Avoid nil dereference if passing a nil resolver

Signed-off-by: Antonio Gamez Diaz <agamez@vmware.com>

pkg/registry/client.go

@@ -88,37 +88,43 @@ func NewClient(options ...ClientOption) (*Client, error) {
 		client.authorizer = authClient
 	}
 
-	if client.resolver == nil {
-		client.resolver = func(ref registry.Reference) (remotes.Resolver, error) {
-			headers := http.Header{}
-			headers.Set("User-Agent", version.GetUserAgent())
-			dockerClient, ok := client.authorizer.(*dockerauth.Client)
-			if ok {
-				username, password, err := dockerClient.Credential(ref.Registry)
-				if err != nil {
-					return nil, errors.New("unable to retrieve credentials")
-				}
-				// A blank returned username and password value is a bearer token
-				if username == "" && password != "" {
-					headers.Set("Authorization", fmt.Sprintf("Bearer %s", password))
-				} else {
-					headers.Set("Authorization", fmt.Sprintf("Basic %s", basicAuth(username, password)))
-				}
+	resolverFn := client.resolver // copy for avoiding recursive call
+	client.resolver = func(ref registry.Reference) (remotes.Resolver, error) {
+		if resolverFn != nil {
+			// validate if the resolverFn returns a valid resolver
+			if resolver, err := resolverFn(ref); resolver != nil && err == nil {
+				return resolver, nil
 			}
+		}
 
-			opts := []auth.ResolverOption{auth.WithResolverHeaders(headers)}
-			if client.httpClient != nil {
-				opts = append(opts, auth.WithResolverClient(client.httpClient))
-			}
-			if client.plainHTTP {
-				opts = append(opts, auth.WithResolverPlainHTTP())
-			}
-			resolver, err := client.authorizer.ResolverWithOpts(opts...)
+		headers := http.Header{}
+		headers.Set("User-Agent", version.GetUserAgent())
+		dockerClient, ok := client.authorizer.(*dockerauth.Client)
+		if ok {
+			username, password, err := dockerClient.Credential(ref.Registry)
 			if err != nil {
-				return nil, err
+				return nil, errors.New("unable to retrieve credentials")
 			}
-			return resolver, nil
+			// A blank returned username and password value is a bearer token
+			if username == "" && password != "" {
+				headers.Set("Authorization", fmt.Sprintf("Bearer %s", password))
+			} else {
+				headers.Set("Authorization", fmt.Sprintf("Basic %s", basicAuth(username, password)))
+			}
+		}
+
+		opts := []auth.ResolverOption{auth.WithResolverHeaders(headers)}
+		if client.httpClient != nil {
+			opts = append(opts, auth.WithResolverClient(client.httpClient))
+		}
+		if client.plainHTTP {
+			opts = append(opts, auth.WithResolverPlainHTTP())
+		}
+		resolver, err := client.authorizer.ResolverWithOpts(opts...)
+		if err != nil {
+			return nil, err
 		}
+		return resolver, nil
 	}
 
 	// allocate a cache if option is set