BUG/MINOR: mux-h2: rely on trailers output not input to turn them to empty data

When sending trailers, we may face an empty HTX trailers block or even
have to discard some of the headers there and be left with nothing to
send. RFC7540 forbids sending of empty HEADERS frames, so in this case
we turn to DATA frames (which is possible since after other DATA).

The code used to only check the input frame's contents to decide whether
or not to switch to a DATA frame, it didn't consider the possibility that
the frame only used to contain headers discarded later, thus it could still
emit an empty HEADERS frame in such a case. This patch makes sure that the
output frame size is checked instead to take the decision.

This patch must be backported to 1.9. In practice this situation is never
encountered since the discarded headers have really nothing to do in a
trailers block.

diff --git a/src/mux_h2.c b/src/mux_h2.c
index 55d69a28e0dbce01c1ddc4eb1ab53fe807578084..baf84d4ef8b76f77dab3d06692b9bfd94d630405 100644 (file)
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -5096,11 +5096,12 @@ static size_t h2s_htx_make_trailers(struct h2s *h2s, struct htx *htx)
                }
        }
 
-       if (!hdr) {
-               /* here we have a problem, we've received an empty trailers
-                * block followed by an EOM. Because of this we can't send a
-                * HEADERS frame, so we have to cheat and instead send an empty
-                * DATA frame conveying the ES flag.
+       if (outbuf.data == 9) {
+               /* here we have a problem, we have nothing to emit (either we
+                * received an empty trailers block followed or we removed its
+                * contents above). Because of this we can't send a HEADERS
+                * frame, so we have to cheat and instead send an empty DATA
+                * frame conveying the ES flag.
                 */
                outbuf.area[3] = H2_FT_DATA;
                outbuf.area[4] = H2_F_DATA_END_STREAM;