Instance-wide (global) info banner and maintenance mode (#36571)

The banner allows site operators to communicate important announcements
(e.g., maintenance windows, policy updates, service notices) directly
within the UI.

The maintenance mode only allows admin to access the web UI.

* Fix #2345
* Fix #9618

---------

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

modules/setting/config/value.go

@@ -5,6 +5,7 @@ package config
 
 import (
 	"context"
+	"reflect"
 	"sync"
 
 	"code.gitea.io/gitea/modules/json"
@@ -16,18 +17,31 @@ type CfgSecKey struct {
 	Sec, Key string
 }
 
-type Value[T any] struct {
+// OptionInterface is used to overcome Golang's generic interface limitation
+type OptionInterface interface {
+	GetDefaultValue() any
+}
+
+type Option[T any] struct {
 	mu sync.RWMutex
 
 	cfgSecKey CfgSecKey
 	dynKey    string
 
-	def, value T
+	value      T
+	defSimple  T
+	defFunc    func() T
+	emptyAsDef bool
+	has        bool
 	revision   int
 }
 
-func (value *Value[T]) parse(key, valStr string) (v T) {
-	v = value.def
+func (opt *Option[T]) GetDefaultValue() any {
+	return opt.DefaultValue()
+}
+
+func (opt *Option[T]) parse(key, valStr string) (v T) {
+	v = opt.DefaultValue()
 	if valStr != "" {
 		if err := json.Unmarshal(util.UnsafeStringToBytes(valStr), &v); err != nil {
 			log.Error("Unable to unmarshal json config for key %q, err: %v", key, err)
@@ -36,7 +50,35 @@ func (value *Value[T]) parse(key, valStr string) (v T) {
 	return v
 }
 
-func (value *Value[T]) Value(ctx context.Context) (v T) {
+func (opt *Option[T]) HasValue(ctx context.Context) bool {
+	_, _, has := opt.ValueRevision(ctx)
+	return has
+}
+
+func (opt *Option[T]) Value(ctx context.Context) (v T) {
+	v, _, _ = opt.ValueRevision(ctx)
+	return v
+}
+
+func isZeroOrEmpty(v any) bool {
+	if v == nil {
+		return true // interface itself is nil
+	}
+	r := reflect.ValueOf(v)
+	if r.IsZero() {
+		return true
+	}
+
+	if r.Kind() == reflect.Slice || r.Kind() == reflect.Map {
+		if r.IsNil() {
+			return true
+		}
+		return r.Len() == 0
+	}
+	return false
+}
+
+func (opt *Option[T]) ValueRevision(ctx context.Context) (v T, rev int, has bool) {
 	dg := GetDynGetter()
 	if dg == nil {
 		// this is an edge case: the database is not initialized but the system setting is going to be used
@@ -44,55 +86,96 @@ func (value *Value[T]) Value(ctx context.Context) (v T) {
 		panic("no config dyn value getter")
 	}
 
-	rev := dg.GetRevision(ctx)
+	rev = dg.GetRevision(ctx)
 
 	// if the revision in the database doesn't change, use the last value
-	value.mu.RLock()
-	if rev == value.revision {
-		v = value.value
-		value.mu.RUnlock()
-		return v
+	opt.mu.RLock()
+	if rev == opt.revision {
+		v = opt.value
+		has = opt.has
+		opt.mu.RUnlock()
+		return v, rev, has
 	}
-	value.mu.RUnlock()
+	opt.mu.RUnlock()
 
 	// try to parse the config and cache it
 	var valStr *string
-	if dynVal, has := dg.GetValue(ctx, value.dynKey); has {
+	if dynVal, hasDbValue := dg.GetValue(ctx, opt.dynKey); hasDbValue {
 		valStr = &dynVal
-	} else if cfgVal, has := GetCfgSecKeyGetter().GetValue(value.cfgSecKey.Sec, value.cfgSecKey.Key); has {
+	} else if cfgVal, has := GetCfgSecKeyGetter().GetValue(opt.cfgSecKey.Sec, opt.cfgSecKey.Key); has {
 		valStr = &cfgVal
 	}
 	if valStr == nil {
-		v = value.def
+		v = opt.DefaultValue()
+		has = false
 	} else {
-		v = value.parse(value.dynKey, *valStr)
+		v = opt.parse(opt.dynKey, *valStr)
+		if opt.emptyAsDef && isZeroOrEmpty(v) {
+			v = opt.DefaultValue()
+		} else {
+			has = true
+		}
 	}
 
-	value.mu.Lock()
-	value.value = v
-	value.revision = rev
-	value.mu.Unlock()
+	opt.mu.Lock()
+	opt.value = v
+	opt.revision = rev
+	opt.has = has
+	opt.mu.Unlock()
+	return v, rev, has
+}
+
+func (opt *Option[T]) DynKey() string {
+	return opt.dynKey
+}
+
+// WithDefaultFunc sets the default value with a function
+// The "def" value might be changed during runtime (e.g.: Unmarshal with default), so it shouldn't use the same pointer or slice
+func (opt *Option[T]) WithDefaultFunc(f func() T) *Option[T] {
+	opt.defFunc = f
+	return opt
+}
+
+func (opt *Option[T]) WithDefaultSimple(def T) *Option[T] {
+	v := any(def)
+	switch v.(type) {
+	case string, bool, int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
+	default:
+		// TODO: use reflect to support convertable basic types like `type State string`
+		r := reflect.ValueOf(v)
+		if r.Kind() != reflect.Struct {
+			panic("invalid type for default value, use WithDefaultFunc instead")
+		}
+	}
+	opt.defSimple = def
+	return opt
+}
+
+func (opt *Option[T]) WithEmptyAsDefault() *Option[T] {
+	opt.emptyAsDef = true
+	return opt
+}
+
+func (opt *Option[T]) DefaultValue() T {
+	if opt.defFunc != nil {
+		return opt.defFunc()
+	}
+	return opt.defSimple
+}
+
+func (opt *Option[T]) WithFileConfig(cfgSecKey CfgSecKey) *Option[T] {
+	opt.cfgSecKey = cfgSecKey
+	return opt
+}
+
+var allConfigOptions = map[string]OptionInterface{}
+
+func NewOption[T any](dynKey string) *Option[T] {
+	v := &Option[T]{dynKey: dynKey}
+	allConfigOptions[dynKey] = v
 	return v
 }
 
-func (value *Value[T]) DynKey() string {
-	return value.dynKey
-}
-
-func (value *Value[T]) WithDefault(def T) *Value[T] {
-	value.def = def
-	return value
-}
-
-func (value *Value[T]) DefaultValue() T {
-	return value.def
-}
-
-func (value *Value[T]) WithFileConfig(cfgSecKey CfgSecKey) *Value[T] {
-	value.cfgSecKey = cfgSecKey
-	return value
-}
-
-func ValueJSON[T any](dynKey string) *Value[T] {
-	return &Value[T]{dynKey: dynKey}
+func GetConfigOption(dynKey string) OptionInterface {
+	return allConfigOptions[dynKey]
 }