Introduce FS.CompressRoot (#1331)

* Introduce FS.CompressRoot

* Avoid duplicated filepath.FromSlash

* Introduce filePathToCompressed

* Revert openIndexFile manually

* Join root and path, and then calls filepath.FromSlash

fs.go

@@ -276,6 +276,10 @@ type FS struct {
 	// Brotli encoding is disabled by default.
 	CompressBrotli bool
 
+	// Path to the compressed root directory to serve files from. If this value
+	// is empty, Root is used.
+	CompressRoot string
+
 	// Enables byte range requests if set to true.
 	//
 	// Byte range requests are disabled by default.
@@ -388,9 +392,7 @@ func (fs *FS) NewRequestHandler() RequestHandler {
 	return fs.h
 }
 
-func (fs *FS) initRequestHandler() {
-	root := fs.Root
-
+func (fs *FS) normalizeRoot(root string) string {
 	// Serve files from the current working directory if Root is empty or if Root is a relative path.
 	if (!fs.AllowEmptyRoot && len(root) == 0) || (len(root) > 0 && !filepath.IsAbs(root)) {
 		path, err := os.Getwd()
@@ -406,6 +408,18 @@ func (fs *FS) initRequestHandler() {
 	for len(root) > 0 && root[len(root)-1] == os.PathSeparator {
 		root = root[:len(root)-1]
 	}
+	return root
+}
+
+func (fs *FS) initRequestHandler() {
+	root := fs.normalizeRoot(fs.Root)
+
+	compressRoot := fs.CompressRoot
+	if len(compressRoot) == 0 {
+		compressRoot = root
+	} else {
+		compressRoot = fs.normalizeRoot(compressRoot)
+	}
 
 	cacheDuration := fs.CacheDuration
 	if cacheDuration <= 0 {
@@ -430,6 +444,7 @@ func (fs *FS) initRequestHandler() {
 		generateIndexPages:     fs.GenerateIndexPages,
 		compress:               fs.Compress,
 		compressBrotli:         fs.CompressBrotli,
+		compressRoot:           compressRoot,
 		pathNotFound:           fs.PathNotFound,
 		acceptByteRange:        fs.AcceptByteRange,
 		cacheDuration:          cacheDuration,
@@ -478,6 +493,7 @@ type fsHandler struct {
 	generateIndexPages     bool
 	compress               bool
 	compressBrotli         bool
+	compressRoot           string
 	acceptByteRange        bool
 	cacheDuration          time.Duration
 	compressedFileSuffixes map[string]string
@@ -780,6 +796,20 @@ func cleanCacheNolock(cache map[string]*fsFile, pendingFiles, filesToRelease []*
 	return pendingFiles, filesToRelease
 }
 
+func (h *fsHandler) pathToFilePath(path string) string {
+	return filepath.FromSlash(h.root + path)
+}
+
+func (h *fsHandler) filePathToCompressed(filePath string) string {
+	if h.root == h.compressRoot {
+		return filePath
+	}
+	if !strings.HasPrefix(filePath, h.root) {
+		return filePath
+	}
+	return filepath.FromSlash(h.compressRoot + filePath[len(h.root):])
+}
+
 func (h *fsHandler) handleRequest(ctx *RequestCtx) {
 	var path []byte
 	if h.pathRewrite != nil {
@@ -831,7 +861,7 @@ func (h *fsHandler) handleRequest(ctx *RequestCtx) {
 
 	if !ok {
 		pathStr := string(path)
-		filePath := filepath.FromSlash(h.root + pathStr)
+		filePath := h.pathToFilePath(pathStr)
 
 		var err error
 		ff, err = h.openFSFile(filePath, mustCompress, fileEncoding)
@@ -1153,7 +1183,14 @@ func (h *fsHandler) compressAndOpenFSFile(filePath string, fileEncoding string)
 		return h.newFSFile(f, fileInfo, false, "")
 	}
 
-	compressedFilePath := filePath + h.compressedFileSuffixes[fileEncoding]
+	compressedFilePath := h.filePathToCompressed(filePath)
+	if compressedFilePath != filePath {
+		if err := os.MkdirAll(filepath.Dir(compressedFilePath), os.ModePerm); err != nil {
+			return nil, err
+		}
+	}
+	compressedFilePath += h.compressedFileSuffixes[fileEncoding]
+
 	absPath, err := filepath.Abs(compressedFilePath)
 	if err != nil {
 		_ = f.Close()