Re: Always run GDB command post-hook after pre-hook has been run

[Stephen Cross <scross@undo-software.com>]

Hi Pedro,

> Implementation approach, or on the idea in the first place?

I'm mostly looking for input on the idea, because I know that the
proposed change affects the behaviour of post-hooks.

The only relevant point about the implementation was that I added:

+static void
+call_post_hook_cleanup(void* p)
+{
+    execute_cmd_post_hook (p);
+}

I added this in response to a comment in 'cleanups.h' that says
(emphasis on the last line):

/* NOTE: cagney/2000-03-04: This typedef is strictly for the
   make_cleanup function declarations below.  Do not use this typedef
   as a cast when passing functions into the make_cleanup() code.
   Instead either use a bounce function or add a wrapper function.
   Calling a f(char*) function with f(void*) is non-portable.  */

I had thought that calling f(char*) via f(void*) would be portable,
but I've added the wrapper function just in case. What do you think?

> I think it'd help if you told us the motivation.  What's the intent
> of running the hookpost even on error?  What are you trying to use the
> hooks for?

Our focus here is on commands that can perform inferior calls. We have
a replacement for gdbserver that by default doesn't support inferior
calls, since we can be part way through a debuggee's history (our core
product is a reversible debugger). So for inferior calls we have to
issue a command to tell our server to fork the current debuggee
process and then GDB can make arbitrary modifications to the fork
child process; once the inferior call is complete we then issue
another command to tell the server to drop the fork child process and
switch back to the parent process.

We're currently using the inferior call events added by my colleague
(Nick Bull) and these work for most cases. However we've found that if
GDB performs an inferior call which returns a pointer and then prints
that, GDB will access the memory *after* issuing the inferior call end
event. If the inferior call returns a buffer it allocated/modified
then this can cause us to print the old value of the buffer.

Hooks solve this problem because we can keep the fork child process
around long enough for GDB to read the correct value. Unfortunately
this means that if the 'print' command fails for any reason then we
won't have been notified to drop the child process, affecting the rest
of the debug session.

> Playing devil's advocate, isn't it reasonable to say that existing
> hookpost scripts out there may be assuming that they only run if
> the hooked command finished successfully?

The online docs say "Whenever you run the command ‘foo’, if the
user-defined command ‘hookpost-foo’ exists, it is executed (with no
arguments) after that command.". They don't seem to mention that
sometimes the post-hook might not be run. Having said that, users may
have observed this happening in practice.

> Wonder whether we should have that.  Alternatively, guess we could have
> a new hookerror hook, that would run on error instead of hookpost.

Yes, I think having a new 'hookerror' hook would be reasonable. This
would ensure existing users wouldn't be affected, but the naming might
cause confusion, so hookpost would need to be clearly documented as
only being run in the success case. I'm happy to augment the patch to
do this.

> What happens / should happen if the hookpost itself throws an error?  Do
> we lose the original hooked-command's error?  Is that OK?

I previously tested this case with the patch applied and it appears
that the error in the post-hook is what appears. So yes, we do lose
the original hooked command's error. It looks like this is because the
throw_exception() function inside GDB first calls 'do_cleanups
(all_cleanups ());' and do_cleanups() allows cleanup functions to
throw (and it updates the list of cleanups before running each
cleanup).

This behaviour seems OK to me, particularly if we added a new
'hookerror' and warned in the documentation that this occurs.
Presumably this should also have a testcase.

Thanks,
Stephen

On Thu, Dec 3, 2015 at 11:54 AM, Pedro Alves <palves@redhat.com> wrote:
> On 12/02/2015 06:24 PM, Stephen Cross wrote:
>> I was wondering if anyone has had a chance to look at this patch?
>> (I've also CC'ed the gdb list since I'm looking for input on the
>> approach.)
>
> Implementation approach, or on the idea in the first place?
>
> I think it'd help if you told us the motivation.  What's the intent
> of running the hookpost even on error?  What are you trying to use the
> hooks for?
>
>>> As you can see the post-hook is now always being called, even if the
>>> command fails.
>
> At first blush, it looks reasonable.  But as always, the devil's in the
> details.  I think only defining what happens around the corner cases
> can we be sure.
>
> Playing devil's advocate, isn't it reasonable to say that existing
> hookpost scripts out there may be assuming that they only run if
> the hooked command finished successfully?
>
> Curiously, the existing documentation actually has a related comment:
>
>> @cindex hooks, post-command
>> @kindex hookpost
>> A hook may also be defined which is run after the command you executed.
>> Whenever you run the command @samp{foo}, if the user-defined command
>> @samp{hookpost-foo} exists, it is executed (with no arguments) after
>> that command.  Post-execution hooks may exist simultaneously with
>> pre-execution hooks, for the same command.
>>
>> It is valid for a hook to call the command which it hooks.  If this
>> occurs, the hook is not re-executed, thereby avoiding infinite recursion.
>>
>> @c It would be nice if hookpost could be passed a parameter indicating
>> @c if the command it hooks executed properly or not.  FIXME!
>
> Wonder whether we should have that.  Alternatively, guess we could have
> a new hookerror hook, that would run on error instead of hookpost.
>
> What happens / should happen if the hookpost itself throws an error?  Do
> we lose the original hooked-command's error?  Is that OK?
>
> Thanks,
> Pedro Alves
>



-- 
Stephen Cross

Software Engineer at Undo Software