The Wiert Corner – irregular stream of stuff

Since about a decade, TThread has a few overloaded [WayBack] Synchronize methods which all allow some specified synchronised method to run in the context of the main thread:

• [WayBack] TThread.Synchronize Method (TThreadMethod)
• [WayBack] TThread.Synchronize Method (TThreadProcedure)
• [WayBack] TThread.Synchronize Method (TThread, TThreadMethod)
• [WayBack] TThread.Synchronize Method (TThread, TThreadProcedure)

Any exceptions raised in that methods are caught using [WayBack] System.AcquireExceptionObject and re-raised in the calling thread.

If that happens, you loose a piece of the stack information. I knew that, but found out the hard way that it does because I had to hunt for bugs through inherited code written by people that did not know.

This was part of the stack trace that code would show during an exception:

Exception EAccessViolation at $004D732F: Access violation at address $00409174 in module ''.....exe''.
Read of address 80808080
StackTrace:
(000D632F){.....exe} [004D732F] System.Classes.TThread.Synchronize$qqrp41System.Classes.TThread.TSynchronizeRecordo (Line 14975, "System.Classes.pas" + 40) + $0
(000D6430){.....exe} [004D7430] System.Classes.TThread.Synchronize$qqrxp22System.Classes.TThreadynpqqrv$v (Line 15007, "System.Classes.pas" + 9) + $A
(005D6E61){.....exe} [009D7E61] IdSync.DoThreadSync$qqrp18Idthread.TIdThreadynpqqrv$v (Line 281, "IdSync.pas" + 21) + $6
(005D6E87){.....exe} [009D7E87] IdSync.TIdSync.SynchronizeMethod$qqrynpqqrv$v (Line 326, "IdSync.pas" + 2) + $8

Exception EAccessViolation at $00409174: Access violation at address $00409174 in module ''.....exe''. Read of address $80808080 with StackTrace
(00008174){.....exe} [00409174] System.@IsClass$qqrxp14System.TObjectp17System.TMetaClass + $C


The first exception has a different address than the one in the exception message.

Which means that you miss the whole stack path to the _IsClass call (the underlying method implementing the as keyword) that the actual exception was initiated at.

And yes: the $80808080 is the FastMM4 marker for freed memory, so this was a use-after-free scenario.

A simple wrapper like this using a central logging facility gave much more insight in the actual cause:

procedure RunLoggedMethod(AMethod: TMethod);
begin
  try
    AMethod();
  except
    on E: Exception do
    begin
      Logger.LogExceptionDuringMethod(E, AMethod);
      raise; // mandatory to stay compatible with the old un-logged code
    end;
  end;
end;

Then call it like this inside a thread descendant:

Synchronize(RunLoggedMethod(MethodToRunInMainThread));

The old code was like this:

Synchronize(MethodToRunInMainThread);

This was quite easy to change, as I already had boiler code around exported DLL functions that had a similar construct (without the raise; as exceptions cannot pass DLL boundaries unless very specific circumstances hold).

Similar methods are needed to encapsulate procedure TIdSync.Synchronize(), procedure TIdSync.SynchronizeMethod, procedure TIdThread.Synchronize(Method: TThreadMethod) and [WayBack] Queue overloads:

• [WayBack] TThread.Queue Method (TThreadMethod)
• [WayBack] TThread.Queue Method (TThreadProcedure)
• [WayBack] TThread.Queue Method (TThread, TThreadMethod)
• [WayBack] TThread.Queue Method (TThread, TThreadProcedure)

–jeroen