[PATCH v2 1/3] tcp: Retransmit FINs like data segments

[David Gibson <david@gibson.dropbear.id.au>]

RFC 9293 doesn't distinguish between regular data segments and FIN segments
for the purposes of retransmissions.  Our existing retransmission logic
will also work for FIN segments, except for one detail: we don't currently
set the ACK_FROM_TAP_DUE flag when we send a FIN.  Add the flag, so that
we'll properly retransmit FIN segments like data segments.

Remove the section from the theory of operation comment that describes a
different way of handling FIN timeouts which (a) isn't correct behaviour
and (b) doesn't appear to be implemented.

I've tested this by adding logic to suppress sending the FIN if retries <
some non-zero value.  We correctly resend the FIN and close normally after
the expected timeouts.

Link: https://bugs.passt.top/show_bug.cgi?id=195
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c     | 4 ----
 tcp_buf.c | 1 +
 tcp_vu.c  | 1 +
 3 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/tcp.c b/tcp.c
index 17e5b006..dbfde2e0 100644
--- a/tcp.c
+++ b/tcp.c
@@ -190,10 +190,6 @@
  * - RTO_INIT_AFTER_SYN_RETRIES: if SYN retries happened during handshake and
  *   RTO is less than this, re-initialise RTO to this for data retransmissions
  *
- * - FIN_TIMEOUT: if a FIN segment was sent to tap/guest (flag ACK_FROM_TAP_DUE
- *   with TAP_FIN_SENT event), and no ACK is received within this time, reset
- *   the connection
- *
  * - FIN_TIMEOUT: if a FIN segment was acknowledged by tap/guest and a FIN
  *   segment (write shutdown) was sent via socket (events SOCK_FIN_SENT and
  *   TAP_FIN_ACKED), but no socket activity is detected from the socket within
diff --git a/tcp_buf.c b/tcp_buf.c
index 5d419d36..d2925410 100644
--- a/tcp_buf.c
+++ b/tcp_buf.c
@@ -407,6 +407,7 @@ int tcp_buf_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 			}
 
 			conn_event(c, conn, TAP_FIN_SENT);
+			conn_flag(c, conn, ACK_FROM_TAP_DUE);
 		}
 
 		return 0;
diff --git a/tcp_vu.c b/tcp_vu.c
index db9db78a..b9e9b55e 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -425,6 +425,7 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 			}
 
 			conn_event(c, conn, TAP_FIN_SENT);
+			conn_flag(c, conn, ACK_FROM_TAP_DUE);
 		}
 
 		return 0;
-- 
2.52.0