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Re: [Qemu-block] [Qemu-devel] [PULL 3/4] nbd/client: Don't hard-disconne

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PULL 3/4] nbd/client: Don't hard-disconnect on ESHUTDOWN from server
Date: Wed, 6 Dec 2017 11:01:34 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 12/06/2017 10:55 AM, Michael Roth wrote:
> Quoting Eric Blake (2017-11-17 09:05:55)
>> The NBD spec says that a server may fail any transmission request
>> with ESHUTDOWN when it is apparent that no further request from
>> the client can be successfully honored.  The client is supposed
>> to then initiate a soft shutdown (wait for all remaining in-flight
>> requests to be answered, then send NBD_CMD_DISC).  However, since
>> qemu's server never uses ESHUTDOWN errors, this code was mostly
>> untested since its introduction in commit b6f5d3b5.
>> More recently, I learned that nbdkit as the NBD server is able to
>> send ESHUTDOWN errors, so I finally tested this code, and noticed
>> that our client was special-casing ESHUTDOWN to cause a hard
>> shutdown (immediate disconnect, with no NBD_CMD_DISC), but only
>> if the server sends this error as a simple reply.  Further
>> investigation found that commit d2febedb introduced a regression
>> where structured replies behave differently than simple replies -
>> but that the structured reply behavior is more in line with the
>> spec (even if we still lack code in nbd-client.c to properly quit
>> sending further requests).  So this patch reverts the portion of
>> b6f5d3b5 that introduced an improper hard-disconnect special-case
>> at the lower level, and leaves the future enhancement of a nicer
>> soft-disconnect at the higher level for another day.
>> CC: address@hidden
> Since 2.10.x lacks d2febedb, the patch would look something like:
> diff --git a/nbd/client.c b/nbd/client.c
> index 4caff77119..f04e95542f 100644
> --- a/nbd/client.c
> +++ b/nbd/client.c
> @@ -945,12 +945,6 @@ ssize_t nbd_receive_reply(QIOChannel *ioc, NBDReply 
> *reply, Error **errp)
>      reply->handle = ldq_be_p(buf + 8);
>      reply->error = nbd_errno_to_system_errno(reply->error);
> -
> -    if (reply->error == ESHUTDOWN) {
> -        /* This works even on mingw which lacks a native ESHUTDOWN */
> -        error_setg(errp, "server shutting down");
> -        return -EINVAL;
> -    }
>      trace_nbd_receive_reply(magic, reply->error, reply->handle);
>      if (magic != NBD_REPLY_MAGIC) {
> If that seems reasonable I can apply it as such.

Your backport looks correct to me; thanks.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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