Refactor system setting (#27000) (#27452)

Backport #27000 by @wxiaoguang

This PR reduces the complexity of the system setting system.

It only needs one line to introduce a new option, and the option can be
used anywhere out-of-box.

It is still high-performant (and more performant) because the config
values are cached in the config system.


![image](f8cdd743-1145-41ab-9f8f-3996aa97d440)

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

modules/setting/config/value.go

@@ -0,0 +1,81 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package config
+
+import (
+	"context"
+	"strconv"
+	"sync"
+)
+
+type CfgSecKey struct {
+	Sec, Key string
+}
+
+type Value[T any] struct {
+	mu sync.RWMutex
+
+	cfgSecKey CfgSecKey
+	dynKey    string
+
+	def, value T
+	revision   int
+}
+
+func (value *Value[T]) parse(s string) (v T) {
+	switch any(v).(type) {
+	case bool:
+		b, _ := strconv.ParseBool(s)
+		return any(b).(T)
+	default:
+		panic("unsupported config type, please complete the code")
+	}
+}
+
+func (value *Value[T]) Value(ctx context.Context) (v T) {
+	dg := GetDynGetter()
+	if dg == nil {
+		// this is an edge case: the database is not initialized but the system setting is going to be used
+		// it should panic to avoid inconsistent config values (from config / system setting) and fix the code
+		panic("no config dyn value getter")
+	}
+
+	rev := dg.GetRevision(ctx)
+
+	// if the revision in database doesn't change, use the last value
+	value.mu.RLock()
+	if rev == value.revision {
+		v = value.value
+		value.mu.RUnlock()
+		return v
+	}
+	value.mu.RUnlock()
+
+	// try to parse the config and cache it
+	var valStr *string
+	if dynVal, has := dg.GetValue(ctx, value.dynKey); has {
+		valStr = &dynVal
+	} else if cfgVal, has := GetCfgSecKeyGetter().GetValue(value.cfgSecKey.Sec, value.cfgSecKey.Key); has {
+		valStr = &cfgVal
+	}
+	if valStr == nil {
+		v = value.def
+	} else {
+		v = value.parse(*valStr)
+	}
+
+	value.mu.Lock()
+	value.value = v
+	value.revision = rev
+	value.mu.Unlock()
+	return v
+}
+
+func (value *Value[T]) DynKey() string {
+	return value.dynKey
+}
+
+func Bool(def bool, cfgSecKey CfgSecKey, dynKey string) *Value[bool] {
+	return &Value[bool]{def: def, cfgSecKey: cfgSecKey, dynKey: dynKey}
+}