Re: [Qemu-devel] [Qemu-block] [PATCH v2 21/40] job: Convert block_job_cancel_async() to Job

[Max Reitz]

On 2018-05-29 13:59, Kashyap Chamarthy wrote:
> On Fri, May 25, 2018 at 10:00:35AM +0200, Kevin Wolf wrote:
>> Am 24.05.2018 um 19:42 hat John Snow geschrieben:
> 
> [...]
> 
> (Randomly chiming in for a small clarification.)
> 
>>>>> or some other mechanism that accomplishes the same type of behavior. It
>>>>> would be nice if it did not have to be determined at job creation time
>>>>> but instead could be determined later.
>>>>
>>>> I agree. We already made sure that job-cancel really means cancel on the
>>>> QAPI level, so we're free to do that. We just need to keep supporting
>>>> block-job-cancel with the old semantics, so what I have is the easy
>>>> conversion. We can change the internal implementation when we actually
>>>> implement the selection of a completion mode.
>>>>
>>>> Kevin
>>>>
>>>
>>> We need this before 3.0 though, yeah? unless we make job-cancel
>>> x-job-cancel or some other warning that the way it works might change, yeah?
>>>
>>> Or do I misunderstand our leeway to change this at a later point in time?
>>>
>>> (can job-cancel apply to block jobs created with the legacy
>>> infrastructure? My reading was "yes.")
>>
>> It can, and it already has its final semantics, so nothing has to change
>> before 3.0. job-cancel is equivalent to block-job-cancel with fixed
>> force=true. If you want the complete-by-cancel behaviour of mirror, you
>> have to use block-job-cancel for now, because job-cancel doesn't provide
>> that functionality.
> 
> I think by "complete-by-cancel" you are referring to this[*] magic
> behaviour, mentioned in the last sentence here:
> 
>     When you cancel an in-progress ‘mirror’ job before the source and
>     target are synchronized, block-job-cancel will emit the event
>     BLOCK_JOB_CANCELLED. However, note that if you cancel a ‘mirror’ job
>     after it has indicated (via the event BLOCK_JOB_READY) that the
>     source and target have reached synchronization, then the event
>     emitted by block-job-cancel changes to BLOCK_JOB_COMPLETED.
> 
> 
> [*] 
> https://git.qemu.org/?p=qemu.git;a=blob;f=docs/interop/live-block-operations.rst#l515
> 
>> So what we can change later is adding a way to initiate this secondary
>> completion mode with a job-* command (probably with a new option for
>> job-complete). But we wouldn't change the semantics of exisiting
>> commands.
> 
> Ah, so if I'm understanding it correctly, the existing subtle magic of
> "complete-by-cancel" will be rectified by separting the two distinct
> operations: 'job-cancel' and 'job-complete'.

Not really, because we already had those two operations for block jobs.
The thing is that block-job-complete (and thus job-complete) will pivot
to the target disk, but block-job-cancel doesn't.

The special behavior is that you can use block-job-cancel after
BLOCK_JOB_READY to complete the job, but not pivot to it.  I don't think
we have a real plan on how to represent that with the generic job
commands, we just know that we don't want to use job-cancel.

(Maybe we can add a flag to job-complete (which to me does not sound
like a good idea), or you could set flags on jobs while they are
running, so you can set a do-not-pivot flag on the mirror job before you
complete it.)

Max

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