This small change takes care of a race condition

that can occur when both sides close at the same time.
If that occurs, without this fix the connection enters
FIN1 on both sides and they will forever send FIN|ACK at
each other until the connection times out. This is because
we stopped processing the FIN|ACK and thus did not advance
the sequence and so never ACK'd each others FIN. This
fix adjusts it so we *do* process the FIN properly and
the race goes away ;-)

MFC after:	1 month

1 files changed, 30 insertions, 0 deletions

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 1840b08..015b1ff 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -2414,6 +2414,16 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 						}
 					} else
 						tp->snd_cwnd += tp->t_maxseg;
+					if ((thflags & TH_FIN) &&
+					    (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
+						/* 
+						 * If its a fin we need to process
+						 * it to avoid a race where both
+						 * sides enter FIN-WAIT and send FIN|ACK
+						 * at the same time.
+						 */
+						break;
+					}
 					(void) tcp_output(tp);
 					goto drop;
 				} else if (tp->t_dupacks == tcprexmtthresh) {
@@ -2453,6 +2463,16 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 					}
 					tp->snd_nxt = th->th_ack;
 					tp->snd_cwnd = tp->t_maxseg;
+					if ((thflags & TH_FIN) &&
+					    (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
+						/* 
+						 * If its a fin we need to process
+						 * it to avoid a race where both
+						 * sides enter FIN-WAIT and send FIN|ACK
+						 * at the same time.
+						 */
+						break;
+					}
 					(void) tcp_output(tp);
 					KASSERT(tp->snd_limited <= 2,
 					    ("%s: tp->snd_limited too big",
@@ -2479,6 +2499,16 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 					    (tp->snd_nxt - tp->snd_una) +
 					    (tp->t_dupacks - tp->snd_limited) *
 					    tp->t_maxseg;
+					if ((thflags & TH_FIN) &&
+					    (TCPS_HAVERCVDFIN(tp->t_state) == 0)) {
+						/* 
+						 * If its a fin we need to process
+						 * it to avoid a race where both
+						 * sides enter FIN-WAIT and send FIN|ACK
+						 * at the same time.
+						 */
+						break;
+					}
 					(void) tcp_output(tp);
 					sent = tp->snd_max - oldsndmax;
 					if (sent > tp->t_maxseg) {