Refactor dockerResolver to make code simple

1.add resolveDockerBase helper
2.dockerBase header copy with header.Clone()

Signed-off-by: songjiayang <songjiayang@jd.com>

remotes/docker/resolver.go

@@ -219,20 +219,15 @@ func (r *countingReader) Read(p []byte) (int, error) {
 var _ remotes.Resolver = &dockerResolver{}
 
 func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocispec.Descriptor, error) {
-	refspec, err := reference.Parse(ref)
+	base, err := r.resolveDockerBase(ref)
 	if err != nil {
 		return "", ocispec.Descriptor{}, err
 	}
-
+	refspec := base.refspec
 	if refspec.Object == "" {
 		return "", ocispec.Descriptor{}, reference.ErrObjectRequired
 	}
 
-	base, err := r.base(refspec)
-	if err != nil {
-		return "", ocispec.Descriptor{}, err
-	}
-
 	var (
 		lastErr error
 		paths   [][]string
@@ -387,12 +382,7 @@ func (r *dockerResolver) Resolve(ctx context.Context, ref string) (string, ocisp
 }
 
 func (r *dockerResolver) Fetcher(ctx context.Context, ref string) (remotes.Fetcher, error) {
-	refspec, err := reference.Parse(ref)
-	if err != nil {
-		return nil, err
-	}
-
-	base, err := r.base(refspec)
+	base, err := r.resolveDockerBase(ref)
 	if err != nil {
 		return nil, err
 	}
@@ -403,23 +393,27 @@ func (r *dockerResolver) Fetcher(ctx context.Context, ref string) (remotes.Fetch
 }
 
 func (r *dockerResolver) Pusher(ctx context.Context, ref string) (remotes.Pusher, error) {
-	refspec, err := reference.Parse(ref)
-	if err != nil {
-		return nil, err
-	}
-
-	base, err := r.base(refspec)
+	base, err := r.resolveDockerBase(ref)
 	if err != nil {
 		return nil, err
 	}
 
 	return dockerPusher{
 		dockerBase: base,
-		object:     refspec.Object,
+		object:     base.refspec.Object,
 		tracker:    r.tracker,
 	}, nil
 }
 
+func (r *dockerResolver) resolveDockerBase(ref string) (*dockerBase, error) {
+	refspec, err := reference.Parse(ref)
+	if err != nil {
+		return nil, err
+	}
+
+	return r.base(refspec)
+}
+
 type dockerBase struct {
 	refspec    reference.Spec
 	repository string
@@ -451,10 +445,8 @@ func (r *dockerBase) filterHosts(caps HostCapabilities) (hosts []RegistryHost) {
 }
 
 func (r *dockerBase) request(host RegistryHost, method string, ps ...string) *request {
-	header := http.Header{}
-	for key, value := range r.header {
-		header[key] = append(header[key], value...)
-	}
+	header := r.header.Clone()
+
 	for key, value := range host.Header {
 		header[key] = append(header[key], value...)
 	}