Remove extra new line in client error messages (#65206)

* Fix formatErrorWriter adding duplicate new lines.

* Address comments.

* Address claude comment.

lib/utils/cli.go

@@ -137,7 +137,7 @@ func InitLogger(purpose LoggingPurpose, level slog.Level, opts ...LoggerOption)
 // FatalError is for CLI front-ends: it detects gravitational/trace debugging
 // information, sends it to the logger, strips it off and prints a clean message to stderr
 func FatalError(err error) {
-	fmt.Fprintln(os.Stderr, UserMessageFromError(err))
+	fmt.Fprint(os.Stderr, UserMessageFromError(err))
 	os.Exit(1)
 }
 
@@ -158,13 +158,17 @@ func GetIterations() int {
 
 // UserMessageFromError returns user-friendly error message from error.
 // The error message will be formatted for output depending on the debug
-// flag
+// flag and will always end with a new line.
 func UserMessageFromError(err error) string {
 	if err == nil {
 		return ""
 	}
 	if slog.Default().Enabled(context.Background(), slog.LevelDebug) {
-		return trace.DebugReport(err)
+		msg := trace.DebugReport(err)
+		if !strings.HasSuffix(msg, "\n") {
+			msg += "\n"
+		}
+		return msg
 	}
 	var buf bytes.Buffer
 	if runtime.GOOS == constants.WindowsOS {
@@ -181,37 +185,37 @@ func UserMessageFromError(err error) string {
 }
 
 // FormatErrorWithNewline returns user friendly error message from error.
-// The error message is escaped if necessary. A newline is added if the error text
-// does not end with a newline.
+// The message will always end with a new line.
 func FormatErrorWithNewline(err error) string {
 	var buf bytes.Buffer
 	formatErrorWriter(err, &buf)
-	message := buf.String()
-	if !strings.HasSuffix(message, "\n") {
-		message = message + "\n"
-	}
-	return message
+	return buf.String()
 }
 
 // formatErrorWriter formats the specified error into the provided writer.
-// The error message is escaped if necessary
+// The error message is escaped if necessary. A newline is added if the
+// error text does not end with a newline.
 func formatErrorWriter(err error, w io.Writer) {
 	if err == nil {
 		return
 	}
-	if certErr := formatCertError(err); certErr != "" {
-		fmt.Fprintln(w, certErr)
-		return
-	}
 
 	msg := trace.UserMessage(err)
+	if certErr := formatCertError(err); certErr != "" {
+		msg = certErr
+	}
+
 	// Error can be of type trace.proxyError where error message didn't get captured.
 	if msg == "" {
 		fmt.Fprintln(w, "please check Teleport's log for more details")
 		return
 	}
 
-	fmt.Fprintln(w, AllowWhitespace(msg))
+	msg = AllowWhitespace(msg)
+	fmt.Fprint(w, msg)
+	if !strings.HasSuffix(msg, "\n") {
+		w.Write([]byte("\n"))
+	}
 }
 
 func formatCertError(err error) string {

tool/tsh/common/tsh.go

@@ -4214,7 +4214,7 @@ func retryWithAccessRequest(
 	}
 
 	// Print and log the original AccessDenied error.
-	fmt.Fprintln(os.Stderr, utils.UserMessageFromError(origErr))
+	fmt.Fprint(os.Stderr, utils.UserMessageFromError(origErr))
 	fmt.Fprintf(os.Stdout, "You do not currently have access to %q, attempting to request access.\n\n", resource)
 	if err := promptUserForAccessRequestDetails(cf, req); err != nil {
 		return trace.Wrap(err)
@@ -4599,7 +4599,7 @@ func convertSSHExitCode(tc *client.TeleportClient, err error) error {
 		}
 
 		// For unexpected errors, print the error message before converting to an exitCodeError.
-		fmt.Fprintln(tc.Stderr, utils.UserMessageFromError(err))
+		fmt.Fprint(tc.Stderr, utils.UserMessageFromError(err))
 		return trace.Wrap(&common.ExitCodeError{Code: status})
 	}
 	return trace.Wrap(err)
@@ -4618,7 +4618,7 @@ func onBenchmark(cf *CLIConf, suite benchmark.Suite) error {
 
 	result, err := cnf.Benchmark(cf.Context, tc, suite)
 	if err != nil {
-		fmt.Fprintln(os.Stderr, utils.UserMessageFromError(err))
+		fmt.Fprint(os.Stderr, utils.UserMessageFromError(err))
 		return trace.Wrap(&common.ExitCodeError{Code: 255})
 	}
 	fmt.Fprintf(cf.Stdout(), "\n")
@@ -4642,7 +4642,7 @@ func onBenchmark(cf *CLIConf, suite benchmark.Suite) error {
 	if cf.BenchExport {
 		path, err := benchmark.ExportLatencyProfile(cf.Context, cf.BenchExportPath, result.Histogram, cf.BenchTicks, cf.BenchValueScale)
 		if err != nil {
-			fmt.Fprintf(cf.Stderr(), "failed exporting latency profile: %s\n", utils.UserMessageFromError(err))
+			fmt.Fprintf(cf.Stderr(), "failed exporting latency profile: %s\n", trace.UserMessage(err))
 		} else {
 			fmt.Fprintf(cf.Stdout(), "latency profile saved: %v\n", path)
 		}