http2: Use NO_ERROR instead of CANCEL when responding before the request is finished

This is required by RFC 7540 Section 8.1.
Bug pointed out by Mike Bishop.

Updates golang/go#19948

Change-Id: I58d4f499609fd493d33115f4e1f64caf4bcbbc03
Reviewed-on: https://go-review.googlesource.com/40630
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/server.go

@@ -1049,7 +1049,11 @@ func (sc *serverConn) wroteFrame(res frameWriteResult) {
 			// stateClosed after the RST_STREAM frame is
 			// written.
 			st.state = stateHalfClosedLocal
-			sc.resetStream(streamError(st.id, ErrCodeCancel))
+			// Section 8.1: a server MAY request that the client abort
+			// transmission of a request without error by sending a
+			// RST_STREAM with an error code of NO_ERROR after sending
+			// a complete response.
+			sc.resetStream(streamError(st.id, ErrCodeNo))
 		case stateHalfClosedRemote:
 			sc.closeStream(st, errHandlerComplete)
 		}

http2/server_test.go

@@ -2352,7 +2352,7 @@ func TestServer_NoCrash_HandlerClose_Then_ClientClose(t *testing.T) {
 
 		// Sent when the a Handler closes while a client has
 		// indicated it's still sending DATA:
-		st.wantRSTStream(1, ErrCodeCancel)
+		st.wantRSTStream(1, ErrCodeNo)
 
 		// Now the handler has ended, so it's ended its
 		// stream, but the client hasn't closed its side