feat(setting): add shared [redis] section as default for redis-backed subsystems (#38550)

Adds a shared `[redis]` config section with a single `CONN_STR` key that
acts as the default Redis connection for every redis-backed subsystem
whose own connection string is empty: `[cache].HOST`,
`[session].PROVIDER_CONFIG`, `[queue].CONN_STR` and
`[global_lock].SERVICE_CONN_STR`.

Precedence: per-subsystem value > shared `[redis].CONN_STR` > existing
built-in defaults. Fully backward compatible: when `[redis]` is not set,
every code path behaves exactly as before.

Motivation: today an admin with one Redis instance has to repeat the
same connection string in up to four places (five once WebSocket lands).
Requested by @wxiaoguang in [this review
discussion](https://github.com/go-gitea/gitea/pull/36965#discussion_r3613306976)
as a prerequisite for #36965. This follows the pattern used by GitLab
and Mastodon: one global Redis definition, per-subsystem overrides fall
back to it.

Since #12385 all identical connection strings already share one client
via `nosql.Manager`, so pointing all subsystems at `[redis].CONN_STR`
naturally converges onto a single shared connection pool.

Covered by table-driven unit tests per subsystem (fallback applies, own
value wins, absent `[redis]` = unchanged behavior). A config-cheat-sheet
update for gitea/docs will follow once this is merged.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>

modules/setting/queue_test.go

@@ -0,0 +1,92 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"testing"
+
+	"gitea.dev/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestQueueRedisSharedConnFallback(t *testing.T) {
+	tests := []struct {
+		name      string
+		iniStr    string
+		queueName string
+		wantConn  string
+	}{
+		{
+			name: "redis queue with empty CONN_STR falls back to shared [redis]",
+			iniStr: `
+[redis]
+CONN_STR = redis://127.0.0.1:6379/0
+[queue]
+TYPE = redis
+`,
+			queueName: "test",
+			wantConn:  "redis://127.0.0.1:6379/0",
+		},
+		{
+			name: "base [queue] CONN_STR wins over shared [redis]",
+			iniStr: `
+[redis]
+CONN_STR = redis://127.0.0.1:6379/0
+[queue]
+TYPE = redis
+CONN_STR = redis://10.0.0.1:6379/1
+`,
+			queueName: "test",
+			wantConn:  "redis://10.0.0.1:6379/1",
+		},
+		{
+			name: "per-queue CONN_STR wins over shared [redis]",
+			iniStr: `
+[redis]
+CONN_STR = redis://127.0.0.1:6379/0
+[queue]
+TYPE = redis
+[queue.test]
+CONN_STR = redis://10.0.0.1:6379/2
+`,
+			queueName: "test",
+			wantConn:  "redis://10.0.0.1:6379/2",
+		},
+		{
+			name: "other queues still fall back to shared [redis] when only one is overridden",
+			iniStr: `
+[redis]
+CONN_STR = redis://127.0.0.1:6379/0
+[queue]
+TYPE = redis
+[queue.test]
+CONN_STR = redis://10.0.0.1:6379/2
+`,
+			queueName: "other",
+			wantConn:  "redis://127.0.0.1:6379/0",
+		},
+		{
+			name: "no shared [redis] keeps previous behavior (built-in localhost default)",
+			iniStr: `
+[queue]
+TYPE = redis
+`,
+			queueName: "test",
+			wantConn:  "redis://127.0.0.1:6379/0",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			defer test.MockVariableValue(&Redis)()
+			cfg, err := NewConfigProviderFromData(tt.iniStr)
+			assert.NoError(t, err)
+
+			loadRedisFrom(cfg)
+			q, err := GetQueueSettings(cfg, tt.queueName)
+			assert.NoError(t, err)
+			assert.Equal(t, tt.wantConn, q.ConnStr)
+		})
+	}
+}