ngtcp2: add a comment explaining write result handling

The choice to continue processing incoming data although the
writeout of the headers/data failed is not obvious. Add a comment
explaining why this is done.

Closes #19093

diff --git a/lib/vquic/curl_ngtcp2.c b/lib/vquic/curl_ngtcp2.c
index 53665b1a3d1df97079ac5ca83470e214aba23912..56ca976b9f34053d3ed34cafe03e1bfd30dc58ea 100644 (file)
--- a/lib/vquic/curl_ngtcp2.c
+++ b/lib/vquic/curl_ngtcp2.c
@@ -1018,8 +1018,12 @@ static void h3_xfer_write_resp_hd(struct Curl_cfilter *cf,
                                   struct h3_stream_ctx *stream,
                                   const char *buf, size_t blen, bool eos)
 {
-
-  /* If we already encountered an error, skip further writes */
+  /* This function returns no error intentionally, but records
+   * the result at the stream, skipping further writes once the
+   * `result` of the transfer is known.
+   * The stream is subsequently cancelled "higher up" in the filter's
+   * send/recv callbacks. Closing the stream here leads to SEND/RECV
+   * errors in other places that then overwrite the transfer's result. */
   if(!stream->xfer_result) {
     stream->xfer_result = Curl_xfer_write_resp_hd(data, buf, blen, eos);
     if(stream->xfer_result)
@@ -1033,8 +1037,12 @@ static void h3_xfer_write_resp(struct Curl_cfilter *cf,
                                struct h3_stream_ctx *stream,
                                const char *buf, size_t blen, bool eos)
 {
-
-  /* If we already encountered an error, skip further writes */
+  /* This function returns no error intentionally, but records
+   * the result at the stream, skipping further writes once the
+   * `result` of the transfer is known.
+   * The stream is subsequently cancelled "higher up" in the filter's
+   * send/recv callbacks. Closing the stream here leads to SEND/RECV
+   * errors in other places that then overwrite the transfer's result. */
   if(!stream->xfer_result) {
     stream->xfer_result = Curl_xfer_write_resp(data, buf, blen, eos);
     /* If the transfer write is errored, we do not want any more data */