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/public/serve_static.go

@@ -6,75 +6,19 @@
 package public
 
 import (
-	"bytes"
-	"io"
-	"net/http"
-	"os"
-	"path/filepath"
 	"time"
 
+	"code.gitea.io/gitea/modules/assetfs"
 	"code.gitea.io/gitea/modules/timeutil"
 )
 
+var _ GzipBytesProvider = (*vfsgen۰CompressedFileInfo)(nil)
+
 // GlobalModTime provide a global mod time for embedded asset files
 func GlobalModTime(filename string) time.Time {
 	return timeutil.GetExecutableModTime()
 }
 
-func fileSystem(dir string) http.FileSystem {
-	return Assets
-}
-
-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
-		}
-	}
-}
-
-// serveContent serve http content
-func serveContent(w http.ResponseWriter, req *http.Request, fi os.FileInfo, modtime time.Time, content io.ReadSeeker) {
-	encodings := parseAcceptEncoding(req.Header.Get("Accept-Encoding"))
-	if encodings.Contains("gzip") {
-		if cf, ok := fi.(*vfsgen۰CompressedFileInfo); ok {
-			rdGzip := bytes.NewReader(cf.GzipBytes())
-			// all static files are managed by Gitea, so we can make sure every file has the correct ext name
-			// then we can get the correct Content-Type, we do not need to do http.DetectContentType on the decompressed data
-			mimeType := detectWellKnownMimeType(filepath.Ext(fi.Name()))
-			if mimeType == "" {
-				mimeType = "application/octet-stream"
-			}
-			w.Header().Set("Content-Type", mimeType)
-			w.Header().Set("Content-Encoding", "gzip")
-			http.ServeContent(w, req, fi.Name(), modtime, rdGzip)
-			return
-		}
-	}
-
-	http.ServeContent(w, req, fi.Name(), modtime, content)
-	return
+func BuiltinAssets() *assetfs.Layer {
+	return assetfs.Bindata("builtin(bindata)", Assets)
 }