Hang due to incorrect exception handling

[NoAnyName]

Hi!

Our team has stumbled upon tricky issue in the OTL, resulting in permanent service hang.

Unfortunately, I do not have minimal example for reproducing the issue (though I can construct one if needed), but I'll try to describe problem in detailed enough manner.

The code enough to reproduce problem is:

Loop := Parallel.ForEach...NoWait;
Loop.Execute;

and add raise.Exception.Create in the DsiAllocateHwnd;

.Execute, after going through several layers, occurs in the TOmniParallelLoopBase.InternalExecuteTask, that initializes loop.
One of the steps is calling task.Unobserved, just before starting the loop process.
Unobserved results in creation of monitor, which needs a handle, that is allocated in DsiAllocateHwnd.

The trick is that DsiAllocateHwnd fails to create window for some reason (didn't figure out why yet, probably, handles/atom leak, will figure out later). As a result, it raises an exception. Hence, it is an exception in the constructor; this results in destroying of the object. Destructor tries to access emMonitoredTasks that hasn't yet been initialized, that results in AV while handling an exception, which in its turn complicates debugging.

constructor TOmniEventMonitor.Create(AOwner: TComponent);
begin
  inherited;
  {$IFDEF MSWINDOWS}
  emMessageWindow := DSiAllocateHWnd(WndProc);
  Win32Check(emMessageWindow <> 0);
  {$ENDIF}
  emMonitoredTasks := CreateInterfaceDictionary;
  emMonitoredPools := CreateInterfaceDictionary;
  emThreadID := {$IFDEF MSWINDOWS}GetCurrentThreadID{$ELSE}TThread.CurrentThread.ThreadID{$ENDIF};
end; { TOmniEventMonitor.Create }

destructor TOmniEventMonitor.Destroy;
var
  intfKV   : TOmniInterfaceDictionaryPair;
  {$IFDEF MSWINDOWS}
  winHandle: THandle;
  {$ENDIF}
begin
  for intfKV in emMonitoredTasks do
    (intfKV.Value as IOmniTaskControl).RemoveMonitor;

After all, exception gets propagated to the upper level, where the loop has been created; as a result, the interface gets released by hidden finally section, calling destructor:

destructor TOmniParallelLoopBase.Destroy;
begin
  if assigned(FCountStopped) then
    {$IFDEF MSWINDOWS}
    WaitForSingleObject(FCountStopped.Handle, INFINITE);

And here it sleeps forever, since FCountStopped never gets down to 0, since loop hasn't been started. Also, exception that has been raised, never gets to a handler/logger.

For myself, I'm going to add ad-hoc logging to DsiAllocateHwnd to figure out when and why it happens and wrap .Execute in the additional try..except block for logging, though it'd be great to fix initial issue with destructor been stuck in the infinite wait - if cycle hasn't been started, there's no need to wait for it to finish when destroying. Also, exception in the monitor destructor is annoying because it is masking original one. Probably, it'd be enough to check if monitor was properly initialized (maybe emMonitoredTasks been assigned at all) before trying to access its fields.