telegram: exclude link targets from message length

notify/telegram/telegram.go

@@ -22,16 +22,19 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"unicode/utf8"
 
 	commoncfg "github.com/prometheus/common/config"
+	"golang.org/x/net/html"
 	"gopkg.in/telebot.v3"
 
 	"github.com/prometheus/alertmanager/notify"
 	"github.com/prometheus/alertmanager/template"
 	"github.com/prometheus/alertmanager/types"
 )
 
-// Telegram supports 4096 chars max - from https://limits.tginfo.me/en.
+// Telegram supports up to 4096 characters after entity parsing.
+// See https://core.telegram.org/bots/api#sendmessage.
 const maxMessageLenRunes = 4096
 
 // Notifier implements a Notifier for telegram notifications.
@@ -92,7 +95,7 @@ func (n *Notifier) Notify(ctx context.Context, alert ...*types.Alert) (bool, err
 		if err != nil {
 			return false, err
 		}
-		if len([]rune(messageText)) > maxMessageLenRunes {
+		if htmlTextRuneCount(messageText) > maxMessageLenRunes {
 			messageText = `Alertmanager notification could not be sent: message length exceeds Telegram limits.
 			Please check the template used for producing the message content.`
 		}
@@ -147,6 +150,26 @@ func wrapWithFailureReason(err error) error {
 	return err
 }
 
+// htmlTextRuneCount excludes HTML markup and attribute values such as link targets.
+func htmlTextRuneCount(message string) int {
+	tokenizer := html.NewTokenizer(strings.NewReader(message))
+	count := 0
+
+	for {
+		switch tokenizer.Next() {
+		case html.ErrorToken:
+			if len(tokenizer.Raw()) > 0 {
+				return utf8.RuneCountInString(message)
+			}
+			return count
+		case html.CommentToken, html.DoctypeToken:
+			return utf8.RuneCountInString(message)
+		case html.TextToken:
+			count += utf8.RuneCount(tokenizer.Text())
+		}
+	}
+}
+
 func createTelegramClient(apiURL, parseMode string, httpClient *http.Client) (*telebot.Bot, error) {
 	bot, err := telebot.NewBot(telebot.Settings{
 		URL:       apiURL,

notify/telegram/telegram_test.go

@@ -83,6 +83,13 @@ func TestTelegramRetry(t *testing.T) {
 
 func TestTelegramNotify(t *testing.T) {
 	token := "secret"
+	longHTMLLink := `<a href="https://example.com/` + strings.Repeat("x", maxMessageLenRunes) + `">Open</a>`
+	longMalformedHTML := `<a href="` + strings.Repeat("x", maxMessageLenRunes)
+	longMalformedHTMLTemplate := fmt.Sprintf(`{{ %q | safeHtml }}`, longMalformedHTML)
+	longMalformedComment := `<!--` + strings.Repeat("x", maxMessageLenRunes)
+	longMalformedCommentTemplate := fmt.Sprintf(`{{ %q | safeHtml }}`, longMalformedComment)
+	longMalformedDoctype := `<!DOCTYPE ` + strings.Repeat("x", maxMessageLenRunes)
+	longMalformedDoctypeTemplate := fmt.Sprintf(`{{ %q | safeHtml }}`, longMalformedDoctype)
 
 	fileWithToken, err := os.CreateTemp(t.TempDir(), "telegram-bot-token")
 	require.NoError(t, err, "creating temp file failed")
@@ -133,6 +140,49 @@ func TestTelegramNotify(t *testing.T) {
 			expText: `Alertmanager notification could not be sent: message length exceeds Telegram limits.
 			Please check the template used for producing the message content.`,
 		},
+		{
+			name: "HTML mode ignores link targets when enforcing length limit",
+			cfg: TelegramConfig{
+				ParseMode:  "HTML",
+				Message:    longHTMLLink,
+				HTTPConfig: &commoncfg.HTTPClientConfig{},
+				BotToken:   commoncfg.Secret(token),
+			},
+			expText: longHTMLLink,
+		},
+		{
+			name: "HTML mode falls back for too-large malformed tag",
+			cfg: TelegramConfig{
+				ParseMode:  "HTML",
+				Message:    longMalformedHTMLTemplate,
+				HTTPConfig: &commoncfg.HTTPClientConfig{},
+				BotToken:   commoncfg.Secret(token),
+			},
+			expText: `Alertmanager notification could not be sent: message length exceeds Telegram limits.
+			Please check the template used for producing the message content.`,
+		},
+		{
+			name: "HTML mode falls back for too-large malformed comment",
+			cfg: TelegramConfig{
+				ParseMode:  "HTML",
+				Message:    longMalformedCommentTemplate,
+				HTTPConfig: &commoncfg.HTTPClientConfig{},
+				BotToken:   commoncfg.Secret(token),
+			},
+			expText: `Alertmanager notification could not be sent: message length exceeds Telegram limits.
+			Please check the template used for producing the message content.`,
+		},
+		{
+			name: "HTML mode falls back for too-large malformed doctype",
+			cfg: TelegramConfig{
+				ParseMode:  "HTML",
+				Message:    longMalformedDoctypeTemplate,
+				HTTPConfig: &commoncfg.HTTPClientConfig{},
+				BotToken:   commoncfg.Secret(token),
+			},
+			expText: `Alertmanager notification could not be sent: message length exceeds Telegram limits.
+			Please check the template used for producing the message content.`,
+		},
 		{
 			name: "Default mode with too-large message",
 			cfg: TelegramConfig{