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Re: [Qemu-devel] [PATCH for-2.11] nbd/client: Don't hard-disconnect on E

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH for-2.11] nbd/client: Don't hard-disconnect on ESHUTDOWN from server
Date: Fri, 17 Nov 2017 08:34:20 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/13/2017 01:48 PM, Eric Blake wrote:
> 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

I think I meant qemu-stable, not qemu-trivial.  At any rate, this is now
queued on my NBD tree for 2.11-rc2.

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

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