Write interactive prompts to stderr and handle echo-disable failures

Interactive prompts from 'helm registry login' (username/password/token)
and the signing-key passphrase prompts in 'helm package' and
'helm plugin package' are printed to stdout. Anyone capturing stdout
(scripts, pipelines) gets prompt text mixed into the output; the
docker/kubectl/oras convention is to keep prompts on stderr so stdout
stays machine-readable.

Also in registry login's readLine:

- the term.DisableEcho error was silently discarded, so a failure to
  disable echo would print the password to the terminal with no
  warning; it now returns an error instead
- echo handling is only attempted when stdin is actually a terminal,
  so piped input (e.g. 'echo pw | helm registry login ...' without
  --password-stdin) keeps working as before

Signed-off-by: Mukul <nmukul32@gmail.com>

pkg/action/package.go

@@ -206,11 +206,12 @@ func (p *Package) Clearsign(filename string) error {
 
 // promptUser implements provenance.PassphraseFetcher
 func promptUser(name string) ([]byte, error) {
-	fmt.Printf("Password for key %q >  ", name)
+	// Prompts go to stderr so stdout stays clean for scripting.
+	fmt.Fprintf(os.Stderr, "Password for key %q >  ", name)
 	// syscall.Stdin is not an int in all environments and needs to be coerced
 	// into one there (e.g., Windows)
 	pw, err := term.ReadPassword(int(syscall.Stdin))
-	fmt.Println()
+	fmt.Fprintln(os.Stderr)
 	return pw, err
 }
 

pkg/cmd/plugin_package.go

@@ -172,9 +172,10 @@ func (o *pluginPackageOptions) run(out io.Writer) error {
 }
 
 func (o *pluginPackageOptions) promptUser(name string) ([]byte, error) {
-	fmt.Printf("Password for key %q >  ", name)
+	// Prompts go to stderr so stdout stays clean for scripting.
+	fmt.Fprintf(os.Stderr, "Password for key %q >  ", name)
 	pw, err := term.ReadPassword(int(syscall.Stdin))
-	fmt.Println()
+	fmt.Fprintln(os.Stderr)
 	return pw, err
 }
 

pkg/cmd/registry_login.go

@@ -136,14 +136,17 @@ func getUsernamePassword(usernameOpt string, passwordOpt string, passwordFromStd
 
 // Copied/adapted from https://github.com/oras-project/oras
 func readLine(prompt string, silent bool) (string, error) {
-	fmt.Print(prompt)
-	if silent {
+	// Prompts go to stderr so stdout stays clean for scripting.
+	fmt.Fprint(os.Stderr, prompt)
+	if silent && term.IsTerminal(os.Stdin.Fd()) {
 		fd := os.Stdin.Fd()
 		state, err := term.SaveState(fd)
 		if err != nil {
 			return "", err
 		}
-		term.DisableEcho(fd, state)
+		if err := term.DisableEcho(fd, state); err != nil {
+			return "", fmt.Errorf("failed to disable terminal echo for password input: %w", err)
+		}
 		defer term.RestoreTerminal(fd, state)
 	}
 
@@ -153,7 +156,7 @@ func readLine(prompt string, silent bool) (string, error) {
 		return "", err
 	}
 	if silent {
-		fmt.Println()
+		fmt.Fprintln(os.Stderr)
 	}
 
 	return string(line), nil