[1/2] gdb: Restore a settings previous value on error during change

  * Luis Machado <lgustavo@codesourcery.com> [2016-11-11 18:20:24 -0600]:

> On 11/11/2016 12:18 PM, Andrew Burgess wrote:
> > When changing the value of a setting in do_set_command, there are three
> > phases:
> > 
> >  1. The old value is destroyed, and the new value is placed into the
> >  settings variable.
> > 
> >  2. The set-hook, the 'func' field of the setting is called.
> > 
> >  3. A change notification is issued.
> > 
> > There are two problems that I try to address in this commit.
> > 
> >  1. If the set-hook does not like the new value then either the setting
> >  is restored to a default value, or we have to have a complex system
> >  within the set-hook to remember the previous value and restore it
> >  manually when an invalid change is made.
> > 
> >  2. If the set-hook throws an error then the change notification is
> >  skipped, even though the setting might still have changed (say, back to
> >  some default value).
> > 
> > The solution I propose here is to delay destroying the old value of the
> > setting until after the set-hook has been called.  If the set-hook runs
> > successfully then the old value will be destroyed when do_set_command
> > returns, the new value will be left in place, and the change
> > notification will be sent sent out exactly as before.
> > 
> > However, if the set-hook throws an error this is now caught in
> > do_set_command, the new value of the setting is removed, and the old
> > value is restored.
> > 
> > After this change, it is no longer possible in a set-hook to change a
> > setting to a default value _and_ throw an error.  If you throw an error
> > you should assume that gdb will restore the previous value of the
> > setting.  If you want to change a setting in the set-hook and inform the
> > user, then a warning, or just a standard message should be fine.
> > 
> > gdb/ChangeLog:
> > 
> > 	* cli/cli-setshow.c: Add exceptions.h include.
> > 	(do_set_comment): When installing a settings new value, don't
> > 	loose the previous value until the set-hook has been called.  If
> > 	the set-hook throws an error, restore the settings previous value.
> > 
> > gdb/testsuite/ChangeLog:
> > 
> > 	* gdb.base/dcache-line-set-error.exp: New file.
> > ---
> >  gdb/ChangeLog                                    |  7 +++
> >  gdb/cli/cli-setshow.c                            | 80 ++++++++++++++++++++++--
> >  gdb/testsuite/ChangeLog                          |  4 ++
> >  gdb/testsuite/gdb.base/dcache-line-set-error.exp | 67 ++++++++++++++++++++
> >  4 files changed, 153 insertions(+), 5 deletions(-)
> >  create mode 100644 gdb/testsuite/gdb.base/dcache-line-set-error.exp
> > 
> > diff --git a/gdb/ChangeLog b/gdb/ChangeLog
> > index 7fc2b4a..3cb115f 100644
> > --- a/gdb/ChangeLog
> > +++ b/gdb/ChangeLog
> > @@ -1,3 +1,10 @@
> > +2016-11-11  Andrew Burgess  <andrew.burgess@embecosm.com>
> > +
> > +	* cli/cli-setshow.c: Add exceptions.h include.
> > +	(do_set_comment): When installing a settings new value, don't
> > +	loose the previous value until the set-hook has been called.  If
> > +	the set-hook throws an error, restore the settings previous value.
> > +
> 
> "a setting's new ..."
> "don't lose ..."
> "restore the setting's..."
> 
> > @@ -151,8 +152,20 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
> >  {
> >    /* A flag to indicate the option is changed or not.  */
> >    int option_changed = 0;
> > +  /* Cleanups created to free the previous value of the setting.  */
> > +  struct cleanup *cleanups;
> > +  /* The previous value of the setting.  Restored if the set-hook function
> > +     throws an error.  */
> > +  union
> > +  {
> > +    enum auto_boolean b;
> > +    int i;
> > +    const char *s;
> > +    unsigned int u;
> > +  } prev_value;
> 
> I wonder if we can share and potentially simplify the storage mechanism for
> these settings with what struct cmd_list_element already has? As opposed to
> declaring something new just for this particular function?

