[PATCH v2] tap: Drop frames if no client connected

[Yumei Huang <yuhuang@redhat.com>]

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>
---
 tap.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tap.c b/tap.c
index 7ba6399..2e371b3 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;
 
-- 
@@ -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;
 
-- 
2.47.0