fix h2 big response not sending bug

src/brpc/policy/http2_rpc_protocol.cpp

@@ -1672,8 +1672,15 @@ H2UnsentResponse::AppendAndDestroySelf(butil::IOBuf* out, Socket* socket) {
     // NOTE: Currently the stream context is definitely removed and updating
     // window size is useless, however it's not true when progressive request
     // is supported.
+    // TODO(zhujiashun): Instead of just returning error to client, a better
+    // solution to handle not enough window size is to wait until WINDOW_UPDATE
+    // is received, and then retry those failed response again.
     if (!MinusWindowSize(&ctx->_remote_window_left, _data.size())) {
-        return butil::Status(ELIMIT, "Remote window size is not enough");
+        char rstbuf[FRAME_HEAD_SIZE + 4];
+        SerializeFrameHead(rstbuf, 4, H2_FRAME_RST_STREAM, 0, _stream_id);
+        SaveUint32(rstbuf + FRAME_HEAD_SIZE, H2_FLOW_CONTROL_ERROR);
+        out->append(rstbuf, sizeof(rstbuf));
+        return butil::Status::OK();
     }
 
     HPacker& hpacker = ctx->hpacker();