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/options/static.go

@@ -6,98 +6,9 @@
 package options
 
 import (
-	"fmt"
-	"io"
-
-	"code.gitea.io/gitea/modules/setting"
-	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/modules/assetfs"
 )
 
-// Dir returns all files from custom directory or bindata.
-func Dir(name string) ([]string, error) {
-	if directories.Filled(name) {
-		return directories.Get(name), nil
-	}
-
-	result, err := listLocalDirIfExist([]string{setting.CustomPath}, "options", name)
-	if err != nil {
-		return nil, err
-	}
-
-	files, err := AssetDir(name)
-	if err != nil {
-		return []string{}, fmt.Errorf("unable to read embedded directory %q. %w", name, err)
-	}
-
-	result = append(result, files...)
-	return directories.AddAndGet(name, result), nil
-}
-
-func AssetDir(dirName string) ([]string, error) {
-	d, err := Assets.Open(dirName)
-	if err != nil {
-		return nil, err
-	}
-	defer d.Close()
-
-	files, err := d.Readdir(-1)
-	if err != nil {
-		return nil, err
-	}
-	results := make([]string, 0, len(files))
-	for _, file := range files {
-		results = append(results, file.Name())
-	}
-	return results, nil
-}
-
-// fileFromOptionsDir is a helper to read files from custom path or bindata.
-func fileFromOptionsDir(elems ...string) ([]byte, error) {
-	// only try custom dir, no static dir
-	if data, err := readLocalFile([]string{setting.CustomPath}, "options", elems...); err == nil {
-		return data, nil
-	}
-
-	f, err := Assets.Open(util.PathJoinRelX(elems...))
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-	return io.ReadAll(f)
-}
-
-func Asset(name string) ([]byte, error) {
-	f, err := Assets.Open("/" + name)
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-	return io.ReadAll(f)
-}
-
-func AssetNames() []string {
-	realFS := Assets.(vfsgen۰FS)
-	results := make([]string, 0, len(realFS))
-	for k := range realFS {
-		results = append(results, k[1:])
-	}
-	return results
-}
-
-func AssetIsDir(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
-		}
-	}
-}
-
-// IsDynamic will return false when using embedded data (-tags bindata)
-func IsDynamic() bool {
-	return false
+func BuiltinAssets() *assetfs.Layer {
+	return assetfs.Bindata("builtin(bindata)", Assets)
 }