[m-users.] Calling exit(3) from a foreign predicate

[Sean Charles (emacstheviking)]

*Your* emails are anything but boring Volker!


> On 25 Jan 2024, at 14:24, Volker Wysk <post at volker-wysk.de> wrote:
> 
> Am Donnerstag, dem 25.01.2024 um 07:53 +0000 schrieb Sean Charles
> (emacstheviking):
>> What a module accessible flag for global control then I guess?
>> 
>> :- module foo.
>> 
>> :- pred is_exit_requested.
>> 
>> :- implementation.
>> 
>> :- mutable (stop_flag, bool, no, ground, [...whats your app needs...]).
>> 
>> :- pragma promise_pure(stop_threads/0). 
>> :- pred stop_threads :-
>>     impure set_stop_flag(yes).
>> 
>> 
>> :- pragma promise_pure(iis_exit_requested/0). 
>> :- pred is_exit_requested is semidet.
>> is_exit_requested :-
>>     semipure get_stop_flag(yes).
>> 
>> 
>> then inside your threads you can just called   foo.is_exit_requested ?
>> This is off the top of my head, so I have probably made a few m istakes,
>> it's early and I've not had my porridge yet, those are my excuses!
>> 
>> That's kind of what I was hinting at nefpre; external control, but I
>> assumed you were passing two-way control structures but if the threads
>> have no ears then thisd might be the solution / a solution?
>> 
>> Good luck VOlker, you always figure out!
> 
> Thanks for your consideration. But the infrastructure for shutting down the
> threads is already in place (except for one piece).
> 
> It works like this: There's a master thread (the main program), which spawns
> a feeder thread and a number of worker threads. 
> 
> The feeder is connected to the workers and to master by a channel each. It
> feeds work items (paths to files) to the workers. Those do something with
> it. They bring the database up to date with respect to the path. They also
> (re)calculate hashes of the files, when needed. The workers send the work
> results (hashes) to master, which does something with it. 
> 
> The feeder also sends messages to master about the succession in which the
> files occur. And when all work has been feeded, the feeder sends an "end"
> message to all workers. This message causes the worker to send an "end"
> message to master, on his part, and to leave the predicate which it has been
> started with. Meaning the worker terminates.
> 
> Master does something with the received work results. It depends on which
> operation has been chosen. Such as "print the hashes of the files".
> 
> When there happens a database error, the whole process, meaning all threads,
> should be terminated. It can occur in one worker, or occur in all workers
> (such as authentication failure when establishing the database connection). 
> 
> Now, in order terminate the process, I can go the easy way and just call
> exit(3). The downside is, that the Mercury shutdown isn't carried out
> (except for what exit() does).
> 
> The other way would be to cause the threads (workers and feeder) to
> terminate themselves. The feeder would need to be instructed to stop feeding
> and terminate. It should send "end" messages to all workers, and terminate
> itself afters. Then master needs to collect all messages (work
> results/errors), which are still pending. And it needs to collect and count
> the "end" messages from the workers to master, so it knows when all workers
> have terminated.
> 
> Informing the feeder to stop feeding, could be done with an mvar
> (thread.mvar module).
> 
> The downside of this is, that it's more complicated than the easy way. (It's
> hard to debug inter-thread-communication). And it would mean waiting until
> all workers are done with their work. That can take a long time, when a
> worker is calculating the hash of a large file...
> 
> So, which way should I take?  I guess it depends on how bad it is to
> terminate the process the hard way by calling exit(3) and not calling
> mercury_runtime_terminate().
> 
> I hope I haven't bored you.
> 
> Cheers,
> Volker
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