BUG/MEDIUM: mux-quic: properly trim HTX buffer on snd_buf reset

MUX QUIC snd_buf operation whill return early if a qcs instance is
resetted. In this case, HTX is left untouched and the callback returns
the whole bufer size. This lead to an undefined behavior as the stream
layer is notified about a transfer but does not see its HTX buffer
emptied. In the end, the transfer may stall which will lead to a leak on
session.

To fix this, HTX buffer is now resetted when snd_buf is short-circuited.
This should fix the issue as now the stream layer can continue the
transfer until its completion.

This patch has already been tested by Tristan and is reported to solve
the github issue #1801.

This should be backported up to 2.6.

(cherry picked from commit 0ed617ac2ff377ce60bd9c8fd97fd9da32d43971)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>

include/haproxy/qmux_http.h

@@ -10,6 +10,7 @@ size_t qcs_http_rcv_buf(struct qcs *qcs, struct buffer *buf, size_t count,
                         char *fin);
 size_t qcs_http_snd_buf(struct qcs *qcs, struct buffer *buf, size_t count,
                         char *fin);
+size_t qcs_http_reset_buf(struct qcs *qcs, struct buffer *buf, size_t count);
 
 #endif /* USE_QUIC */
 

src/mux_quic.c

@@ -2110,7 +2110,7 @@ static size_t qc_snd_buf(struct stconn *sc, struct buffer *buf,
 	BUG_ON_HOT(qcs->flags & QC_SF_DETACH);
 
 	if (qcs_is_close_local(qcs) || (qcs->flags & QC_SF_TO_RESET)) {
-		ret = count;
+		ret = qcs_http_reset_buf(qcs, buf, count);
 		goto end;
 	}
 

src/qmux_http.c

@@ -87,3 +87,23 @@ size_t qcs_http_snd_buf(struct qcs *qcs, struct buffer *buf, size_t count,
 
 	return ret;
 }
+
+/* QUIC MUX snd_buf reset. HTX data stored in <buf> of length <count> will be
+ * cleared. This can be used when data should not be transmitted any longer.
+ *
+ * Return the size in bytes of cleared data.
+ */
+size_t qcs_http_reset_buf(struct qcs *qcs, struct buffer *buf, size_t count)
+{
+	struct htx *htx;
+
+	TRACE_ENTER(QMUX_EV_STRM_SEND, qcs->qcc->conn, qcs);
+
+	htx = htx_from_buf(buf);
+	htx_reset(htx);
+	htx_to_buf(htx, buf);
+
+	TRACE_LEAVE(QMUX_EV_STRM_SEND, qcs->qcc->conn, qcs);
+
+	return count;
+}