I'm not sure how we'd do that.  The 'struct cmd_list_element' holds a
'void *' pointer to a variable of _some type_ the interpretation of
that 'void *' depends on the 'var_type' field within the 'struct
cmd_list_element'.  But we never actually hold the _value_ of the
setting in the 'struct cmd_list_element'.

So I think that we will always end up introducing _some_ new structure
to hold the previous value.  I'm open to suggestions though, just
because I can't see the solution doesn't mean it's not there...

> Since we're already touching this function, it may be worthwhile to get it
> cleaned up a bit if that's not too much effort.

I can do some clean up if you'd like, I can even make it part of this
patch series, though obviously it'll be a new patch in this series.
But you'll need to give some more specific indication of what you'd
like to see changed....

> 
> I also see a lot of repetition trying to save the old value. If we could get
> it saved at the function's entry, that would save many lines of code, but i
> think this is a nice-to-have and not required for this patch.

I'm not sure I'd agree with "many", but in the new version I've tried
to reduce the number of new lines added.  As I discuss above, given
that the 'struct cmd_list_element' only has a 'void *' pointer,
backing up the previous value has to be done on a case by case basis.
Again, though, if you have a specific idea in mind that I'm not
seeing, feel free to make a suggestion, I'm happy to change things.

> 
> > 
> >    gdb_assert (c->type == set_cmd);
> > +  cleanups = make_cleanup (null_cleanup, NULL);
> > 
> >    switch (c->var_type)
> >      {
> > @@ -200,7 +213,8 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
> >  	if (*(char **) c->var == NULL
> >  	    || strcmp (*(char **) c->var, newobj) != 0)
> >  	  {
> > -	    xfree (*(char **) c->var);
> > +	    prev_value.s = *(const char **) c->var;
> > +	    make_cleanup (xfree, *(char **) c->var);
> >  	    *(char **) c->var = newobj;
> 
> For example, this type of procedure of saving the old value repeats itself
> over and...
> > 
> >  	    option_changed = 1;
> > @@ -215,7 +229,8 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
> > 
> >        if (*(char **) c->var == NULL || strcmp (*(char **) c->var, arg) != 0)
> >  	{
> > -	  xfree (*(char **) c->var);
> > +	  prev_value.s = *(const char **) c->var;
> > +	  make_cleanup (xfree, *(char **) c->var);
> >  	  *(char **) c->var = xstrdup (arg);
> > 
> 
> ... over and ...
> >  	  option_changed = 1;
> > @@ -248,7 +263,8 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
> >  	if (*(char **) c->var == NULL
> >  	    || strcmp (*(char **) c->var, val) != 0)
> >  	  {
> > -	    xfree (*(char **) c->var);
> > +	    prev_value.s = *(const char **) c->var;
> > +	    make_cleanup (xfree, *(char **) c->var);
> >  	    *(char **) c->var = val;
> > 
> 
> ... over again.
> 
> > @@ -452,7 +474,51 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
> >      default:
> >        error (_("gdb internal error: bad var_type in do_setshow_command"));
> >      }
> > -  c->func (c, NULL, from_tty);
> > +
> > +  TRY
> > +    {
> > +      c->func (c, NULL, from_tty);
> > +    }
> > +  CATCH (ex, RETURN_MASK_ERROR)
> > +    {
> > +      /* The only cleanups in place free the previous value, which we're
> > +	 about to restore.  */
> 
> Maybe change this comment a bit to be more to the point?
> 
> "Discard the cleanups since we failed to set the new value."
> 
> > +      discard_cleanups (cleanups);
> > +      /* Now restore the previous value, and free the new value.  */
> > +      if (option_changed)
> > +	switch (c->var_type)
> > +	  {
> > +	  case var_string:
> > +	  case var_string_noescape:
> > +	  case var_filename:
> > +	  case var_optional_filename:
> > +	    xfree (*(char **) c->var);
> > +	    *(char **) c->var = (char *) prev_value.s;
> > +	    break;
> > +
> > +	  case var_boolean:
> > +	  case var_integer:
> > +	  case var_zinteger:
> > +	  case var_zuinteger_unlimited:
> > +	    *(int *) c->var = prev_value.i;
> > +	    break;
> > +
> > +	  case var_auto_boolean:
> > +	    *(enum auto_boolean *) c->var = prev_value.b;
> > +	    break;
> > +
> > +	  case var_uinteger:
> > +	  case var_zuinteger:
> > +	    *(unsigned int *) c->var = prev_value.u;
> > +	    break;
> > +
> > +	  case var_enum:
> > +	    *(char **) c->var = (char *) prev_value.s;
> > +	    break;
> > +	  }
> > +      throw_exception (ex);
> > +    }
> > +  END_CATCH
> 
> Can we make a cleanup that just restores some old value regardless of its
> type so we can call it once without having to check what kind of variable
> we're dealing with (since we would have checked that before getting here
> already and before putting such cleanup in place?).
> 
> It sounds like we could use a couple cleanups? One to free the old value due
> to having a new valid value in place and another to restore the old value in
> case the new value is invalid?
> 
> Not sure how much work that adds, but sounds like it would make this code a
> bit more clean?

