Question

Given this example, how do we ensure that the thread instance has been freed? What happens to the thread instance when Thread.FreeOnTerminate=true and you blow up in your Thread.OnTerminate event? Is the thread orphaned unless you handle the exception and free the thread explicitey in OnTerminate?

// Thread constructor

constructor TMyThread.Create(CreateSuspended: Boolean);
begin
  inherited Create(CreateSuspended);
  Self.FreeOnTerminate := True;
end;

// TMyThread OnTerminate event

procedure TMyThread.OnTerminate(Sender: TObject);
var o: TObject;     
begin
  o:=nil;
  showmessage(o.classname); // guaranteed AV
end;

Answer 1

You should handle all exceptions in your OnTerminate handler because an unhandled exception will cause that the instance of the thread will not be freed (see Classes.ThreadProc implementation).
Just enclose the body of your handler in try..except and handle all exceptions.

But your example for ‘guaranteed AV’ is wrong: Free does not cause an AV if the instance is nil.