http: new error for reading a body after it's been closed

R=adg
CC=golang-dev
https://golang.org/cl/4433094

src/pkg/http/transfer.go

@@ -439,9 +439,29 @@ type body struct {
 	hdr     interface{}   // non-nil (Response or Request) value means read trailer
 	r       *bufio.Reader // underlying wire-format reader for the trailer
 	closing bool          // is the connection to be closed after reading body?
+	closed  bool
+}
+
+// ErrBodyReadAferClose is returned when reading a Request Body after
+// the body has been closed. This typically happens when the body is
+// read after an HTTP Handler calls WriteHeader or Write on its
+// ResponseWriter.
+var ErrBodyReadAferClose = os.NewError("http: invalid Read on closed request Body")
+
+func (b *body) Read(p []byte) (n int, err os.Error) {
+	if b.closed {
+		return 0, ErrBodyReadAferClose
+	}
+	return b.Reader.Read(p)
 }
 
 func (b *body) Close() os.Error {
+	if b.closed {
+		return nil
+	}
+	defer func() {
+		b.closed = true
+	}()
 	if b.hdr == nil && b.closing {
 		// no trailer and closing the connection next.
 		// no point in reading to EOF.