Re: Patch to propagate GDB's knowledge of the executing state to frontend

[Pedro Alves <palves@redhat.com>]

On 11/02/2012 06:46 PM, ali_anwar wrote:
> On 11/02/2012 09:15 PM, dje@google.com wrote:
>> Yao Qi writes:
>>   >  On 10/25/2012 07:09 PM, ali_anwar wrote:
>>   >  >  [...]
>>   >  >  @@ -1,3 +1,13 @@
>>   >  >  +2012-10-25  Ali Anwar<ali_anwar@codesourcery.com>
>>   >  >  +
>>   >  >  +    * infrun.c (handle_inferior_event_stub, regcache_dup_stub):
>>   >  >  +    New functions.
>>   >  >  +    (normal_stop): Change to propagate GDB's knowledge of the
>>   >  >  +    executing state to frontend when not able to fetch registers.
>>   >  >  +    (wait_for_inferior): Chnage to propagate GDB's knowledge of
>>   >                                ^^^^^^ typo
>>   >
>>   >
>>   >  >  +    the executing state if not able to fetch backtrace once the
>>   >  >  +    step has already occured.
>>   >                            ^^^^^^^ typo.
>>   >
>>   >  In each changelog entry, we'll put 'what do we change' instead of 'why
>>   >  do we change in this way'.  So this entry can be simplified.
>>
>> Hi.
>>
>> I agree with your first sentence, and would add that if such an
>> explanation is needed, it belongs in the code not the changelog.
>> [We don't have enough comments in the code explaining *why* things
>> are the way they are.]
>>
>> But I'd say that's not the case here, at least for the changelog entries.
>> Instead, I would remove the leading "Change to", and just say "Propagate ...".
>>
>> Also, I would add a comment to the code explaining *why* the calls are wrapped
>> in catch_error (and I would have the comment live at the call to catch_error,
>> not in the definition of the two new stubs).
>>
>> One could also say the two new functions also require comments,
>> but they're pretty simple and hook_stop_stub doesn't have a comment,
>> so I'd be ok with leaving them out.
> 
> Thanks for the review. Please find attached the modified patch.
> 
> -Ali
> 
> target_state.patch
> 
> 
> Index: gdb/ChangeLog
> ===================================================================
> RCS file: /cvs/src/src/gdb/ChangeLog,v
> retrieving revision 1.14760
> diff -u -r1.14760 ChangeLog
> --- gdb/ChangeLog	24 Oct 2012 19:08:15 -0000	1.14760
> +++ gdb/ChangeLog	2 Nov 2012 18:41:48 -0000
> @@ -1,3 +1,11 @@
> +2012-10-25  Ali Anwar  <ali_anwar@codesourcery.com>
> +
> +	* infrun.c (handle_inferior_event_stub, regcache_dup_stub):
> +	New functions.
> +	(normal_stop): Propagate GDB's knowledge of the executing
> +	state to frontend.
> +	(wait_for_inferior): Likewise.
> +
>  2012-10-24  Tristan Gingold  <gingold@adacore.com>
>  
>  	* ravenscar-sparc-thread.c (ravenscar_sparc_fetch_registers):
> Index: gdb/infrun.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/infrun.c,v
> retrieving revision 1.559
> diff -u -r1.559 infrun.c
> --- gdb/infrun.c	17 Sep 2012 07:26:55 -0000	1.559
> +++ gdb/infrun.c	2 Nov 2012 18:41:49 -0000
> @@ -73,6 +73,10 @@
>  
>  static int hook_stop_stub (void *);
>  
> +static int regcache_dup_stub (void *);
> +
> +static int handle_inferior_event_stub (void *);
> +
>  static int restore_selected_frame (void *);
>  
>  static int follow_fork (void);
> @@ -2700,8 +2704,11 @@
>  	 state.  */
>        old_chain = make_cleanup (finish_thread_state_cleanup, &minus_one_ptid);
>  
> -      /* Now figure out what to do with the result of the result.  */
> -      handle_inferior_event (ecs);
> +      /* Now figure out what to do with the result of the result. If an
> +         error happens while handling the event, catch it to propagate
> +         GDB's knowledge of the executing state.  */
> +      catch_errors (handle_inferior_event_stub, ecs,
> +                    "Error while handling inferior event:\n", RETURN_MASK_ALL);
>  
>        /* No error, don't finish the state yet.  */
>        discard_cleanups (old_chain);

I don't understand this.  The point of the finish_thread_state_cleanup
is doing exactly what you say is missing.  If you swallow errors, then the
cleanup doesn't run at all (it's discarded immediately afterwards).

> @@ -6080,9 +6087,12 @@
>        if (stop_registers)
>  	regcache_xfree (stop_registers);
>  
> -      /* NB: The copy goes through to the target picking up the value of
> -	 all the registers.  */
> -      stop_registers = regcache_dup (get_current_regcache ());
> +      /* NB: The copy goes through to the target picking up the value
> +         of all the registers. Catch error to propagate GDB's knowledge
> +         of the executing state to frontend even when not able to fetch
> +         registers.  */
> +      catch_errors (regcache_dup_stub, NULL,
> +                    "Error while running regcache_dup:\n", RETURN_MASK_ALL);

normal_stop has the finish_thread_state_cleanup installed too at the
top, and it has been run already above:

  /* Let the user/frontend see the threads as stopped.  */
  do_cleanups (old_chain);

So I'm afraid I don't understand exactly what this is fixing, and _how_
this is fixing it.  On an error, frontends should just no longer trust
any of their state, and issue a full refresh.

> +      catch_errors (regcache_dup_stub, NULL,
> +                    "Error while running regcache_dup:\n", RETURN_MASK_ALL);

So this also not okay for the related reason that it would swallow all
kinds of errors (even ctrl-c), so that the error is no longer propagated
to the user/frontend.

-- 
Pedro Alves