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Re: [PATCH] tui: replace deprecated_register_changed_hook with observer


On 07/08/2015 02:37 PM, Patrick Palka wrote:
> On Wed, Jul 8, 2015 at 8:48 AM, Pedro Alves <palves@redhat.com> wrote:
>> On 07/08/2015 01:30 PM, Patrick Palka wrote:
>>> On Wed, Jul 8, 2015 at 7:41 AM, Pedro Alves <palves@redhat.com> wrote:
>>>> On 07/06/2015 02:17 AM, Patrick Palka wrote:
>>>>> This is a straightforward replacement of the TUI's use of the
>>>>> aforementioned hook with the register_changed observer.  Since this was
>>>>> the only user of the hook, this patch also removes the hook.
>>>>>
>>>>> [ I am not sure if the changes to the function tui_register_changed are
>>>>>   correct.  In particular, the inputted frame argument is now passed down
>>>>>   to tui_check_data_values instead of the frame returned by
>>>>>   get_selected_frame.  The frame argument passed to each register_changed
>>>>>   observer corresponds to the VALUE_FRAME_ID of the register being
>>>>>   modified within a register assignment, e.g. the $rax in "print $rax =
>>>>>   FOO".  When would the frame corresponding to the VALUE_FRAME_ID of a
>>>>>   register not be the currently selected frame?  ]
>>>>>
>>>>
>>>> Grepping for value_assign callers finds e.g., varobjs:
>>>>
>>>>   varobj.c:      val = value_assign (var->value, value);
>>>>
>>>> Adding an assertion like this:
>>>>
>>>> @@ -1169,6 +1169,7 @@ value_assign (struct value *toval, struct value *fromval)
>>>>               }
>>>>           }
>>>>
>>>> +       gdb_assert (frame == get_selected_frame (NULL));
>>>>         observer_notify_register_changed (frame, value_reg);
>>>>         if (deprecated_register_changed_hook)
>>>>           deprecated_register_changed_hook (-1);
>>>>
>>>> and playing with varobjs shows the assertion failing:
>>>>
>>>>  (gdb) interpreter-exec mi "-var-create - * $rax"
>>>>  ^done,name="var1",numchild="0",value="6295640",type="int64_t",has_more="0"
>>>>  (gdb) up
>>>>  #1  0x000000000040082a in thread_function0 (arg=0x0) at threads.c:69
>>>>  69              usleep (1);  /* Loop increment.  */
>>>>  (gdb) up
>>>>  #2  0x0000003616a07ee5 in start_thread (arg=0x7ffff7fc1700) at pthread_create.c:309
>>>>  309           THREAD_SETMEM (pd, result, CALL_THREAD_FCT (pd));
>>>>  (gdb) interpreter-exec mi "-var-assign var1 1"
>>>>  ~"/home/pedro/gdb/mygit/build/../src/gdb/valops.c:1172: internal-error: value_assign: Assertion `frame == get_selected_frame (NULL)' failed.\nA problem internal to GDB has been detected,\nfurther debugging may prove unreliable.\nQuit this debugging session? (y or n) "
>>>>
>>>> The TUI doesn't use MI, but there are probably other similar cases
>>>> in the tree.  E.g., I'd assume you can create a register Value with Python,
>>>> and then assign to it when the selected frame is not
>>>> the register's frame.
>>>
>>> Ah okay.. So it seems to me that if the frame argument !=
>>> get_selected_frame, then we should not update the register window at
>>> all since the register window is supposed to show the register values
>>> of the currently selected frame.
>>
>> Yes, I think so.
>>
>>> Or instead, just ignore the frame argument and always pass
>>> get_selected_frame to tui_check_data_values, even if frame !=
>>> get_selected_frame.  Seems to me that this is the safest option.
>>
>> That'd be a 1-1 with the current code.  Though, I believe
>> that results in spuriously clearing the highlight of
>> previously changed registers (of the selected frame), because
>> nothing will have changed.  So seems like the other option
>> actually fixes a bug.
> 
> Is it actually the case that a register change made on one frame can
> not show up on some other frame?

Oh, you're right, good point.   Registers can well be at the
same physical location across frames.

> 
> If I debug gdb with gdb, doing "start" followed by "step" a couple
> dozen times, do "layout regs", then select the outermost frame and do
> "print $rbx = 50", the regs window shows that $rbx has not changed on
> the (selected) outermost frame but if i select the innermost frame,
> $rbx has changed to 50.  And the frame_id of the register $rbx was
> indeed the (selected) outermost frame, yet the registers of the
> selected frame did not change after the value assignment and the
> registers of some other frame did.  I don't know why this particular
> example behaves this way, but it seems to illustrates that it's
> possible that a register change made in one frame can affect the
> register values of another frame.  So I don't know if the "frame !=
> get_selected_frame ()" check is 100% correct.
> 

Yeah.  Sorry, somehow forgot this...

Thanks,
Pedro Alves


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