Revisiting an older thread
On Mon, Sep 13, 2021 at 04:19:36PM +0100, Richard W.M. Jones wrote:
> $ rm -f /tmp/sock /tmp/pid
> $ qemu-img create -f qcow2 /tmp/disk.qcow2 1M
> $ qemu-nbd -t --format=qcow2 --socket=/tmp/sock --pid-file=/tmp/pid
> /tmp/disk.qcow2 &
> $ nbdsh -u 'nbd+unix:///?socket=/tmp/sock' -c 'h.get_size()'
> qemu-nbd: Disconnect client, due to: Failed to send reply: Unable to write to
> socket: Broken pipe
> $ killall qemu-nbd
>
> nbdsh is abruptly dropping the NBD connection here which is a valid
> way to close the connection. It seems unnecessary to print an error
> in this case so this commit suppresses it.
I've investigated this a bit further, and found that we have a
regression in 6.0 when the abrupt disconnect occurs over TCP instead
of Unix sockets. Prior to that point, if a client does a hard
disconnect with no pending message, a 5.2 server reports:
$ qemu-nbd -f raw file &
[1] 1059670
$ nbdsh -u nbd://localhost:10809 -c 'h.trim(1,0)'
qemu-nbd: Disconnect client, due to: Failed to read request: Unexpected
end-of-file before all bytes were read
[1]+ Done qemu-nbd -f raw file
but in 6.0, we started reporting:
qemu-nbd: Disconnect client, due to: Request handling failed in intermediate
state
and looking further, I discovered that if you also use --trace=nbd_\*,
qemu-nbd ends up tracing uninitialized memory; it is now failing
because it sees an unexpected magic number from the uninitialized
memory, rather than its earlier detection of early EOF.
It's interesting that a Unix socket sees EPIPE when a TCP socket does
not, but it also means that your patch in isolation won't solve the
TCP issue, so I'm trying to come up with something today.
>
> Note that if you call the nbdsh h.shutdown() method then the message
> was not printed:
>
> $ nbdsh -u 'nbd+unix:///?socket=/tmp/sock' -c 'h.get_size()' -c 'h.shutdown()'
>
> Signed-off-by: Richard W.M. Jones <rjo...@redhat.com>
> ---
> nbd/server.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/nbd/server.c b/nbd/server.c
> index 3927f7789d..e0a43802b2 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -2669,7 +2669,12 @@ static coroutine_fn void nbd_trip(void *opaque)
> ret = nbd_handle_request(client, &request, req->data, &local_err);
> }
> if (ret < 0) {
> - error_prepend(&local_err, "Failed to send reply: ");
> + if (errno != EPIPE) {
> + error_prepend(&local_err, "Failed to send reply: ");
> + } else {
> + error_free(local_err);
> + local_err = NULL;
> + }
> goto disconnect;
> }
>
> --
> 2.32.0
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
