[PATCH v2 4/6] tcp: Fix closing logic for half-closed connections

[Stefano Brivio <sbrivio@redhat.com>]

First off, don't close connections half-closed by the guest before
our own FIN is acknowledged by the guest itself.

That is, after we receive a FIN from the guest (TAP_FIN_RCVD), if we
don't have any data left to send from the socket (SOCK_FIN_RCVD, or
EPOLLHUP), we send a FIN segment to the guest (TAP_FIN_SENT), but we
need to actually have it acknowledged (and have no pending
retransmissions) before we can close the connection: check for
TAP_FIN_ACKED, first.

Then, if we set TAP_FIN_SENT, and we receive an ACK segment from the
guest, set TAP_FIN_ACKED. This was entirely missing for the
TAP_FIN_RCVD case, and as we fix the problem described above, this
becomes relevant as well.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/tcp.c b/tcp.c
index 11c9c84..fccea35 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2090,9 +2090,14 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 		tcp_tap_window_update(c, conn, ntohs(th->window));
 		tcp_data_from_sock(c, conn);
 
-		if (conn->events & SOCK_FIN_RCVD &&
-		    conn->seq_ack_from_tap == conn->seq_to_tap)
-			conn_event(c, conn, CLOSED);
+		if (conn->seq_ack_from_tap == conn->seq_to_tap) {
+			if (th->ack && conn->events & TAP_FIN_SENT)
+				conn_event(c, conn, TAP_FIN_ACKED);
+
+			if (conn->events & SOCK_FIN_RCVD &&
+			    conn->events & TAP_FIN_ACKED)
+				conn_event(c, conn, CLOSED);
+		}
 
 		return 1;
 	}
@@ -2372,7 +2377,7 @@ void tcp_sock_handler(const struct ctx *c, union epoll_ref ref,
 		return;
 	}
 
-	if ((conn->events & TAP_FIN_SENT) && (events & EPOLLHUP)) {
+	if ((conn->events & TAP_FIN_ACKED) && (events & EPOLLHUP)) {
 		conn_event(c, conn, CLOSED);
 		return;
 	}
-- 
@@ -2090,9 +2090,14 @@ int tcp_tap_handler(const struct ctx *c, uint8_t pif, sa_family_t af,
 		tcp_tap_window_update(c, conn, ntohs(th->window));
 		tcp_data_from_sock(c, conn);
 
-		if (conn->events & SOCK_FIN_RCVD &&
-		    conn->seq_ack_from_tap == conn->seq_to_tap)
-			conn_event(c, conn, CLOSED);
+		if (conn->seq_ack_from_tap == conn->seq_to_tap) {
+			if (th->ack && conn->events & TAP_FIN_SENT)
+				conn_event(c, conn, TAP_FIN_ACKED);
+
+			if (conn->events & SOCK_FIN_RCVD &&
+			    conn->events & TAP_FIN_ACKED)
+				conn_event(c, conn, CLOSED);
+		}
 
 		return 1;
 	}
@@ -2372,7 +2377,7 @@ void tcp_sock_handler(const struct ctx *c, union epoll_ref ref,
 		return;
 	}
 
-	if ((conn->events & TAP_FIN_SENT) && (events & EPOLLHUP)) {
+	if ((conn->events & TAP_FIN_ACKED) && (events & EPOLLHUP)) {
 		conn_event(c, conn, CLOSED);
 		return;
 	}
-- 
2.43.0