[fpc-devel] TThread.WaitFor causes deadlock under Linux

[Peter Popov]

It may not be relevant, but do you specify a timeout for the wait?

Peter

On Wed, 17 Sep 2008 05:54:03 -0500, Graeme Geldenhuys  
<graemeg.lists at gmail.com> wrote:

> Hi,
>
> I posted the original message in the fpc-users mailing list, but after
> a few replies, I seem to think it's a bug in FPC. Hence I decided to
> post the message here as well.
>
> -------------------[ original mail ]---------------------
> I have a thread based logging framework (part of tiOPF).  I created a
> "Log to GUI" viewer for the log framework using fpGUI toolkit. While
> the application runs, the cached logging to the GUI window works
> perfectly - no problems there.  For some reason, the call to the
> internal TThread.WaitFor doesn't always return when the application
> terminates. The viewer is based on code for Delphi's VCL, which never
> showed this issue.
>
> I'm not sure how to debug threads and don't have a clue as to what
> could cause the problem.
>
> My application is built with FPC 2.2.3 and fpGUI toolkit, running
> under Ubuntu Linux 7.10 (2.6 based kernel).
>
> Example of a successful termination:
>
> graemeg at graemeg:tutoradmin$ ./tutoradmin.exe -lv
> TtiLogToGUI.FormClose called
>>> TfpgApplication.Destroy
>   destroying components... DONE
>   killing message queue
> << TfpgApplication.Destroy
>>> TtiLogToCacheAbs.Terminate
>   called .Terminate() and now calling .WaitFor()
> << TtiLogToCacheAbs.Terminate
> graemeg at graemeg:tutoradmin$
>
>
> An example, where the application doesn't terminate.
>
> graemeg at graemeg:tutoradmin$ ./tutoradmin.exe -lv
> TtiLogToGUI.FormClose called
>>> TfpgApplication.Destroy
>   destroying components... DONE
>   killing message queue
> << TfpgApplication.Destroy
>>> TtiLogToCacheAbs.Terminate
>   called .Terminate() and now calling .WaitFor()
>
> I have to press Ctrl+C to terminate the application fully.
>
> TtiLogToCacheAbs is a abstract base class used for cached logging. It
> contains a TCriticalSection and a TThread.
>
> TtiLogToGUI is a descendant of TtiLogToCacheAbs and contains the GUI
> Form instance (visual logging). The GUI Form is closed and destroyed
> before the Application or thread terminates. It's the first output
> line (....FormClose) shown above...
>
> Here is the section of code which doesn't complete 100% and hangs the
> application when the application gets terminated.
>
> procedure TtiLogToCacheAbs.Terminate;
> begin
>  writeln('>> TtiLogToCacheAbs.Terminate');
>  FThrdLog.Terminate;
>  writeln('   called .Terminate() and now calling .WaitFor()');
>  FThrdLog.WaitFor;
>  writeln('<< TtiLogToCacheAbs.Terminate');
> end;
>
> Any ideas?
>
>
>
> ---------------[  and some responses ]-------------------------
>
>>> When you call synchronize from inside of you thread, this sends a  
>>> message
>>> to
>>> application thread with the procedure that will be used to synchronize  
>>> AND
>>> WAIT FOR a response from application thread of the execution of sync
>>> procedure. BUT when you call Thread.WaitFor in application thread, the
>>> application don't process any messages because it is waiting the thread
>>> finish, and thread don't finish because it is waiting a response of
>>> application thread of the execution of sync procedure... and you have  
>>> your
>>> aplication freeze!
>>
>> This is exactly what seems to be happening... I'll try your
>> work-around and see how it goes. Thanks.
>>
>> Note:
>> I have no such issues in Delphi, using the exact same threading code.
>> So does that mean FPC has a bug in threading?
>
> Delphi has code in its WaitFor function that prevents this deadlock
> from occuring.  They basically wait on the SyncEvent and the handle of
> the thread, calling CheckSynchonize when the SyncEvent triggers.
>
> I'm not sure if this is possible with fpc, but I'm sure there must be
> a solution.
> -------------------------[ end ]---------------------------
>
>
>
> What can I do to resolve this problem?  I'm not very knowledable with
> threads so bare with me...  TThread.WaitFor is not virtual, so I can't
> override it and try and place a CheckSynchronize call in there.  What
> else could I try?  Is this a know bug in FPC and TThread under
> Linux/Unix?
>
> I have the UseCThreads define enabled, which pulls in the 'cthreads'
> unit in my program unit.
>
> Regards,
>   - Graeme -
>
>
> _______________________________________________
> fpGUI - a cross-platform Free Pascal GUI toolkit
> http://opensoft.homeip.net/fpgui/
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> http://lists.freepascal.org/mailman/listinfo/fpc-devel