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Re: [Qemu-devel] [PATCH v3] Improve error handling in do_snapshot_blkdev

From: Jes Sorensen
Subject: Re: [Qemu-devel] [PATCH v3] Improve error handling in do_snapshot_blkdev()
Date: Tue, 08 Mar 2011 09:24:48 +0100
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101209 Fedora/3.1.7-0.35.b3pre.fc14 Thunderbird/3.1.7

On 03/07/11 18:47, Anthony Liguori wrote:
> On 03/07/2011 10:39 AM, Jes Sorensen wrote:
>> On 03/07/11 17:34, Anthony Liguori wrote:
>>> You should restore the original image file before doing qerror_report()
>>> and just drop the error_printf()s as it's all redundant information.
>> I would hardly consider it redundant information that it failed and we
>> try to go back to the original image. That is an error in itself, even
>> though rolling back is better than abort()ing.
>> If qerror_report() is a fatal situation, that is problematic.
> It's fatal for the command, yes.  You should do qerror_report() in the
> exit path.
>>   Then we
>> need qerror_warn() or something as well, which can return non fatal
>> information.
> In your case, it's definitely a fatal error for the command.  The
> command is failing and you're just printing out information about the
> rollback information you're taking.

I guess the disconnect here is the definition of fatal. Fatal in my book
means we're dead, toast, gone ..... hardly the case if we manage to fail
back to the previous image.

>> The printfs are very valuable for the human monitor, but it isn't really
>> clear to me what is the ideal return value.
> But error_printf() is meaningless in the context of QMP.  You can
> reproduce these printfs in HMP based on the errors returned by QMP.
> But if you're just doing an HMP command (and don't care about QMP) then
> you shouldn't use qerror_report().  But you need to care about QMP so
> you should focus on making it a well behaved QMP command.

The question here is then how to propagate the message back that we
failed to switch to the new image, but stayed on the old one, as opposed
to both of them failing? This part of QMP is really black magic and
there doesn't seem to be a good error for this. Time for a new QMP error?

> BTW, there shouldn't be an abort() in any of these paths.  If you fail
> to reopen, just let the failure propagate.

In this particular case it can be argued that the situation is so fatal
that it is better to fail than to let the guest continue.


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