fix(repo): avoid deadlock when updating language stats concurrently (#40167)

The `repo_stats_update` queue only deduplicates waiting items, so
several workers can index the same repository at once, then
`UpdateLanguageStats` can lead to transaction deadlock.

Use a global lock to avoid concurrent updates, and also helps
https://github.com/go-gitea/gitea/pull/39342 to avoid duplicate updates.

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/repo/language_stats.go

@@ -151,7 +151,7 @@ func UpdateLanguageStats(ctx context.Context, repo *Repository, commitID string,
 		var topLang string
 		var s int64
 		for lang, size := range stats {
-			if size > s {
+			if size > s || (size == s && lang < topLang) {
 				s = size
 				topLang = lang
 			}

models/repo/language_stats_test.go

@@ -0,0 +1,28 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo_test
+
+import (
+	"testing"
+
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/models/unittest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestUpdateLanguageStatsPrimaryTie(t *testing.T) {
+	require.NoError(t, unittest.PrepareTestDatabase())
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: 1})
+
+	require.NoError(t, repo_model.UpdateLanguageStats(t.Context(), repo, "1234567890", map[string]int64{"Go": 10, "C": 10}))
+
+	stats, err := repo_model.GetLanguageStats(t.Context(), repo)
+	require.NoError(t, err)
+	require.Len(t, stats, 2)
+	for _, stat := range stats {
+		assert.Equal(t, stat.Language == "C", stat.IsPrimary, stat.Language)
+	}
+}