Use a general approach to access custom/static/builtin assets (#24022)

The idea is to use a Layered Asset File-system (modules/assetfs/layered.go)

For example: when there are 2 layers: "custom", "builtin", when access
to asset "my/page.tmpl", the Layered Asset File-system will first try to
use "custom" assets, if not found, then use "builtin" assets.

This approach will hugely simplify a lot of code, make them testable.

Other changes:

* Simplify the AssetsHandlerFunc code
* Simplify the `gitea embedded` sub-command code

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/templates/static.go

@@ -6,114 +6,17 @@
 package templates
 
 import (
-	"html/template"
-	"io"
-	"io/fs"
-	"os"
-	"path"
-	"path/filepath"
-	"strings"
-	texttmpl "text/template"
 	"time"
 
-	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/assetfs"
 	"code.gitea.io/gitea/modules/timeutil"
 )
 
-var (
-	subjectTemplates = texttmpl.New("")
-	bodyTemplates    = template.New("")
-)
-
 // GlobalModTime provide a global mod time for embedded asset files
 func GlobalModTime(filename string) time.Time {
 	return timeutil.GetExecutableModTime()
 }
 
-// GetAssetFilename returns the filename of the provided asset
-func GetAssetFilename(name string) (string, error) {
-	filename := filepath.Join(setting.CustomPath, name)
-	_, err := os.Stat(filename)
-	if err != nil && !os.IsNotExist(err) {
-		return name, err
-	} else if err == nil {
-		return filename, nil
-	}
-	return "(builtin) " + name, nil
-}
-
-// GetAsset get a special asset, only for chi
-func GetAsset(name string) ([]byte, error) {
-	bs, err := os.ReadFile(filepath.Join(setting.CustomPath, name))
-	if err != nil && !os.IsNotExist(err) {
-		return nil, err
-	} else if err == nil {
-		return bs, nil
-	}
-	return BuiltinAsset(strings.TrimPrefix(name, "templates/"))
-}
-
-// GetFiles calls a callback for each template asset
-func walkTemplateFiles(callback func(path, name string, d fs.DirEntry, err error) error) error {
-	if err := walkAssetDir(filepath.Join(setting.CustomPath, "templates"), true, callback); err != nil && !os.IsNotExist(err) {
-		return err
-	}
-	return nil
-}
-
-// GetTemplateAssetNames only for chi
-func GetTemplateAssetNames() []string {
-	realFS := Assets.(vfsgen۰FS)
-	tmpls := make([]string, 0, len(realFS))
-	for k := range realFS {
-		if strings.HasPrefix(k, "/mail/") {
-			continue
-		}
-		tmpls = append(tmpls, "templates/"+k[1:])
-	}
-
-	customDir := path.Join(setting.CustomPath, "templates")
-	customTmpls := getDirTemplateAssetNames(customDir)
-	return append(tmpls, customTmpls...)
-}
-
-func walkMailerTemplates(callback func(path, name string, d fs.DirEntry, err error) error) error {
-	if err := walkAssetDir(filepath.Join(setting.CustomPath, "templates", "mail"), false, callback); err != nil && !os.IsNotExist(err) {
-		return err
-	}
-	return nil
-}
-
-// BuiltinAsset reads the provided asset from the builtin embedded assets
-func BuiltinAsset(name string) ([]byte, error) {
-	f, err := Assets.Open("/" + name)
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-	return io.ReadAll(f)
-}
-
-// BuiltinAssetNames returns the names of the built-in embedded assets
-func BuiltinAssetNames() []string {
-	realFS := Assets.(vfsgen۰FS)
-	results := make([]string, 0, len(realFS))
-	for k := range realFS {
-		results = append(results, k[1:])
-	}
-	return results
-}
-
-// BuiltinAssetIsDir returns if a provided asset is a directory
-func BuiltinAssetIsDir(name string) (bool, error) {
-	if f, err := Assets.Open("/" + name); err != nil {
-		return false, err
-	} else {
-		defer f.Close()
-		if fi, err := f.Stat(); err != nil {
-			return false, err
-		} else {
-			return fi.IsDir(), nil
-		}
-	}
+func BuiltinAssets() *assetfs.Layer {
+	return assetfs.Bindata("builtin(bindata)", Assets)
 }