fix(http): enhance Content-Length handling in ResponseModifier

- Introduced origContentLength and bodyModified fields to track original content length and body modification status.
- Updated ContentLength and ContentLengthStr methods to return accurate content length based on body modification state.
- Adjusted Write and FlushRelease methods to ensure proper handling of Content-Length header.
- Modified middleware to use the new ContentLengthStr method.

internal/net/gphttp/middleware/middleware.go

@@ -210,8 +210,8 @@ func (m *Middleware) ServeHTTP(next http.HandlerFunc, w http.ResponseWriter, r *
 		// override the response status code
 		rm.WriteHeader(currentResp.StatusCode)
 
-		// overriding the response header is not necessary
-		// modifyResponse is supposed to write to Header directly instead of assigning a new header map)
+		// overriding the response header
+		maps.Copy(rm.Header(), currentResp.Header)
 
 		// override the content length and body if changed
 		if currentResp.Body != currentBody {

internal/route/rules/response_modifier.go

@@ -23,6 +23,9 @@ type ResponseModifier struct {
 	statusCode int
 	shared     Cache
 
+	origContentLength int64 // from http.Response in ResponseAsRW
+	bodyModified      bool
+
 	hijacked bool
 
 	errs gperr.Builder
@@ -64,8 +67,9 @@ func (r responseAsRW) Header() http.Header {
 
 func ResponseAsRW(resp *http.Response) *ResponseModifier {
 	return &ResponseModifier{
-		statusCode: resp.StatusCode,
-		w:          responseAsRW{resp},
+		statusCode:        resp.StatusCode,
+		w:                 responseAsRW{resp},
+		origContentLength: resp.ContentLength,
 	}
 }
 
@@ -121,6 +125,9 @@ func (rm *ResponseModifier) BodyReader() io.ReadCloser {
 }
 
 func (rm *ResponseModifier) ResetBody() {
+	if !rm.bodyModified {
+		return
+	}
 	if rm.buf == nil {
 		return
 	}
@@ -134,6 +141,8 @@ func (rm *ResponseModifier) SetBody(r io.ReadCloser) error {
 		rm.buf.Reset()
 	}
 
+	rm.bodyModified = true
+
 	_, err := io.Copy(rm.buf, r)
 	if err != nil {
 		return fmt.Errorf("failed to copy body: %w", err)
@@ -143,12 +152,26 @@ func (rm *ResponseModifier) SetBody(r io.ReadCloser) error {
 }
 
 func (rm *ResponseModifier) ContentLength() int {
-	if rm.buf == nil {
-		return 0
+	if !rm.bodyModified {
+		if rm.origContentLength > 0 {
+			return int(rm.origContentLength)
+		}
+		contentLength, _ := strconv.Atoi(rm.ContentLengthStr())
+		return contentLength
 	}
 	return rm.buf.Len()
 }
 
+func (rm *ResponseModifier) ContentLengthStr() string {
+	if !rm.bodyModified {
+		if rm.origContentLength > 0 {
+			return strconv.FormatInt(rm.origContentLength, 10)
+		}
+		return rm.w.Header().Get("Content-Length")
+	}
+	return strconv.Itoa(rm.buf.Len())
+}
+
 func (rm *ResponseModifier) Content() []byte {
 	if rm.buf == nil {
 		return nil
@@ -172,6 +195,11 @@ func (rm *ResponseModifier) Response() Response {
 }
 
 func (rm *ResponseModifier) Write(b []byte) (int, error) {
+	if len(b) == 0 {
+		return 0, nil
+	}
+
+	rm.bodyModified = true
 	if rm.buf == nil {
 		rm.buf = rm.bufPool.GetBuffer()
 	}
@@ -200,26 +228,24 @@ func (rm *ResponseModifier) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 func (rm *ResponseModifier) FlushRelease() (int, error) {
 	n := 0
 	if !rm.hijacked {
-		h := rm.w.Header()
-		// for k := range h {
-		// 	if strings.EqualFold(k, "content-length") {
-		// 		h.Del(k)
-		// 	}
-		// }
-		contentLength := rm.ContentLength()
-		h.Set("Content-Length", strconv.Itoa(rm.ContentLength()))
-		h.Del("Transfer-Encoding")
-		h.Del("Trailer")
+		if rm.bodyModified {
+			h := rm.w.Header()
+			h.Set("Content-Length", rm.ContentLengthStr())
+			h.Del("Transfer-Encoding")
+			h.Del("Trailer")
+		}
 		rm.w.WriteHeader(rm.StatusCode())
 
-		if contentLength > 0 {
-			nn, werr := rm.w.Write(rm.Content())
-			n += nn
-			if werr != nil {
-				rm.errs.Addf("write error: %w", werr)
-			}
-			if err := http.NewResponseController(rm.w).Flush(); err != nil && !errors.Is(err, http.ErrNotSupported) {
-				rm.errs.Addf("flush error: %w", err)
+		if rm.bodyModified {
+			if content := rm.Content(); len(content) > 0 {
+				nn, werr := rm.w.Write(content)
+				n += nn
+				if werr != nil {
+					rm.errs.Addf("write error: %w", werr)
+				}
+				if err := http.NewResponseController(rm.w).Flush(); err != nil && !errors.Is(err, http.ErrNotSupported) {
+					rm.errs.Addf("flush error: %w", err)
+				}
 			}
 		}
 	}

internal/route/rules/vars_static.go

@@ -88,7 +88,7 @@ var staticReqVarSubsMap = map[string]reqVarGetter{
 
 var staticRespVarSubsMap = map[string]respVarGetter{
 	VarRespContentType: func(resp *ResponseModifier) string { return resp.Header().Get("Content-Type") },
-	VarRespContentLen:  func(resp *ResponseModifier) string { return strconv.Itoa(resp.ContentLength()) },
+	VarRespContentLen:  func(resp *ResponseModifier) string { return resp.ContentLengthStr() },
 	VarRespStatusCode:  func(resp *ResponseModifier) string { return strconv.Itoa(resp.StatusCode()) },
 }