Re: [RFC] Python Finish Breakpoints

[Kevin Pouget <kevin.pouget@gmail.com>]

On Wed, Nov 9, 2011 at 3:09 PM, Kevin Pouget <kevin.pouget@gmail.com> wrote:
>
> On Fri, Nov 4, 2011 at 10:03 PM, Tom Tromey <tromey@redhat.com> wrote:
> >
> > >>>>> "Kevin" == Kevin Pouget <kevin.pouget@gmail.com> writes:
> >
> > Tom> It seems like it should be an error to try to compute the return value
> > Tom> when not stopped at this breakpoint.
> >
> > Kevin> I'm not totally convinced ...
> > Kevin> what would you think about throwing an AttributeError("return_value
> > Kevin> not available yet") when accessing the attribute before the breakpoint
> > Kevin> is hit, but keep the cached value afterwards?
> >
> > What I meant was that accessing the cached value any time is fine --
> > just that attempting to compute the value for the first time at any
> > point other than the breakpoint location was wrong, just because
> > whatever we compute then will be unrelated to what the user might want.
> >
> > It is hard to be sure that the current code handles this properly.
> > See below.
> ...
> > Kevin> +static PyObject *
> > Kevin> +bpfinishpy_get_returnvalue (PyObject *self, void *closure)
> > Kevin> +{
> > [...]
> > Kevin> +      for (bs = inferior_thread ()->control.stop_bpstat;
> > Kevin> +          bs; bs = bs->next)
> > Kevin> +        {
> > Kevin> +          struct breakpoint *bp = bs->breakpoint_at;
> > Kevin> +
> > Kevin> +          if (bp != NULL && (PyObject *) bp->py_bp_object == self)
> > Kevin> +            {
> > Kevin> +              bpfinish_stopped_at_finish_bp (self_finishbp);
> > Kevin> +              if (PyErr_Occurred ())
> > Kevin> +                return NULL;
> >
> > This seems to try to do the right thing -- but is
> > inferior_thread()->control even valid at all points that can reach this?
> >
> > What about just computing the value before calling the 'stop' method?
> > As long as it computes a lazy value this won't be very expensive.
>
> This part is a bit tricky, and your suggestion has highlighted a
> problem I didn't consider/test:
>
> py-breakpoint.c::gdbpy_should_stop is executed _before_ we have the
> chance to save the stop_registers, used to compute the return value
> (in infrun.c::normal_stop).
> (the problem existed since the beginning, but I never faced it before)
>
> I've updated the function infcmd.c::get_return_value to take this
> situation into account, using the current registers if the
> 'stop_registers' are not set, based on what is done in
> infrun.c::normal_stop:
>
> > struct value *
> > get_return_value (struct type *func_type, struct type *value_type)
> > ...
> >   /* If stop_registers where not saved, use the current registers.  */
> >   if (!stop_regs)
> >     {
> >       stop_regs = regcache_dup (get_current_regcache ());
> >       cleanup = make_cleanup_regcache_xfree (stop_regs);
> >     }
>
> but I can't say that I'm confident with regcache handling, and I don't
> know if these lines would have unexpected side effects ...
>
>
> The patch enclosed passes the testsuite with no regression on x86_64/fedora 15
>
> > Kevin> +  TRY_CATCH (except, RETURN_MASK_ALL)
> > Kevin> +    {
> > Kevin> +      struct value *ret =
> > Kevin> +          get_return_value (type_object_to_type (py_bp->function_type),
> > Kevin> +                            type_object_to_type (py_bp->return_type));
> > Kevin> +
> > Kevin> +      if (ret)
> > Kevin> +        py_bp->return_value = value_to_value_object (ret);
> > Kevin> +      else
> > Kevin> +        py_bp->return_value = Py_None;
> > Kevin> +    }
> > Kevin> +  if (except.reason < 0)
> > Kevin> +    gdbpy_convert_exception(except);
> >
> > Missing a space.
> >
> > I think you need to Py_INCREF Py_None here.
> >
> >
> > Kevin> +  if (except.reason < 0)
> > Kevin> +    {
> > Kevin> +      gdbpy_convert_exception(except);
> >
> > Missing space.
> >
> > Kevin> +  if (except.reason < 0
> > Kevin> +      || !self_bpfinish->return_type || !self_bpfinish->function_type)
> > Kevin> +    {
> > Kevin> +      /* Won't be able to compute return value.  */
> > Kevin> +      self_bpfinish->return_type = NULL;
> > Kevin> +      self_bpfinish->function_type = NULL;
> >
> > Need decrefs here.
>
> all fixed, thanks
>
> > Kevin> +                  self_bpfinish->return_type = type_to_type_object (ret_type);
> > Kevin> +                  self_bpfinish->function_type =
> > Kevin> +                      type_to_type_object (SYMBOL_TYPE (function));
> >
> > These can fail with NULL and must be handled, probably by returning.
> > But you can't return out of a TRY_CATCH.
>
> I think that I handle that in the following lines:
>
> > Kevin> +  if (except.reason < 0
> > Kevin> +      || !self_bpfinish->return_type || !self_bpfinish->function_type)
>
> I think I wrote a word about that in the previous mail, anyway, my
> feeling was that I don't want to abort the FinishBreakpoint creation
> just because of a return value not computable, so I currently nullify
> these fields and silently disable return value computation. I can't
> see any straightforward way to notify Python about that,
> warning/exceptions won't suite ... otherwise, I could expose the
