net/http: add MaxBytesError

Fixes #30715

Change-Id: Ia3712d248b6dc86abef71ccea6e705a571933d53
GitHub-Last-Rev: 6ae68402a5
GitHub-Pull-Request: golang/go#49359
Reviewed-on: https://go-review.googlesource.com/c/go/+/361397
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

api/next/30715.txt

@@ -0,0 +1,3 @@
+pkg net/http, type MaxBytesError struct #30715
+pkg net/http, type MaxBytesError struct, Limit int64 #30715
+pkg net/http, method (*MaxBytesError) Error() string #30715

src/net/http/request.go

@@ -1126,21 +1126,34 @@ 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
-// sending a large request and wasting server resources.
+// sending a large request and wasting server resources. If possible,
+// it tells the ResponseWriter to close the connection after the limit
+// has been reached.
 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
 }
@@ -1182,7 +1195,7 @@ 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)
 		}
+		mbErr, ok := err.(*MaxBytesError)
+		if !ok {
+			t.Errorf("expected MaxBytesError, got %T", err)
+		}
+		if mbErr.Limit != limit {
+			t.Errorf("MaxBytesError.Limit = %d, want %d", mbErr.Limit, limit)
+		}
 	}))
 	defer cst.close()