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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH v3] Improve error handling in do_snapshot_blkdev()
Date: Tue, 08 Mar 2011 11:46:23 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8

On 03/08/2011 10:44 AM, Jes Sorensen wrote:
On 03/08/11 14:42, Anthony Liguori wrote:
On 03/08/2011 02:24 AM, Jes Sorensen wrote:
On 03/07/11 18:47, Anthony Liguori wrote:
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.
Let me put it another way, you can't call qerror_report twice because
there is only one QMP error object sent in the protocol.  You
potentially call qerror_report twice which breaks QMP.

The way you ought to structure things is to return to the old image, and
then throw an error saying that you couldn't open the new image.
I see, I had the impression QMP would create multiple objects and pass
them along. Guess not.

No, this is made clearer in QAPI because an error pointer is passed and you explicitly set the object.

If FileOpenFailed has the filename of the new image, then opening the
file failed and we're using the old image.  If FileOpenFailed has the
filename of the old image, we're toast.

That basically covers it, no?
It kinda sorta covers it. The problem with that is that you then have to
do a string match of the return values to determine which of the cases
happened, which isn't very nice. But I guess we can do that for now.

Right, but this can be done in the HMP command such that the HMP command still prints out the warning message.

The key is to have well documented error semantics where the various cases can be distinguished because then we can ensure that we can not only print out a nice error message in HMP, but that a remote QMP client (like libvirt) can also generate a high quality error message.


Anthony Liguori

I'll have a look.


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