limit < 0 {
+				panic("tag lookup concurrency limit should be a non-negative integer value")
+			}
+			options = append(options, storage.TagLookupConcurrencyLimit(limit))
+		}
+	}
+
 	// configure redirects
 	var redirectDisabled bool
 	if redirectConfig, ok := config.Storage["redirect"]; ok {