tap: Drop frames if no client connected

If no client is attached, discard outgoing frames and report them as
sent. Without this we will get an EBADF in either writev() (pasta) or
sendmsg() (passt). That's basically harmless, but a bit ugly.
Explicitly catching this case results in behaviour that's probably a
bit clearer to debug if we hit it.

There are several different approaches we can take here. Here's some
reasoning as David explained:

* Don't listen() until the tap connection is ready

 - It's not clear that the host rejecting the connection is better
   than the host accepting, then the connection stalling until the
   guest is ready.
 - Would require substantial rework because we currently listen() as
   we parse the command line and don't store the information we'd need
   to do it later.

* Don't accept() until the tap connection is ready

 - To the peer, will behave basically the same as this patch - the
   host will complete the TCP handshake, then the connection will stall
   until the guest is ready.
 - More work to implement, because essentially every sock-side handler
   has to check fd_tap and abort early

* Drop packets in tap_send_frames(), but return 0

 - To the peer, would behave basically the same
 - Would make the TCP code do a bunch of busy work attempting to
   resend, probably to no avail
 - Handling of errors returned by tap_send_frames() is on the basis
   that it's probably a transient fault (buffer full) and we want to
   resend very soon.  That approach doesn't make sense for a missing
   guest.

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Yumei Huang <yuhuang@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/tap.c b/tap.c
index 399eeaa..95d309b 100644
--- a/tap.c
+++ b/tap.c
@@ -507,13 +507,16 @@ static size_t tap_send_frames_passt(const struct ctx *c,
  * @iov must have total length @bufs_per_frame * @nframes, with each set of
  * @bufs_per_frame contiguous buffers representing a single frame.
  *
- * Return: number of frames actually sent
+ * Return: number of frames actually sent, or accounted as sent
  */
 size_t tap_send_frames(const struct ctx *c, const struct iovec *iov,
 		       size_t bufs_per_frame, size_t nframes)
 {
 	size_t m;
 
+	if (c->fd_tap == -1)
+		return nframes;
+
 	if (!nframes)
 		return 0;