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Re: [Qemu-devel] [Qemu-block] [PATCH v2] qapi: block-core: Clarify event

From: John Snow
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v2] qapi: block-core: Clarify events emitted by 'block-job-cancel'
Date: Wed, 15 Nov 2017 16:56:13 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/15/2017 04:54 PM, Kashyap Chamarthy wrote:
> On Wed, Nov 15, 2017 at 02:15:57PM -0500, John Snow wrote:
>> On 11/15/2017 04:09 AM, Kashyap Chamarthy wrote:
>>> When you cancel an in-progress live block operation with QMP
>>> `block-job-cancel`, it emits the event: BLOCK_JOB_CANCELLED.  However,
>>> when `block-job-cancel` is issued after `drive-mirror` has indicated (by
>>> emitting the event BLOCK_JOB_READY) that the source and destination
>>> remain synchronized:
> [...]
>>> But this is expected behaviour, where the _COMPLETED event indicates
>>> that synchronization has successfully ended (and the destination has a
>>> point-in-time copy, which is at the time of cancel).
>>> So add a small note to this effect.  (Thanks: Max Reitz for reminding
>>> me of this on IRC.)
>> I suppose this difference probably isn't covered in what was the
>> bitmaps.md doc file (we don't bother covering mirror there, only
>> backup); 
> Your supposition is correct: Looking at the now-renamed
> 'bitmaps.rst'[1], it isn't covered there.

Makes sense, I don't think we need to correct this, and

>> is it covered sufficiently in live-block-operations.rst ?
> I looked in there[2] too.  Short answer: no.  Long: In the "Live disk
> synchronization — drive-mirror and blockdev-mirror" section, I simply
> seemed to declare: 
>     "Issuing the command ``block-job-cancel`` after it emits the event
> As if that's the *only* event it emits, which is clearly not the case.
> So while at it, wonder if should I also update it
> ('live-block-operations.rst') too.

It's an interesting gotcha that I wasn't really acutely aware of myself,
so having it in the doc format for API programmers who aren't
necessarily digging through our source sounds like a pleasant courtesy.

> [1] https://git.qemu.org/?p=qemu.git;a=blob;f=docs/interop/bitmaps.rst
> [2] 
> https://git.qemu.org/?p=qemu.git;a=blob;f=docs/interop/live-block-operations.rst#l513

Thanks Kashyap :)

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