Added test and fixed formatting.

internal/cli/monitor/monitor.go

@@ -24,7 +24,7 @@ import (
 	"os"
 	"sort"
 	"strings"
-        "time"
+	"time"
 
 	"github.com/arduino/arduino-cli/commands/monitor"
 	"github.com/arduino/arduino-cli/configuration"
@@ -250,33 +250,33 @@ func contains(s []string, searchterm string) bool {
 }
 
 type timeStampWriter struct {
-	writer io.Writer
+	writer            io.Writer
 	sendTimeStampNext bool
 }
 
 func newTimeStampWriter(writer io.Writer) *timeStampWriter {
 	return &timeStampWriter{
-		writer: writer,
+		writer:            writer,
 		sendTimeStampNext: true,
 	}
 }
 
 func (t *timeStampWriter) Write(p []byte) (int, error) {
 	written := 0
 	for _, b := range p {
-		if (t.sendTimeStampNext) {
+		if t.sendTimeStampNext {
 			_, err := t.writer.Write([]byte(time.Now().Format("[2006-01-02 15:04:05] ")))
 			if err != nil {
 				return written, err
 			}
-			t.sendTimeStampNext = false;
+			t.sendTimeStampNext = false
 		}
 		n, err := t.writer.Write([]byte{b})
 		written += n
 		if err != nil {
 			return written, err
 		}
-		t.sendTimeStampNext = b == '\n';
+		t.sendTimeStampNext = b == '\n'
 	}
 	return written, nil
 }

internal/cli/monitor/monitor_test.go

@@ -0,0 +1,37 @@
+// This file is part of arduino-cli.
+//
+// Copyright 2023 ARDUINO SA (http://www.arduino.cc/)
+//
+// This software is released under the GNU General Public License version 3,
+// which covers the main part of arduino-cli.
+// The terms of this license can be found at:
+// https://www.gnu.org/licenses/gpl-3.0.en.html
+//
+// You can be released from the requirements of the above licenses by purchasing
+// a commercial license. Buying such a license is mandatory if you want to
+// modify or otherwise use the software for commercial activities involving the
+// Arduino software without disclosing the source code of your own applications.
+// To purchase a commercial license, send an email to [email protected].
+
+package monitor
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestTimeStampWriter(t *testing.T) {
+	buf := &bytes.Buffer{}
+	writer := newTimeStampWriter(buf)
+
+	writer.Write([]byte("foo"))
+	// The first received bytes get a timestamp prepended
+	require.Regexp(t, `^\[\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}\] foo$`, buf)
+
+	buf.Reset()
+	writer.Write([]byte("\nbar\n"))
+	// A timestamp should be inserted before the first char of the next line
+	require.Regexp(t, "^\n"+`\[\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}\] bar` + "\n$", buf)
+}