Introduce path Clean/Join helper functions (#23495)

Since #23493 has conflicts with latest commits, this PR is my proposal
for fixing #23371

Details are in the comments

And refactor the `modules/options` module, to make it always use
"filepath" to access local files.

Benefits:

* No need to do `util.CleanPath(strings.ReplaceAll(p, "\\", "/"))),
"/")` any more (not only one before)
* The function behaviors are clearly defined

modules/options/base.go

@@ -7,36 +7,38 @@ import (
 	"fmt"
 	"io/fs"
 	"os"
-	"path"
 	"path/filepath"
 
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 )
 
+var directories = make(directorySet)
+
 // Locale reads the content of a specific locale from static/bindata or custom path.
 func Locale(name string) ([]byte, error) {
-	return fileFromDir(path.Join("locale", util.CleanPath(name)))
+	return fileFromOptionsDir("locale", name)
 }
 
 // Readme reads the content of a specific readme from static/bindata or custom path.
 func Readme(name string) ([]byte, error) {
-	return fileFromDir(path.Join("readme", util.CleanPath(name)))
+	return fileFromOptionsDir("readme", name)
 }
 
 // Gitignore reads the content of a gitignore locale from static/bindata or custom path.
 func Gitignore(name string) ([]byte, error) {
-	return fileFromDir(path.Join("gitignore", util.CleanPath(name)))
+	return fileFromOptionsDir("gitignore", name)
 }
 
 // License reads the content of a specific license from static/bindata or custom path.
 func License(name string) ([]byte, error) {
-	return fileFromDir(path.Join("license", util.CleanPath(name)))
+	return fileFromOptionsDir("license", name)
 }
 
 // Labels reads the content of a specific labels from static/bindata or custom path.
 func Labels(name string) ([]byte, error) {
-	return fileFromDir(path.Join("label", util.CleanPath(name)))
+	return fileFromOptionsDir("label", name)
 }
 
 // WalkLocales reads the content of a specific locale
@@ -79,17 +81,54 @@ func walkAssetDir(root string, callback func(path, name string, d fs.DirEntry, e
 	return nil
 }
 
-func statDirIfExist(dir string) ([]string, error) {
-	isDir, err := util.IsDir(dir)
+// mustLocalPathAbs coverts a path to absolute path
+// FIXME: the old behavior (StaticRootPath might not be absolute), not ideal, just keep the same as before
+func mustLocalPathAbs(s string) string {
+	abs, err := filepath.Abs(s)
 	if err != nil {
-		return nil, fmt.Errorf("unable to check if static directory %s is a directory. %w", dir, err)
+		// This should never happen in a real system. If it happens, the user must have already been in trouble: the system is not able to resolve its own paths.
+		log.Fatal("Unable to get absolute path for %q: %v", s, err)
 	}
-	if !isDir {
-		return nil, nil
+	return abs
+}
+
+func joinLocalPaths(baseDirs []string, subDir string, elems ...string) (paths []string) {
+	abs := make([]string, len(elems)+2)
+	abs[1] = subDir
+	copy(abs[2:], elems)
+	for _, baseDir := range baseDirs {
+		abs[0] = mustLocalPathAbs(baseDir)
+		paths = append(paths, util.FilePathJoinAbs(abs...))
 	}
-	files, err := util.StatDir(dir, true)
-	if err != nil {
-		return nil, fmt.Errorf("unable to read directory %q. %w", dir, err)
+	return paths
+}
+
+func listLocalDirIfExist(baseDirs []string, subDir string, elems ...string) (files []string, err error) {
+	for _, localPath := range joinLocalPaths(baseDirs, subDir, elems...) {
+		isDir, err := util.IsDir(localPath)
+		if err != nil {
+			return nil, fmt.Errorf("unable to check if path %q is a directory. %w", localPath, err)
+		} else if !isDir {
+			continue
+		}
+
+		dirFiles, err := util.StatDir(localPath, true)
+		if err != nil {
+			return nil, fmt.Errorf("unable to read directory %q. %w", localPath, err)
+		}
+		files = append(files, dirFiles...)
 	}
 	return files, nil
 }
+
+func readLocalFile(baseDirs []string, subDir string, elems ...string) ([]byte, error) {
+	for _, localPath := range joinLocalPaths(baseDirs, subDir, elems...) {
+		data, err := os.ReadFile(localPath)
+		if err == nil {
+			return data, nil
+		} else if !os.IsNotExist(err) {
+			log.Error("Unable to read file %q. Error: %v", localPath, err)
+		}
+	}
+	return nil, os.ErrNotExist
+}