fix(repo): avoid deadlock when updating language stats concurrently (#40167) (#40202)

Backport #40167 by @perfectra1n

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: Jon Fuller <jonfuller2012@gmail.com>
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)
+	}
+}

modules/indexer/stats/db.go

@@ -9,6 +9,7 @@ import (
 	repo_model "gitea.dev/models/repo"
 	"gitea.dev/modules/git"
 	"gitea.dev/modules/git/languagestats"
+	"gitea.dev/modules/globallock"
 	"gitea.dev/modules/graceful"
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/process"
@@ -23,6 +24,13 @@ func (db *DBIndexer) Index(id int64) error {
 	ctx, _, finished := process.GetManager().AddContext(graceful.GetManager().ShutdownContext(), fmt.Sprintf("Stats.DB Index Repo[%d]", id))
 	defer finished()
 
+	// the unique queue doesn't stop two workers from indexing the same repository at once
+	releaser, err := globallock.Lock(ctx, fmt.Sprintf("repo_stats_index_%d", id))
+	if err != nil {
+		return err
+	}
+	defer releaser()
+
 	repo, err := repo_model.GetRepositoryByID(ctx, id)
 	if err != nil {
 		return err