BUG/MINOR: mux_quic: fix max stream ID reuse estimation

The following patch adjusts QUIC mux avail_streams() to ensure maximum
stream ID is never exceeded.

  commit 143d0034c912f1490812b6302f0dffb37f3ec02d
  BUG/MINOR: mux_quic: limit avail_streams() to 2^62

However, the calcul is incorrect, as <next_bidi_l> member value is set
to the next ID available, not the last one in use. Also, when the last
stream is closed, it will be greater than QCS_ID_MAX_STRM_CL_BIDI,
resulting in a substraction wrapping.

Fix this by using the simplest approach. Return value of avail_streams()
is only reduced if either the maximum stream ID limit is already
exceeded, or there is only a single stream still usable. In other cases,
return value is left as is.

Note that this bug is unlikely to have any impact as the maximum stream
ID is a very large value.

This should be backported up to 3.3.

diff --git a/src/mux_quic.c b/src/mux_quic.c
index ce27cc728e459e1394e6f59c3e8ef018b75b3c63..234762362c5530841af03df02fe0a6667722f70b 100644 (file)
--- a/src/mux_quic.c
+++ b/src/mux_quic.c
@@ -3287,9 +3287,11 @@ static int qmux_avail_streams(struct connection *conn)
                ret = MIN(ret, max_reuse);
        }
 
-       /* Ensure we do not exceed the maximum usable stream ID. */
-       if (unlikely(ret > QCS_ID_MAX_STRM_CL_BIDI - qcc->next_bidi_l))
-               ret = QCS_ID_MAX_STRM_CL_BIDI - qcc->next_bidi_l;
+       /* Do not exceed maximum usable stream ID. To simplify the calcul,
+        * limit is only applied when one or zero stream remains.
+        */
+       if (ret && unlikely(qcc->next_bidi_l >= QCS_ID_MAX_STRM_CL_BIDI))
+               ret = qcc->next_bidi_l == QCS_ID_MAX_STRM_CL_BIDI ? 1 : 0;
 
        return ret;
 }