> return_type to the Python interface, this way, one could check that
> it's not None and now if GDB will/might be able to compute the return
> value when the FinishBP is hit
>
> > Kevin> +  BPPY_SET_REQUIRE_VALID (&self_bpfinish->py_bp);
> >
> > Hm, why is this here?
>
> no reason apparently, the try/catch above shall ensure that the BP is valid
>
> > Kevin> +static void
> > Kevin> +bpfinishpy_out_of_scope (struct finish_breakpoint_object *bpfinish_obj)
> > Kevin> +{
> > [...]
> > Kevin> +  delete_breakpoint (bpfinish_obj->py_bp.bp);
> >
> > I think it needs a TRY_CATCH.
> >
> > Kevin> +      else if (b->pspace == current_inferior ()->pspace
> > Kevin> +           && (!target_has_registers
> > Kevin> +               || frame_find_by_id (b->frame_id) == NULL))
> > Kevin> +        {
> > Kevin> +          bpfinishpy_out_of_scope (finish_bp);
> > Kevin> +        }
> > Kevin> +    }
> >
> > This too, I think.
>
> right, done
>
> > Kevin> +static void
> > Kevin> +bpfinishpy_handle_stop (struct bpstats *bs, int print_frame)
> > Kevin> +{
> > Kevin> +  struct cleanup *cleanup = ensure_python_env (get_current_arch (),
> > Kevin> +                                               current_language);
> > Kevin> +
> > Kevin> +  iterate_over_breakpoints (bpfinishpy_detect_out_scope_cb,
> > Kevin> +                            bs == NULL ? NULL : bs->breakpoint_at);
> >
> > bpfinishpy_detect_out_scope_cb still acquires the GIL (via ensure_python_env),
> > but should not.
>
> I'm not exactly sure what was you concern here, as far as I
> understand, the GIL was acquired in bpfinishpy_handle_stop, so it
> should have no effect in detect_out_scope_cb. So I've removed it from
> detect_out_scope_cb as it was useless.
>
> I've also made a litlle modification in the FinishBreakpoint __init__,
> which now default it's frame argument to gdb.current_frame(), instead
> of making it mandatory. I've updated the documentation and testsuite
> accordingly.
>
>
> Thanks for the time you spend reviewing my patches,
>
> Kevin
>
> --
>
> 2011-11-09  Kevin Pouget  <kevin.pouget@st.com>
>
>        Introduce gdb.FinishBreakpoint in Python
>
>        * Makefile.in (SUBDIR_PYTHON_OBS): Add py-finishbreakpoint.o.
>        (SUBDIR_PYTHON_SRCS): Add python/py-finishbreakpoint.c.
>        Add build rule for this file.
>        * infcmd.c (print_return_value): Split to create get_return_value.
>        (get_return_value): New function based on print_return_value. Handle
>        case where stop_registers are not set.
>        * inferior.h (get_return_value): New prototype.
>        * python/py-breakpoint.c (bppy_pending_object): Make non-static.
>        (gdbpy_breakpoint_created): Set is_py_finish_bp is necessary.
>        (struct breakpoint_object): Move to python-internal.h
>        (BPPY_REQUIRE_VALID): Likewise.
>        (BPPY_SET_REQUIRE_VALID): Likewise.
>        (gdbpy_breakpoint_created): Initialize is_finish_bp.
>        (gdbpy_should_stop): Add  pre/post hooks before/after calling stop
>        method.
>        * python/python-internal.h (breakpoint_object_type): Add as extern.
>        (bppy_pending_object): Likewise.
>        (typedef struct breakpoint_object) Removed.
>        (struct breakpoint_object): Moved from py-breakpoint.c.
>        Add field is_finish_bp.
>        (BPPY_REQUIRE_VALID): Moved from py-breakpoint.c.
>        (BPPY_SET_REQUIRE_VALID): Likewise.
>        (frame_object_to_frame_info): New prototype.
>        (gdbpy_initialize_finishbreakpoints): New prototype.
>        (bpfinishpy_is_finish_bp): Likewise.
>        (bpfinishpy_pre_stop_hook): Likewise.
>        (bpfinishpy_post_stop_hook): Likewise.
>        * python/py-finishbreakpoint.c: New file.
>        * python/py-frame.c(frame_object_to_frame_info): Make non-static and
>        accept PyObject instead of frame_object.
>        (frapy_is_valid): Don't cast to frame_object.
>        (frapy_name): Likewise.
>        (frapy_type): Likewise.
>        (frapy_unwind_stop_reason): Likewise.
>        (frapy_pc): Likewise.
>        (frapy_block): Likewise.
>        (frapy_function): Likewise.
>        (frapy_older): Likewise.
>        (frapy_newer): Likewise.
>        (frapy_find_sal): Likewise.
>        (frapy_read_var): Likewise.
>        (frapy_select): Likewise.
>        * python/python.c (gdbpy_is_stopped_at_finish_bp): New noop function.
>        (_initialize_python): Add gdbpy_initialize_finishbreakpoints.
>        * python/python.h: Include breakpoint.h
>        (gdbpy_is_stopped_at_finish_bp): New prototype.
>
> doc/
>        * gdb.texinfo (Breakpoints In Python): New subsection: Finish
>        Breakpoints.
>        (Python API): Add menu entry for Finish Breakpoints
>
> testsuite/
>        * gdb.python/py-breakpoint.exp (mult_line): Define and use variable
>        instead of line number.
>        * gdb.python/py-finish-breakpoint.c: New file.
>        * gdb.python/py-finish-breakpoint.exp: New file.
>        * gdb.python/py-finish-breakpoint.py: New file.
>        * gdb.python/py-finish-breakpoint2.cc: New file.
>        * gdb.python/py-finish-breakpoint2.exp: New file.
>        * gdb.python/py-finish-breakpoint2.py: New file.

ping after one week, if you have a bit of time


Thanks,

Kevin