[fpc-devel] Exceptions in threads

[Mattias Gärtner]

Zitat von Michael Van Canneyt <michael at freepascal.org>:

>
>
> On Thu, 11 Dec 2008, Mattias Gärtner wrote:
>
> > Zitat von Florian Klaempfl <florian at freepascal.org>:
> >
> > > Mattias Gärtner schrieb:
> > > > I'm writing a unit to simplify parallel methods/procedures.
> > > >
> > > > For example:
> > > >   DoParallel(@AMethod,StartIndex,EndIndex,Data);
> > > >
> > > > The AMethod is executed with several threads in parallel.
> > > >
> > > > What should happen when an exception occurs?
> > > > It would be nice if the exception can be transfered to the main thread.
> > > > Is this possible? How?
> > >
> > > See synchronize/checksynchronize implementation.
> >
> > Sorry, I misused the term 'main thread'.
> >
> > DoParallel starts AMethod with several threads in parallel and waits until
> all
> > threads finished. DoParallel can be called by any thread (e.g. the main
> thread
> > of the process *or* any other sub thread). Even the threads of the pool can
> > call again DoParallel. This is only limited by the memory, as each call
> creates
> > a threadgroup object.
> >
> > The goal is to simplify parallelizing procedures so for the user DoParallel
> > should work like a normal procedure call.
> > That's why it would be nice if the following could work:
> >
> > try
> >   ...
> >   DoParallel(...);
> >   ...
> > except
> > end;
> >
> > The exception can occur in any of the subthreads including the starting
> thread
> > (which I named 'main thread', because it is the main thread of the group.
> Maybe
> > I should call it 'group thread' or 'starter thread').
> >
> > When an exception occurs, the group waits for all other threads of this
> group to
> > finish and then the exception should happen 'as if the exception occured in
> the
> > starting thread'.
>
> The way I uderstand threads this can never work, because then there should
> be a synchronization wait loop after the doparallel.  Without the wait loop
> it'll never work, because where (at which statement) should the exception be
> raised ?

It should be raised in the starter thread. The question is how to
- stop an exception in a thread (try..except)
- give it to another thread - the starter thread (giving the object is easy, but
...)
- continue the helper thread (this will normally free the exception object, I
need a way to avoid this automatism)
- raise the exception in the starter thread (raise FetchedException).


> To make it operable, the thread should catch the exception, and then do a
> synchronize with a procedure that raises the exception, so that when
> synchronize is called, the exception is raised.

I don't see how this is possible.

Mattias