tcp: Use EPOLLET for any state of not established connections

Currently, for not established connections, we monitor sockets with
edge-triggered events (EPOLLET) if we are in the TAP_SYN_RCVD state
(outbound connection being established) but not in the
TAP_SYN_ACK_SENT case of it (socket is connected, and we sent SYN,ACK
to the container/guest).

While debugging https://bugs.passt.top/show_bug.cgi?id=94, I spotted
another possibility for a short EPOLLRDHUP storm (10 seconds), which
doesn't seem to happen in actual use cases, but I could reproduce it:
start a connection from a container, while dropping (using netfilter)
ACK segments coming out of the container itself.

On the server side, outside the container, accept the connection and
shutdown the writing side of it immediately.

At this point, we're in the TAP_SYN_ACK_SENT case (not just a mere
TAP_SYN_RCVD state), we get EPOLLRDHUP from the socket, but we don't
have any reasonable way to handle it other than waiting for the tap
side to complete the three-way handshake. So we'll just keep getting
this EPOLLRDHUP until the SYN_TIMEOUT kicks in.

Always enable EPOLLET when EPOLLRDHUP is the only epoll event we
subscribe to: in this case, getting multiple EPOLLRDHUP reports is
totally useless.

In the only remaining non-established state, SOCK_ACCEPTED, for
inbound connections, we're anyway discarding EPOLLRDHUP events until
we established the conection, because we don't know what to do with
them until we get an answer from the tap side, so it's safe to enable
EPOLLET also in that case.

Link: https://bugs.passt.top/show_bug.cgi?id=94
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

tcp.c

@@ -440,7 +440,7 @@ static uint32_t tcp_conn_epoll_events(uint8_t events, uint8_t conn_flags)
 	if (events == TAP_SYN_RCVD)
 		return EPOLLOUT | EPOLLET | EPOLLRDHUP;
 
-	return EPOLLRDHUP;
+	return EPOLLET | EPOLLRDHUP;
 }
 
 /**