MM-68364: Truncate oversized GitHub plugin DM posts to avoid silent drops

server/plugin/plugin.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode/utf8"
 
 	"github.com/google/go-github/v54/github"
 	"github.com/gorilla/mux"
@@ -957,7 +958,7 @@ func (p *Plugin) CreateBotDMPost(userID, message, postType string) {
 	post := &model.Post{
 		UserId:    p.BotUserID,
 		ChannelId: channel.Id,
-		Message:   message,
+		Message:   truncatePostMessage(message),
 		Type:      postType,
 	}
 
@@ -967,6 +968,21 @@ func (p *Plugin) CreateBotDMPost(userID, message, postType string) {
 	}
 }
 
+func truncatePostMessage(message string) string {
+	const truncationMarker = "\n\n_… message truncated_"
+
+	if utf8.RuneCountInString(message) <= model.PostMessageMaxRunesV2 {
+		return message
+	}
+
+	keep := model.PostMessageMaxRunesV2 - utf8.RuneCountInString(truncationMarker)
+	if keep <= 0 {
+		return string([]rune(truncationMarker)[:model.PostMessageMaxRunesV2])
+	}
+
+	return string([]rune(message)[:keep]) + truncationMarker
+}
+
 func (p *Plugin) CheckIfDuplicateDailySummary(userID, text string) (bool, error) {
 	previousSummary, err := p.GetDailySummaryText(userID)
 	if err != nil {

server/plugin/plugin_test.go

@@ -5,7 +5,9 @@ package plugin
 
 import (
 	"encoding/json"
+	"strings"
 	"testing"
+	"unicode/utf8"
 
 	"github.com/golang/mock/gomock"
 	"github.com/pkg/errors"
@@ -374,3 +376,33 @@ func TestForceDisconnectUser_DeleteErrors(t *testing.T) {
 
 	api.AssertExpectations(t)
 }
+
+func TestTruncatePostMessage(t *testing.T) {
+	t.Run("short message is returned unchanged", func(t *testing.T) {
+		msg := "hello world"
+		require.Equal(t, msg, truncatePostMessage(msg))
+	})
+
+	t.Run("message at the rune limit is returned unchanged", func(t *testing.T) {
+		msg := strings.Repeat("a", model.PostMessageMaxRunesV2)
+		require.Equal(t, msg, truncatePostMessage(msg))
+	})
+
+	t.Run("oversized ASCII message is truncated and marker appended", func(t *testing.T) {
+		msg := strings.Repeat("a", model.PostMessageMaxRunesV2+5_000)
+		out := truncatePostMessage(msg)
+
+		require.LessOrEqual(t, utf8.RuneCountInString(out), model.PostMessageMaxRunesV2)
+		require.True(t, strings.HasSuffix(out, "_… message truncated_"))
+	})
+
+	t.Run("oversized multibyte message keeps rune boundaries", func(t *testing.T) {
+		// Each rune is 3 bytes, so byte-based truncation would corrupt the output.
+		msg := strings.Repeat("✓", model.PostMessageMaxRunesV2+1_000)
+		out := truncatePostMessage(msg)
+
+		require.LessOrEqual(t, utf8.RuneCountInString(out), model.PostMessageMaxRunesV2)
+		require.True(t, utf8.ValidString(out), "truncated output should remain valid UTF-8")
+		require.True(t, strings.HasSuffix(out, "_… message truncated_"))
+	})
+}