cli/command: ConfigureAuth: fix terminal state not being restored on error

ConfigureAuth used the readInput() utility to read the username and password.
However, this utility did not return errors it encountered, but instead did
an os.Exit(1). A result of this was that the terminal was not restored if
an error happened. When reading the password, the terminal is configured to
disable echo (i.e. characters are not printed), and failing to restore
the previous state means that the terminal is now "non-functional".

This patch:

- changes readInput() to return errors it encounters
- uses a defer() to restore terminal state

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/registry.go

@@ -113,7 +113,11 @@ func ConfigureAuth(cli Cli, flUser, flPassword string, authconfig *registrytypes
 			fmt.Fprintln(cli.Out(), "Login with your Docker ID to push and pull images from Docker Hub. If you don't have a Docker ID, head over to https://hub.docker.com to create one.")
 		}
 		promptWithDefault(cli.Out(), "Username", authconfig.Username)
-		flUser = readInput(cli.In(), cli.Out())
+		var err error
+		flUser, err = readInput(cli.In())
+		if err != nil {
+			return err
+		}
 		flUser = strings.TrimSpace(flUser)
 		if flUser == "" {
 			flUser = authconfig.Username
@@ -128,12 +132,15 @@ func ConfigureAuth(cli Cli, flUser, flPassword string, authconfig *registrytypes
 			return err
 		}
 		fmt.Fprintf(cli.Out(), "Password: ")
-		term.DisableEcho(cli.In().FD(), oldState)
-
-		flPassword = readInput(cli.In(), cli.Out())
+		_ = term.DisableEcho(cli.In().FD(), oldState)
+		defer func() {
+			_ = term.RestoreTerminal(cli.In().FD(), oldState)
+		}()
+		flPassword, err = readInput(cli.In())
+		if err != nil {
+			return err
+		}
 		fmt.Fprint(cli.Out(), "\n")
-
-		term.RestoreTerminal(cli.In().FD(), oldState)
 		if flPassword == "" {
 			return errors.Errorf("Error: Password Required")
 		}
@@ -145,14 +152,14 @@ func ConfigureAuth(cli Cli, flUser, flPassword string, authconfig *registrytypes
 	return nil
 }
 
-func readInput(in io.Reader, out io.Writer) string {
-	reader := bufio.NewReader(in)
-	line, _, err := reader.ReadLine()
+// readInput reads, and returns user input from in. It tries to return a
+// single line, not including the end-of-line bytes.
+func readInput(in io.Reader) (string, error) {
+	line, _, err := bufio.NewReader(in).ReadLine()
 	if err != nil {
-		fmt.Fprintln(out, err.Error())
-		os.Exit(1)
+		return "", errors.Wrap(err, "error while reading input")
 	}
-	return string(line)
+	return string(line), nil
 }
 
 func promptWithDefault(out io.Writer, prompt string, configDefault string) {