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/base.go

@@ -4,14 +4,10 @@
 package templates
 
 import (
-	"fmt"
-	"io/fs"
-	"os"
-	"path/filepath"
 	"strings"
 	"time"
 
-	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/assetfs"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 )
@@ -47,81 +43,30 @@ func BaseVars() Vars {
 	}
 }
 
-func getDirTemplateAssetNames(dir string) []string {
-	return getDirAssetNames(dir, false)
+func AssetFS() *assetfs.LayeredFS {
+	return assetfs.Layered(CustomAssets(), BuiltinAssets())
 }
 
-func getDirAssetNames(dir string, mailer bool) []string {
-	var tmpls []string
+func CustomAssets() *assetfs.Layer {
+	return assetfs.Local("custom", setting.CustomPath, "templates")
+}
 
-	if mailer {
-		dir += filepath.Join(dir, "mail")
-	}
-	f, err := os.Stat(dir)
+func ListWebTemplateAssetNames(assets *assetfs.LayeredFS) ([]string, error) {
+	files, err := assets.ListAllFiles(".", true)
 	if err != nil {
-		if os.IsNotExist(err) {
-			return tmpls
-		}
-		log.Warn("Unable to check if templates dir %s is a directory. Error: %v", dir, err)
-		return tmpls
-	}
-	if !f.IsDir() {
-		log.Warn("Templates dir %s is a not directory.", dir)
-		return tmpls
+		return nil, err
 	}
+	return util.SliceRemoveAllFunc(files, func(file string) bool {
+		return strings.HasPrefix(file, "mail/") || !strings.HasSuffix(file, ".tmpl")
+	}), nil
+}
 
-	files, err := util.StatDir(dir)
+func ListMailTemplateAssetNames(assets *assetfs.LayeredFS) ([]string, error) {
+	files, err := assets.ListAllFiles(".", true)
 	if err != nil {
-		log.Warn("Failed to read %s templates dir. %v", dir, err)
-		return tmpls
+		return nil, err
 	}
-
-	prefix := "templates/"
-	if mailer {
-		prefix += "mail/"
-	}
-	for _, filePath := range files {
-		if !mailer && strings.HasPrefix(filePath, "mail/") {
-			continue
-		}
-
-		if !strings.HasSuffix(filePath, ".tmpl") {
-			continue
-		}
-
-		tmpls = append(tmpls, prefix+filePath)
-	}
-	return tmpls
-}
-
-func walkAssetDir(root string, skipMail bool, callback func(path, name string, d fs.DirEntry, err error) error) error {
-	mailRoot := filepath.Join(root, "mail")
-	if err := filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
-		name := path[len(root):]
-		if len(name) > 0 && name[0] == '/' {
-			name = name[1:]
-		}
-		if err != nil {
-			if os.IsNotExist(err) {
-				return callback(path, name, d, err)
-			}
-			return err
-		}
-		if skipMail && path == mailRoot && d.IsDir() {
-			return fs.SkipDir
-		}
-		if util.CommonSkip(d.Name()) {
-			if d.IsDir() {
-				return fs.SkipDir
-			}
-			return nil
-		}
-		if strings.HasSuffix(d.Name(), ".tmpl") || d.IsDir() {
-			return callback(path, name, d, err)
-		}
-		return nil
-	}); err != nil && !os.IsNotExist(err) {
-		return fmt.Errorf("unable to get files for template assets in %s: %w", root, err)
-	}
-	return nil
+	return util.SliceRemoveAllFunc(files, func(file string) bool {
+		return !strings.HasPrefix(file, "mail/") || !strings.HasSuffix(file, ".tmpl")
+	}), nil
 }