issuing [block-]job-complete to jobs in STANDBY state

[John Snow]

Hi; downstream we've run into an issue where VMs under heavy load with 
many simultaneously concurrent block jobs running might occasionally 
flicker into the STANDBY state, during which time they will be unable to 
receive JOB COMPLETE commands. I assume this flicker is due to 
child_job_drained_begin().

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1945635

It's safe to just retry this operation again, but it may be difficult to 
understand WHY the job is paused at the application level, since the 
flush event may be asynchronous and unpredictable.

We could define a transition to allow COMPLETE to be applied to STANDBY 
jobs, but is there any risk or drawback to doing so? On QMP's side, we 
do know the difference between a temporary pause and a user pause/error 
pause (Both use the user_pause flag.)

I imagine it's safe to continue rejecting COMPLETE commands if 
user_paused is set ("No, go fix this first!") and we could define a 
pathway for implicitly STANDBY jobs only. However, in this case, we 
don't really know how long STANDBY will last. Do we have the ability to 
easily accept an async "intent" to complete a job without tying up the 
monitor?

ATM I think only mirror uses .complete, but it looks like it tries to 
actually set up the pivot a good deal before delegating to the bottom 
half, so I worry it's not safe to try to run this when we are in the 
middle of a drain.

Any thoughts?

--js