I've folded the whole restore or clean-up the previous value into a
single cleanup function.  I'm not 100% convinced that the new code is
better than the previous, but I don't think it's any worse.

Let me know what you think of the new version.

Thanks,
Andrew

---

gdb: Restore a settings previous value on error during change

When changing the value of a setting in do_set_command, there are three
phases:

 1. The old value is destroyed, and the new value is placed into the
 settings variable.

 2. The set-hook, the 'func' field of the setting is called.

 3. A change notification is issued.

There are two problems that I try to address in this commit.

 1. If the set-hook does not like the new value then either the setting
 is restored to a default value, or we have to have a complex system
 within the set-hook to remember the previous value and restore it
 manually when an invalid change is made.

 2. If the set-hook throws an error then the change notification is
 skipped, even though the setting might still have changed (say, back to
 some default value).

The solution I propose here is to delay destroying the old value of the
setting until after the set-hook has been called.  If the set-hook runs
successfully then the old value will be destroyed when do_set_command
returns, the new value will be left in place, and the change
notification will be sent sent out exactly as before.

However, if the set-hook throws an error this is now caught in
do_set_command, the new value of the setting is removed, and the old
value is restored.

After this change, it is no longer possible in a set-hook to change a
setting to a default value _and_ throw an error.  If you throw an error
you should assume that gdb will restore the previous value of the
setting.  If you want to change a setting in the set-hook and inform the
user, then a warning, or just a standard message should be fine.

gdb/ChangeLog:

	* cli/cli-setshow.c: Add exceptions.h include.
	(struct set_cmd_cleanup_data): New definition.
	(set_cmd_cleanup_or_revert): New function.
	(do_set_comment): When installing a setting's new value, don't
	lose the previous value until the set-hook has been called.  Use a
	cleanup to either delete or restore the previous value.

gdb/testsuite/ChangeLog:

	* gdb.base/dcache-line-set-error.exp: New file.
---
 gdb/ChangeLog                                    |   9 ++
 gdb/cli/cli-setshow.c                            | 142 ++++++++++++++++++++++-
 gdb/testsuite/ChangeLog                          |   4 +
 gdb/testsuite/gdb.base/dcache-line-set-error.exp |  67 +++++++++++
 4 files changed, 217 insertions(+), 5 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/dcache-line-set-error.exp
  

On 11/15/2016 05:02 PM, Andrew Burgess wrote:
> * Luis Machado <lgustavo@codesourcery.com> [2016-11-11 18:20:24 -0600]:
>>> @@ -151,8 +152,20 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
>>>  {
>>>    /* A flag to indicate the option is changed or not.  */
>>>    int option_changed = 0;
>>> +  /* Cleanups created to free the previous value of the setting.  */
>>> +  struct cleanup *cleanups;
>>> +  /* The previous value of the setting.  Restored if the set-hook function
>>> +     throws an error.  */
>>> +  union
>>> +  {
>>> +    enum auto_boolean b;
>>> +    int i;
>>> +    const char *s;
>>> +    unsigned int u;
>>> +  } prev_value;
>>
>> I wonder if we can share and potentially simplify the storage mechanism for
>> these settings with what struct cmd_list_element already has? As opposed to
>> declaring something new just for this particular function?
>
> I'm not sure how we'd do that.  The 'struct cmd_list_element' holds a
> 'void *' pointer to a variable of _some type_ the interpretation of
> that 'void *' depends on the 'var_type' field within the 'struct
> cmd_list_element'.  But we never actually hold the _value_ of the
> setting in the 'struct cmd_list_element'.
>
> So I think that we will always end up introducing _some_ new structure
> to hold the previous value.  I'm open to suggestions though, just
> because I can't see the solution doesn't mean it's not there...
>

What i had in mind was not to declare values with explicit types like 
the above union but reuse 'enum var_types' with a 'void *' pointer that 
is the value.

