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

[Graeme Geldenhuys]

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 -


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