notify: fix panic when truncating multi-byte messages

notify/util.go

@@ -135,8 +135,11 @@ func TruncateInBytes(s string, n int) (string, bool) {
 	r := []rune(s)
 	truncationTarget := n - 3
 
-	// Next, let's truncate the runes to the lower possible number.
-	truncatedRunes := r[:truncationTarget]
+	// A string holding multi-byte characters has fewer runes than bytes, so
+	// truncationTarget can exceed the number of runes available. Start from
+	// the whole rune slice in that case; the loop below still trims it down to
+	// truncationTarget bytes.
+	truncatedRunes := r[:min(truncationTarget, len(r))]
 	for len(string(truncatedRunes)) > truncationTarget {
 		truncatedRunes = r[:len(truncatedRunes)-1]
 	}

notify/util_test.go

@@ -23,6 +23,7 @@ import (
 	"path"
 	"reflect"
 	"runtime"
+	"strings"
 	"testing"
 
 	"github.com/prometheus/common/model"
@@ -99,6 +100,14 @@ func TestTruncate(t *testing.T) {
 			runes: expect{out: "❤️✅🚀🔥❌❤️✅🚀🔥❌❤️✅🚀🔥❌…", trunc: true},
 			bytes: expect{out: "❤️✅🚀…", trunc: true},
 		},
+		{
+			// Two runes, eight bytes: the byte budget left for the text is
+			// larger than the number of runes to pick it from.
+			in:    "😀😀",
+			n:     7,
+			runes: expect{out: "😀😀", trunc: false},
+			bytes: expect{out: "😀…", trunc: true},
+		},
 	}
 
 	type truncateFunc func(string, int) (string, bool)
@@ -130,6 +139,42 @@ func TestTruncate(t *testing.T) {
 	}
 }
 
+// TestTruncateInBytesFewerRunesThanBytes covers strings that are longer than
+// the byte limit while holding fewer runes than it, which is the case for any
+// sufficiently long non-ASCII text. TruncateInBytes used to index the rune
+// slice with the byte budget and panicked on those inputs.
+func TestTruncateInBytesFewerRunesThanBytes(t *testing.T) {
+	// The message limit of the Webex notifier, the only caller of
+	// TruncateInBytes.
+	const n = 7439
+
+	for _, tc := range []struct {
+		name  string
+		char  string
+		count int
+	}{
+		{name: "two-byte runes", char: "д", count: 3720},
+		{name: "three-byte runes", char: "世", count: 2600},
+		{name: "four-byte runes", char: "😀", count: 2000},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			in := strings.Repeat(tc.char, tc.count)
+			require.Greater(t, len(in), n, "the input must be over the byte limit")
+			require.Less(t, len([]rune(in)), n, "the input must hold fewer runes than the byte limit")
+
+			out, truncated := TruncateInBytes(in, n)
+			require.True(t, truncated)
+			require.LessOrEqual(t, len(out), n)
+			require.True(t, strings.HasSuffix(out, truncationMarker))
+
+			kept := strings.TrimSuffix(out, truncationMarker)
+			require.True(t, strings.HasPrefix(in, kept), "the kept text must be a prefix of the input")
+			require.Len(t, []rune(kept), (n-len(truncationMarker))/len(tc.char),
+				"the kept text must hold as many whole runes as the byte budget allows")
+		})
+	}
+}
+
 type brokenReader struct{}
 
 func (b brokenReader) Read([]byte) (int, error) {

notify/webex/webex_test.go

@@ -15,10 +15,12 @@ package webex
 
 import (
 	"context"
+	"encoding/json"
 	"io"
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
 	"time"
 
@@ -169,6 +171,54 @@ func TestWebexTemplating(t *testing.T) {
 	}
 }
 
+// TestWebexTruncatesMultiByteMessage sends a message that is over
+// maxMessageSize in bytes while holding fewer runes than that, the shape any
+// long non-ASCII alert takes. Notify used to panic on it.
+func TestWebexTruncatesMultiByteMessage(t *testing.T) {
+	var out []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(_ http.ResponseWriter, r *http.Request) {
+		var err error
+		out, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+	}))
+	defer srv.Close()
+	u, err := url.Parse(srv.URL)
+	require.NoError(t, err)
+
+	notifier, err := New(
+		&config.WebexConfig{
+			HTTPConfig: &commoncfg.HTTPClientConfig{},
+			APIURL:     &amcommoncfg.URL{URL: u},
+			Message:    "{{ .CommonAnnotations.description }}",
+		},
+		test.CreateTmpl(t),
+		promslog.NewNopLogger(),
+	)
+	require.NoError(t, err)
+
+	// 3720 Cyrillic characters are 7440 bytes, one byte over the limit.
+	description := strings.Repeat("д", 3720)
+	require.Greater(t, len(description), maxMessageSize)
+	require.Less(t, len([]rune(description)), maxMessageSize)
+
+	ctx := notify.WithGroupKey(context.Background(), "1")
+	retry, err := notifier.Notify(ctx, &types.Alert{
+		Alert: model.Alert{
+			Labels:      model.LabelSet{"lbl1": "val1"},
+			Annotations: model.LabelSet{"description": model.LabelValue(description)},
+			StartsAt:    time.Now(),
+			EndsAt:      time.Now().Add(time.Hour),
+		},
+	})
+	require.NoError(t, err)
+	require.False(t, retry)
+
+	var w webhook
+	require.NoError(t, json.Unmarshal(out, &w))
+	require.LessOrEqual(t, len(w.Markdown), maxMessageSize)
+	require.True(t, strings.HasPrefix(description, strings.TrimSuffix(w.Markdown, "…")))
+}
+
 func TestWebexFailureReason(t *testing.T) {
 	for _, tc := range []struct {
 		name           string