Something like:

struct prev_value
{
   void *val;
   enum var_type type;
};

That way we would be able to save the previous value as 'void *' at the 
top of the function regardless of its type and register the cleanup only 
once. Only when we get to the cleanup we decide to free it or not. Then 
we can use the 'enum var_type' information to free whatever needs to be 
freed.

That way we can get rid of all the type-specific assignments, like here:

-	  xfree (*(char **) c->var);
+	  prev_value.s = *(const char **) c->var;
+	  make_cleanup (xfree, *(char **) c->var);
  	  *(char **) c->var = xstrdup (arg);

The above gets repeated a few times along the function.

Hopefully i managed to make it a bit more clear?

>> Since we're already touching this function, it may be worthwhile to get it
>> cleaned up a bit if that's not too much effort.
>
> I can do some clean up if you'd like, I can even make it part of this
> patch series, though obviously it'll be a new patch in this series.
> But you'll need to give some more specific indication of what you'd
> like to see changed....
>
>>
>> I also see a lot of repetition trying to save the old value. If we could get
>> it saved at the function's entry, that would save many lines of code, but i
>> think this is a nice-to-have and not required for this patch.
>
> I'm not sure I'd agree with "many", but in the new version I've tried
> to reduce the number of new lines added.  As I discuss above, given
> that the 'struct cmd_list_element' only has a 'void *' pointer,
> backing up the previous value has to be done on a case by case basis.
> Again, though, if you have a specific idea in mind that I'm not
> seeing, feel free to make a suggestion, I'm happy to change things.
>

These two comments i've made had the same scope as the first. That is, 
to use a 'void *' value plus its type and to save the previous value and 
register the cleanup only once at the top of the function. It was not a 
separate cleanup idea.

In any case, this is just a suggestion.
  

* Luis Machado <lgustavo@codesourcery.com> [2016-11-16 08:17:50 -0600]:

> On 11/15/2016 05:02 PM, Andrew Burgess wrote:
> > * Luis Machado <lgustavo@codesourcery.com> [2016-11-11 18:20:24 -0600]:
> > > > @@ -151,8 +152,20 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
> > > >  {
> > > >    /* A flag to indicate the option is changed or not.  */
> > > >    int option_changed = 0;
> > > > +  /* Cleanups created to free the previous value of the setting.  */
> > > > +  struct cleanup *cleanups;
> > > > +  /* The previous value of the setting.  Restored if the set-hook function
> > > > +     throws an error.  */
> > > > +  union
> > > > +  {
> > > > +    enum auto_boolean b;
> > > > +    int i;
> > > > +    const char *s;
> > > > +    unsigned int u;
> > > > +  } prev_value;
> > > 
> > > I wonder if we can share and potentially simplify the storage mechanism for
> > > these settings with what struct cmd_list_element already has? As opposed to
> > > declaring something new just for this particular function?
> > 
> > I'm not sure how we'd do that.  The 'struct cmd_list_element' holds a
> > 'void *' pointer to a variable of _some type_ the interpretation of
> > that 'void *' depends on the 'var_type' field within the 'struct
> > cmd_list_element'.  But we never actually hold the _value_ of the
> > setting in the 'struct cmd_list_element'.
> > 
> > So I think that we will always end up introducing _some_ new structure
> > to hold the previous value.  I'm open to suggestions though, just
> > because I can't see the solution doesn't mean it's not there...
> > 
> 
> What i had in mind was not to declare values with explicit types like the
> above union but reuse 'enum var_types' with a 'void *' pointer that is the
> value.
> 
> Something like:
> 
> struct prev_value
> {
>   void *val;
>   enum var_type type;
> };
> 
> That way we would be able to save the previous value as 'void *' at the top
> of the function regardless of its type and register the cleanup only
> once.

