net/http: add MaxBytesError

Fixes golang#30715

src/net/http/request.go

@@ -1124,7 +1124,7 @@ func readRequest(b *bufio.Reader) (req *Request, err error) {
 // MaxBytesReader is similar to io.LimitReader but is intended for
 // limiting the size of incoming request bodies. In contrast to
 // io.LimitReader, MaxBytesReader's result is a ReadCloser, returns a
-// non-EOF error for a Read beyond the limit, and closes the
+// MaxBytesError for a Read beyond the limit, and closes the
 // underlying reader when its Close method is called.
 //
 // MaxBytesReader prevents clients from accidentally or maliciously
@@ -1133,12 +1133,23 @@ func MaxBytesReader(w ResponseWriter, r io.ReadCloser, n int64) io.ReadCloser {
 	if n < 0 { // Treat negative limits as equivalent to 0.
 		n = 0
 	}
-	return &maxBytesReader{w: w, r: r, n: n}
+	return &maxBytesReader{w: w, r: r, i: n, n: n}
+}
+
+// MaxBytesError is returned by MaxBytesReader when its read limit is exceeded.
+type MaxBytesError struct {
+	Limit int64
+}
+
+func (e MaxBytesError) Error() string {
+	// Due to Hyrum's law, this text cannot be changed.
+	return "http: request body too large"
 }
 
 type maxBytesReader struct {
 	w   ResponseWriter
 	r   io.ReadCloser // underlying reader
+	i   int64         // max bytes initially, for MaxBytesError
 	n   int64         // max bytes remaining
 	err error         // sticky error
 }
@@ -1180,7 +1191,7 @@ func (l *maxBytesReader) Read(p []byte) (n int, err error) {
 	if res, ok := l.w.(requestTooLarger); ok {
 		res.requestTooLarge()
 	}
-	l.err = errors.New("http: request body too large")
+	l.err = MaxBytesError{l.i}
 	return n, l.err
 }
 

src/net/http/serve_test.go

@@ -3035,6 +3035,13 @@ func testRequestBodyLimit(t *testing.T, h2 bool) {
 		if n != limit {
 			t.Errorf("io.Copy = %d, want %d", n, limit)
 		}
+		mb, ok := err.(MaxBytesError)
+		if !ok {
+			t.Errorf("expected MaxBytesError, got %T", err)
+		}
+		if mb.Limit != limit {
+			t.Errorf("MaxBytesError.Limit = %d, want %d", mb.Limit, limit)
+		}
 	}))
 	defer cst.close()