net/http: add MaxBytesError

Fixes #30715

Change-Id: Ia3712d248b6dc86abef71ccea6e705a571933d53
GitHub-Last-Rev: 6ae68402a5a7c57f7f18e945d48c69ba2b134078
GitHub-Pull-Request: golang/go#49359
Reviewed-on: https://go-review.googlesource.com/c/go/+/361397
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

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()