help wanted: strange / unknown behaviour of gui.create_job

I'm currently writing (or, trying to write 🙄) a lua script that will generate the preview cache for selected images (without leaving dt to start the external program...). Basically, it seems to work but I see two strange things that I don't understand right now.

First, the essential parts of my script:

local function stop_job( job )
  job.valid = false
end

local function create_previews()
  job = dt.gui.create_job( "generating previews", true, stop_job )
  local images = dt.gui.action_images
  for i,image in pairs(images) do
    if job.valid then
      image:generate_cache( true, 0, 8 )
      job.percent = i / #images
      dt.control.sleep( 1 ) -- without this, the job cannot be aborted
    else
      break
    end
  end
  if job.valid then -- without this, dt crashes with an exception in dt_pthread_mutex_destroy() after aborting the job
    job.valid = false
  end
end

the create_previews function will be called from a registered action.

My two questions:

1. Without the sleep() I cannot stop the running job. After starting the job, the progress bar is shown and when I hit the 'X' nothing happens until the for loop processed all images. Then the stop_job() is called. With the sleep the stop_job is called after processing the current image and the job terminates as desired.

2. When I set the job.valid to false after the loop it only works as long as the job is not terminated via the stop_job function. As this already sets the job to invalid I guess it is already destroyed and in the second job.valid=false dt crashes with an exception in dt_pthread_mutex_destroy.

Both phenomena are completely understandable, but: I see a lot of scripts that execute job.valid=false twice (when aborted) and seem to rely on stopping without a call to sleep(). Hmmm. What I'm missing here?

I hope you can clarify this,
thanks a lot!
Christian

Beginne mit der gezeigten registrierten Aktion und reproduziere die Abbruchfunktion von dt.gui.create_job sowohl mit als auch ohne dt.control.sleep. Verfolge den Lebenszyklus von job.valid im Zusammenhang mit stop_job und der abschließenden Zuweisung, unter Verwendung der Implementierung der Lua API oder der im Issue erwähnten vorhandenen Skripte. Als erledigt gilt die Aufgabe, wenn das Abbruchverhalten und die sichere Verwendung des Job-Lebenszyklus geklärt oder korrigiert sind, ohne den gemeldeten Absturz.