Re: [python][patch] Inferior and Thread information support.

[Pedro Alves <pedro at codesourcery dot com>]

On Monday 14 June 2010 13:42:41, Phil Muldoon wrote:
> On 06/10/2010 07:40 PM, Tom Tromey wrote:
> >>>>>> "Phil" == Phil Muldoon <pmuldoon@redhat.com> writes:
> > 
> > Phil> +  THPY_REQUIRE_VALID (thread_obj);
> > Phil> +
> > Phil> +  cleanup = make_cleanup_restore_current_thread ();
> > Phil> +
> > Phil> +  TRY_CATCH (except, RETURN_MASK_ALL)
> > Phil> +    {
> > Phil> +      switch_to_thread (thread_obj->thread->ptid);
> > Phil> +
> > Phil> +      frame = get_current_frame ();
> > Phil> +      frame_obj = frame_info_to_frame_object (frame);
> > Phil> +    }
> > Phil> +  GDB_PY_HANDLE_EXCEPTION (except);
> > 
> > I am really not sure about this.
> > 
> > Doesn't switch_to_thread reset the frame cache?
> > Meaning that the returned frame_obj will immediately be invalid?
> > 
> > You would have to try this with a multi-threaded program, where you are
> > stopped in thread A but then request a frame in thread B.
> 
> I'll investigate this further. I had concerns about this (I think we
> chatted a little about this on irc way back when).  I'll write an
> inferior test and see.  Maybe Pedro or someone else in that area knows
> a little more.

Tom is correct.  You'll not see a problem with single-thread apps
because switch_to_thread does an early return before invalidating
the frame cache when the thread you're switching to is already current.

In short, don't ever store a frame_info pointer like this.  Instead,
if you need to record the frame somewhere, store a frame_id.
(I haven't really looked at the patch and the use case, just
commenting from the what's quoted above)

-- 
Pedro Alves