There are a number of ways an application can check if there are

inbound data waiting on a filedescriptor, such as a pipe or a socket,
for instance by using select(2), poll(2), kqueue(2), ioctl(FIONREAD)
etc.

But we have no way of finding out if written data have yet to be
disposed of, for instance, transmitted (and ack'ed!) to some remote
host, or read by the applicantion at the far end of the pipe.

The closest we get, is calling shutdown(2) on a TCP socket in
non-blocking mode, but this has the undesirable sideeffect of
preventing future communication.

Add a complement to FIONREAD, called FIONWRITE, which returns the
number of bytes not yet properly disposed of.  Implement it for
all sockets.

Background:

A HTTP server will want to time out connections, if no new request
arrives within a certain period after the last transmitted response
has actually been sent (and ack'ed).

For a busy HTTP server, this timeout can be subsecond duration.

In order to signal to a load-balancer that the connection is truly
dead, TCP_RST will be the preferred method, as this avoids the need
for a RTT delay for FIN handshaking, with a client which, surprisingly
often, no longer at the remote IP number.

If a slow, distant client is being served a response which is big
enough to fill the window, but small enough to fit in the socket
buffer, the write(2) call will return immediately.

If the session timeout is armed at that time, all bytes in the
response may not have been transmitted by the time it fires.

FIONWRITE allows the timeout to check that no data is outstanding
on the connection, before it TCP_RST's it.

Input & Idea from: rwatson
Approved by:	re (kib)

1 files changed, 5 insertions, 0 deletions

diff --git a/sys/kern/sys_socket.c b/sys/kern/sys_socket.c
index 93da80d..ecfb4ad 100644
--- a/sys/kern/sys_socket.c
+++ b/sys/kern/sys_socket.c
@@ -169,6 +169,11 @@ soo_ioctl(struct file *fp, u_long cmd, void *data, struct ucred *active_cred,
 		*(int *)data = so->so_rcv.sb_cc;
 		break;
 
+	case FIONWRITE:
+		/* Unlocked read. */
+		*(int *)data = so->so_snd.sb_cc;
+		break;
+
 	case FIOSETOWN:
 		error = fsetown(*(int *)data, &so->so_sigio);
 		break;