listObjects optimized to handle max-keys=1 when prefix is object (#13000)

Some applications albeit poorly written rather than using headObject
rely on listObjects to check for existence of object, this unusual
request always has prefix=(to actual object) and max-keys=1

handle this situation specially such that we can avoid readdir()
on the top level parent to avoid sorting and skipping, ensuring
that such type of listObjects() always behaves similar to a
headObject() call.

cmd/erasure-server-pool.go

@@ -1060,6 +1060,22 @@ func maxKeysPlusOne(maxKeys int, addOne bool) int {
 
 func (z *erasureServerPools) ListObjects(ctx context.Context, bucket, prefix, marker, delimiter string, maxKeys int) (ListObjectsInfo, error) {
 	var loi ListObjectsInfo
+
+	if len(prefix) > 0 && maxKeys == 1 && delimiter == "" && marker == "" {
+		// Optimization for certain applications like
+		// - Cohesity
+		// - Actifio, Splunk etc.
+		// which send ListObjects requests where the actual object
+		// itself is the prefix and max-keys=1 in such scenarios
+		// we can simply verify locally if such an object exists
+		// to avoid the need for ListObjects().
+		objInfo, err := z.GetObjectInfo(ctx, bucket, prefix, ObjectOptions{NoLock: true})
+		if err == nil {
+			loi.Objects = append(loi.Objects, objInfo)
+			return loi, nil
+		}
+	}
+
 	opts := listPathOptions{
 		Bucket:      bucket,
 		Prefix:      prefix,