Enhancement - show old and new thread info when switching during debugging

[pfee@talk21.com]

> > So, I would  think that adding something similar in

> > thread.c:switch_to_thread would  do the job.
> 
> It wouldn't.  What he wants is the last user selected  thread,
> not the thread that happens to be current when switch_to_thread
> is  called.
> 
> - user resumes with thread 1 selected
> - thread 2 hits  breakpoint, gdb switches to thread 2
>   - breakpoint doesn't cause stop,  target is re-resumed
> - thread 3 hits breakpoint, gdb switches to thread  3
>   - breakpoint causes stop, previous_inferior_ptid is
>      thread 1, but the last switch_to_thread would have
>     recorded  $previous_thread as thread 2
> 
> If you want a convenience variable, please  model it on
> $_thread --- see end of thread.c:_initialize_thread.
> 
> Note  that the previously selected thread may not exist anymore,
> it may have exited  meanwhile, for example, or the inferior
> exec'ed.
> 
> -- 
> Pedro  Alves
>

Hi Pedro,

I agree with your example, the previous_inferior_ptid in infrun.c, normal_stop() 
has the information I need.  I'll try and use that to populate the new 
convenience variable.

I'll a try to construct a test program that creates a scenario where the 
previous thread as died by the time normal_stop() executes.

Thanks,
Paul