net/http: call requestTooLarge on unwrapped ResponseWriter in MaxBytesReader

src/net/http/http_test.go

@@ -9,6 +9,7 @@ package http
 import (
 	"bytes"
 	"internal/testenv"
+	"io"
 	"io/fs"
 	"net/url"
 	"os"
@@ -189,6 +190,45 @@ func TestNoUnicodeStrings(t *testing.T) {
 	}
 }
 
+type requestTooLargerResponseWriter struct {
+	called bool
+}
+
+func (rw *requestTooLargerResponseWriter) Header() Header              { return Header{} }
+func (rw *requestTooLargerResponseWriter) Write(b []byte) (int, error) { return len(b), nil }
+func (rw *requestTooLargerResponseWriter) WriteHeader(statusCode int)  {}
+func (rw *requestTooLargerResponseWriter) requestTooLarge() {
+	rw.called = true
+}
+
+type wrapper struct {
+	ResponseWriter
+}
+
+func (w *wrapper) Unwrap() ResponseWriter {
+	return w.ResponseWriter
+}
+
+func TestMaxBytesReaderUnwrapTriggersRequestTooLarge(t *testing.T) {
+	body := strings.NewReader("123456")
+	limit := int64(5)
+
+	innerRw := &requestTooLargerResponseWriter{}
+	wrappedRw := &wrapper{ResponseWriter: innerRw}
+
+	l := MaxBytesReader(wrappedRw, io.NopCloser(body), limit)
+
+	buf := make([]byte, 10)
+	_, err := l.Read(buf)
+
+	if _, ok := err.(*MaxBytesError); !ok {
+		t.Errorf("expected MaxBytesError, got %T", err)
+	}
+	if !innerRw.called {
+		t.Errorf("expected requestTooLarge to be called, but it wasn't")
+	}
+}
+
 func TestProtocols(t *testing.T) {
 	var p Protocols
 	if p.HTTP1() {

src/net/http/request.go

@@ -1243,9 +1243,23 @@ func (l *maxBytesReader) Read(p []byte) (n int, err error) {
 	type requestTooLarger interface {
 		requestTooLarge()
 	}
-	if res, ok := l.w.(requestTooLarger); ok {
-		res.requestTooLarge()
+	// Unwrap the ResponseWriter wrappers until we find one that implements
+	// the server-only requestTooLarger interface, then call requestTooLarge().
+	// This ensures that even if the ResponseWriter is wrapped by a custom implementation,
+	// the underlying server writer can be notified when the request body is too large.
+	rw := l.w
+	for {
+		if res, ok := rw.(requestTooLarger); ok {
+			res.requestTooLarge()
+			break
+		}
+		unwrapper, ok := rw.(rwUnwrapper)
+		if !ok {
+			break
+		}
+		rw = unwrapper.Unwrap()
 	}
+
 	l.err = &MaxBytesError{l.i}
 	return n, l.err
 }