[Interest] [External] Re: How to properly terminate QtConcurrent run

Sean Murphy Sean.M.Murphy at us.kbr.com
Wed Aug 17 17:11:01 CEST 2022

Thanks for the response!

> You should signal processChunk to stop processing early. You can for
> example do this through an atomic variable which is periodically checked, e.g.
> as part of the stop condition in that loop you mentioned.

I'll look into this idea, but some of what my processChunk() does are calls into an
external library that I can't control so my options are limited there. As far as I can
tell, this is an issue for any code that just takes a long time.

Take this hypothetical scenario for example:
 - we have some processing task that would take hours to run if run in a single thread,
    but it can be broken down into chunks
 - once broken into chunks, each chunk takes maybe a minute to run, but most of
    that minute is consumed in an external library that I can't do anything about
 - user starts one of these tasks, but before it finishes they decide they don't care
    about the results and want to completely quit the application
 - I warn the user there's a job in progress, are they sure they want to quit? They say Yes

So at this point, the user really wants to quit the application. From what I've gleaned so
far, and I could be wrong so feel free to correct me, I can tell the QFutureWatcher to
cancel the job. But this only prevents any chunks that hadn't been started from getting
started, it doesn't immediately terminate the chunks that are currently in progress,
those continue until they all finish, at which point the QFutureWatcher signals both
canceled() and finished(), and now I can safely quit the application. But if it were me as
the user, I'd be annoyed that I have to wait up to a minute for the application to quit
after I asked it to quit.

> I may be wrong, but I think terminating a thread forcefully is considered a
> poor design.

In general I agree, but I'm not sure I do in the special case of the user quitting the application
under the conditions I've described above. At that point to me (this is me wearing my user hat
now, not my developer hat), I would want the application to quit immediately and I don't really
care what happens under the hood - as long as it doesn't crash.

(Developer hat back on) I don't see a way to easily accomplish this under the QtConcurrent
API. Most of the examples I can find online show how to do the set up part, but they rarely
give any time to showing how to cleanly abort a run in progress. The only actual solution I
see that can do this cleanly is to ditch QtConcurrent, and write my own chunkProcessor
objects that spawn and manage their own worker threads, which when signaled, can forcefully
terminate regardless of what is being done at the time.

I'm open to suggestions/corrections/etc. Also, if anyone has any references they like that talk
more about quitting/cleaning up threads under all sorts of different scenarios, I'd love to read
more about it - I seem to be struggling to find those.

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