Add context cache as a request level cache (#22294)

To avoid duplicated load of the same data in an HTTP request, we can set
a context cache to do that. i.e. Some pages may load a user from a
database with the same id in different areas on the same page. But the
code is hidden in two different deep logic. How should we share the
user? As a result of this PR, now if both entry functions accept
`context.Context` as the first parameter and we just need to refactor
`GetUserByID` to reuse the user from the context cache. Then it will not
be loaded twice on an HTTP request.

But of course, sometimes we would like to reload an object from the
database, that's why `RemoveContextData` is also exposed.

The core context cache is here. It defines a new context
```go
type cacheContext struct {
	ctx  context.Context
	data map[any]map[any]any
        lock sync.RWMutex
}

var cacheContextKey = struct{}{}

func WithCacheContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, cacheContextKey, &cacheContext{
		ctx:  ctx,
		data: make(map[any]map[any]any),
	})
}
```

Then you can use the below 4 methods to read/write/del the data within
the same context.

```go
func GetContextData(ctx context.Context, tp, key any) any
func SetContextData(ctx context.Context, tp, key, value any)
func RemoveContextData(ctx context.Context, tp, key any)
func GetWithContextCache[T any](ctx context.Context, cacheGroupKey string, cacheTargetID any, f func() (T, error)) (T, error)
```

Then let's take a look at how `system.GetString` implement it.

```go
func GetSetting(ctx context.Context, key string) (string, error) {
	return cache.GetWithContextCache(ctx, contextCacheKey, key, func() (string, error) {
		return cache.GetString(genSettingCacheKey(key), func() (string, error) {
			res, err := GetSettingNoCache(ctx, key)
			if err != nil {
				return "", err
			}
			return res.SettingValue, nil
		})
	})
}
```

First, it will check if context data include the setting object with the
key. If not, it will query from the global cache which may be memory or
a Redis cache. If not, it will get the object from the database. In the
end, if the object gets from the global cache or database, it will be
set into the context cache.

An object stored in the context cache will only be destroyed after the
context disappeared.

2 files changed, 13 insertions, 12 deletions

diff --git a/models/avatars/avatar.go b/models/avatars/avatar.go
index 67f693e82f..6cf05dd284 100644
--- a/models/avatars/avatar.go
+++ b/models/avatars/avatar.go
@@ -147,13 +147,13 @@ func generateRecognizedAvatarURL(u url.URL, size int) string {
 // generateEmailAvatarLink returns a email avatar link.
 // if final is true, it may use a slow path (eg: query DNS).
 // if final is false, it always uses a fast path.
-func generateEmailAvatarLink(email string, size int, final bool) string {
+func generateEmailAvatarLink(ctx context.Context, email string, size int, final bool) string {
 	email = strings.TrimSpace(email)
 	if email == "" {
 		return DefaultAvatarLink()
 	}
 
-	enableFederatedAvatar := system_model.GetSettingBool(system_model.KeyPictureEnableFederatedAvatar)
+	enableFederatedAvatar := system_model.GetSettingBool(ctx, system_model.KeyPictureEnableFederatedAvatar)
 
 	var err error
 	if enableFederatedAvatar && system_model.LibravatarService != nil {
@@ -174,7 +174,7 @@ func generateEmailAvatarLink(email string, size int, final bool) string {
 		return urlStr
 	}
 
-	disableGravatar := system_model.GetSettingBool(system_model.KeyPictureDisableGravatar)
+	disableGravatar := system_model.GetSettingBool(ctx, system_model.KeyPictureDisableGravatar)
 	if !disableGravatar {
 		// copy GravatarSourceURL, because we will modify its Path.
 		avatarURLCopy := *system_model.GravatarSourceURL
@@ -186,11 +186,11 @@ func generateEmailAvatarLink(email string, size int, final bool) string {
 }
 
 // GenerateEmailAvatarFastLink returns a avatar link (fast, the link may be a delegated one: "/avatar/${hash}")
-func GenerateEmailAvatarFastLink(email string, size int) string {
-	return generateEmailAvatarLink(email, size, false)
+func GenerateEmailAvatarFastLink(ctx context.Context, email string, size int) string {
+	return generateEmailAvatarLink(ctx, email, size, false)
 }
 
 // GenerateEmailAvatarFinalLink returns a avatar final link (maybe slow)
-func GenerateEmailAvatarFinalLink(email string, size int) string {
-	return generateEmailAvatarLink(email, size, true)
+func GenerateEmailAvatarFinalLink(ctx context.Context, email string, size int) string {
+	return generateEmailAvatarLink(ctx, email, size, true)
 }
diff --git a/models/avatars/avatar_test.go b/models/avatars/avatar_test.go
index 29be2ea346..a3cb36d0e1 100644
--- a/models/avatars/avatar_test.go
+++ b/models/avatars/avatar_test.go
@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	avatars_model "code.gitea.io/gitea/models/avatars"
+	"code.gitea.io/gitea/models/db"
 	system_model "code.gitea.io/gitea/models/system"
 	"code.gitea.io/gitea/modules/setting"
 
@@ -16,15 +17,15 @@ import (
 const gravatarSource = "https://secure.gravatar.com/avatar/"
 
 func disableGravatar(t *testing.T) {
-	err := system_model.SetSettingNoVersion(system_model.KeyPictureEnableFederatedAvatar, "false")
+	err := system_model.SetSettingNoVersion(db.DefaultContext, system_model.KeyPictureEnableFederatedAvatar, "false")
 	assert.NoError(t, err)
-	err = system_model.SetSettingNoVersion(system_model.KeyPictureDisableGravatar, "true")
+	err = system_model.SetSettingNoVersion(db.DefaultContext, system_model.KeyPictureDisableGravatar, "true")
 	assert.NoError(t, err)
 	system_model.LibravatarService = nil
 }
 
 func enableGravatar(t *testing.T) {
-	err := system_model.SetSettingNoVersion(system_model.KeyPictureDisableGravatar, "false")
+	err := system_model.SetSettingNoVersion(db.DefaultContext, system_model.KeyPictureDisableGravatar, "false")
 	assert.NoError(t, err)
 	setting.GravatarSource = gravatarSource
 	err = system_model.Init()
@@ -47,11 +48,11 @@ func TestSizedAvatarLink(t *testing.T) {
 
 	disableGravatar(t)
 	assert.Equal(t, "/testsuburl/assets/img/avatar_default.png",
-		avatars_model.GenerateEmailAvatarFastLink("gitea@example.com", 100))
+		avatars_model.GenerateEmailAvatarFastLink(db.DefaultContext, "gitea@example.com", 100))
 
 	enableGravatar(t)
 	assert.Equal(t,
 		"https://secure.gravatar.com/avatar/353cbad9b58e69c96154ad99f92bedc7?d=identicon&s=100",
-		avatars_model.GenerateEmailAvatarFastLink("gitea@example.com", 100),
+		avatars_model.GenerateEmailAvatarFastLink(db.DefaultContext, "gitea@example.com", 100),
 	)
 }