[Libguestfs] [PATCH libnbd] lib: Kill subprocess in nbd_close.

[Eric Blake]

On 7/25/19 12:43 PM, Richard W.M. Jones wrote:
> $ time nbdsh -c 'h.connect_command (["nbdkit", "-s", "null", "size=512", "--filter=delay", "delay-read=10"]); b = nbd.aio_buffer(1); h.aio_pread (b, 0); del (h)'
> real	0m10.499s
> user	0m0.065s
> sys	0m0.023s
> 
> With this patch the elapsed time is near instantaneous.
> ---
>  lib/handle.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/handle.c b/lib/handle.c
> index 1fe4467..111cfc5 100644
> --- a/lib/handle.c
> +++ b/lib/handle.c
> @@ -124,8 +124,10 @@ nbd_close (struct nbd_handle *h)
>      freeaddrinfo (h->result);
>    if (h->sock)
>      h->sock->ops->close (h->sock);

Note that this patch is trying to speed up a slow waitpid() issued
_after_ we call h->sock->ops->close().  You'd think that after we
close() our end, that the remote end doing a read() would see an
immediate EOF, and that would be enough for the server to shut down
quickly instead of waiting for the delay to expire and finally notice
during its write() that we are no longer reading.  However, POSIX says
that when sockets are involved, a close() on our end does not trigger an
immediate EOF on the other side's read end, but rather may start an
asynchronous timer to try to finish the TCP connection gracefully to
flush any pending traffic, where both directions have to agree that the
connection is dead before either direction sees EOF/EPIPE.  And that's
where the shutdown(2) API comes in handy: it forces the kernel to
realize that one or both directions of traffic are no longer needed,
making this asynchronous handling in close() much faster.  At least
nbdkit is known to react favorably to a shutdown() request - see nbdkit
commit 430f8141.

We may want to add h->sock->ops->shutdown() (especially if we want to
handle SHUT_WR through adding more states for more graceful shutdown
paths, including when TLS is involved), but even in the short-term,
something as simple as adding shutdown(h->fd, SHUT_RDWR) immediately
before the s->sock->ops->close() would be helpful.

That said, I think we want a two-pronged approach - adding a use of
shutdown(), but also playing with signal:


> -  if (h->pid >= 0) /* XXX kill it? */

We're only doing this for connect_command.  We must NOT kill TCP or Unix
servers, as those are pre-existing and presumably must stick around for
further clients; but when we spawned the server, it is indeed likely
that the server should not continue to exist after our handle goes away
- it is unlikely that a client would ever expect the server they just
created to continue to exist after the handle to serve other clients.
So this makes sense.

> +  if (h->pid >= 0) {
> +    kill (h->pid, SIGHUP);

I don't know if SIGTERM is any better, but think SIGHUP is okay for now.
Also, I don't know if we want to put a time limit on waiting for the
server, and follow up with an even harder SIGKILL if it hasn't
gracefully exited.  Maybe it should be a knob where the user can call
nbd_set_FOO to tune the knob for their desired cleanup behavior (whether
for which signal to send, or whether to follow up with SIGKILL, or even
how long to wait)?  Or are we just reinventing timeout(1) from
coreutils?  So for now this seems okay.

>      waitpid (h->pid, NULL, 0);
> +  }

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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