cli/command: PromptUserForCredentials: remove named output variables

This function has multiple conditional branches, which makes it harder
to see at a glance whether authConfig may be partially populated. This
patch instead returns a fresh instance for error returns to prevent any
confusion.

It also removes the named output variables, as they're now no longer used,
and the returned types should already be descriptive enough to understand
what's returned.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 3a8485085d)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/registry.go

@@ -112,7 +112,7 @@ func ConfigureAuth(ctx context.Context, cli Cli, flUser, flPassword string, auth
 // If defaultUsername is not empty, the username prompt includes that username
 // and the user can hit enter without inputting a username  to use that default
 // username.
-func PromptUserForCredentials(ctx context.Context, cli Cli, argUser, argPassword, defaultUsername, serverAddress string) (authConfig registrytypes.AuthConfig, err error) {
+func PromptUserForCredentials(ctx context.Context, cli Cli, argUser, argPassword, defaultUsername, serverAddress string) (registrytypes.AuthConfig, error) {
 	// On Windows, force the use of the regular OS stdin stream.
 	//
 	// See:
@@ -144,38 +144,41 @@ func PromptUserForCredentials(ctx context.Context, cli Cli, argUser, argPassword
 		} else {
 			prompt = fmt.Sprintf("Username (%s): ", defaultUsername)
 		}
+
+		var err error
 		argUser, err = PromptForInput(ctx, cli.In(), cli.Out(), prompt)
 		if err != nil {
-			return authConfig, err
+			return registrytypes.AuthConfig{}, err
 		}
 		if argUser == "" {
 			argUser = defaultUsername
 		}
 	}
 	if argUser == "" {
-		return authConfig, errors.Errorf("Error: Non-null Username Required")
+		return registrytypes.AuthConfig{}, errors.Errorf("Error: Non-null Username Required")
 	}
 	if argPassword == "" {
 		restoreInput, err := DisableInputEcho(cli.In())
 		if err != nil {
-			return authConfig, err
+			return registrytypes.AuthConfig{}, err
 		}
 		defer restoreInput()
 
 		argPassword, err = PromptForInput(ctx, cli.In(), cli.Out(), "Password: ")
 		if err != nil {
-			return authConfig, err
+			return registrytypes.AuthConfig{}, err
 		}
 		fmt.Fprint(cli.Out(), "\n")
 		if argPassword == "" {
-			return authConfig, errors.Errorf("Error: Password Required")
+			return registrytypes.AuthConfig{}, errors.Errorf("Error: Password Required")
 		}
 	}
 
-	authConfig.Username = argUser
-	authConfig.Password = argPassword
-	authConfig.ServerAddress = serverAddress
-	return authConfig, nil
+	return registrytypes.AuthConfig{
+		Username:      argUser,
+		Password:      argPassword,
+		ServerAddress: serverAddress,
+	}, nil
 }
 
 // RetrieveAuthTokenFromImage retrieves an encoded auth token given a complete