I think that you're suggesting saving the _value_ of the setting into
a 'void *', so a choice of an 'enum auto_boolean', an int (signed and
unsigned) or a 'char *'.  I guess at the moment they'll all fit, but
that seems like a fairly ugly bit of code.

Plus we'd still have to have a switch as there's no guarantee that all
those types are the same size, so when we dereference through the
cmd_list_element to get the value we'd have to have different code for
each var_type, otherwise we risk accessing outside the object type.

So we'd end up with:

  struct
  {
    void *val;
    enum var_type type;
  }  prev_value;

  switch (c->var_type)
    {
    case var_string:
    case var_string_noescape:
    case var_filename:
    case var_optional_filename:
    case var_enum:
      prev_value.val = (void *)(*(char **) c->var);
      break;

    case var_boolean:
    case var_integer:
    case var_zinteger:
    case var_zuinteger_unlimited:
      prev_value.val = (void *)(*(int *) c->var);
      break;

    case var_auto_boolean:
      prev_value.val = (void *)(*(enum auto_boolean *) c->var);
      break;

    case var_uinteger:
    case var_zuinteger:
      prev_value.val = (void *)(*(unsigned int *) c->var);
      break;
    }

I guess we could collapse the int/unsigned int cases as we know the
sizes will match, and you'll still need similar code to copy the value
back into place.

I'm not seeing any particular code saving, nor any improvement in code
quality there, and any time we use casts to place one item into
another there's always scope for bugs to creep in....

> Only when we get to the cleanup we decide to free it or not. Then we can use
> the 'enum var_type' information to free whatever needs to be freed.
> 
> That way we can get rid of all the type-specific assignments, like here:
> 
> -	  xfree (*(char **) c->var);
> +	  prev_value.s = *(const char **) c->var;
> +	  make_cleanup (xfree, *(char **) c->var);
>  	  *(char **) c->var = xstrdup (arg);
> 
> The above gets repeated a few times along the function.

The latest iteration removes the make_cleanup call here.  We now just
backup the previous value, so that's save a little code.

> 
> Hopefully i managed to make it a bit more clear?
> 
> > > Since we're already touching this function, it may be worthwhile to get it
> > > cleaned up a bit if that's not too much effort.
> > 
> > I can do some clean up if you'd like, I can even make it part of this
> > patch series, though obviously it'll be a new patch in this series.
> > But you'll need to give some more specific indication of what you'd
> > like to see changed....
> > 
> > > 
> > > I also see a lot of repetition trying to save the old value. If we could get
> > > it saved at the function's entry, that would save many lines of code, but i
> > > think this is a nice-to-have and not required for this patch.
> > 
> > I'm not sure I'd agree with "many", but in the new version I've tried
> > to reduce the number of new lines added.  As I discuss above, given
> > that the 'struct cmd_list_element' only has a 'void *' pointer,
> > backing up the previous value has to be done on a case by case basis.
> > Again, though, if you have a specific idea in mind that I'm not
> > seeing, feel free to make a suggestion, I'm happy to change things.
> > 
> 
> These two comments i've made had the same scope as the first. That is, to
> use a 'void *' value plus its type and to save the previous value and
> register the cleanup only once at the top of the function. It was not a
> separate cleanup idea.
> 
> In any case, this is just a suggestion.

I've fixed the typos and other minor items you pointed out in your
initial review (thank you for that), and the second iteration is
possibly a little cleaner it its use of cleanups.

I don't currently see the benefit / improvement that using a 'void *'
to store the previous value would offer (over using an union), so I
have no plan to rewrite in that direction.

I'm always open to being convinced though, if you feel I'm still
missing the point :)

Thanks,
Andrew
  

On 11/16/2016 12:01 PM, Andrew Burgess wrote:
> * Luis Machado <lgustavo@codesourcery.com> [2016-11-16 08:17:50 -0600]:
>
>> On 11/15/2016 05:02 PM, Andrew Burgess wrote:
>>> * Luis Machado <lgustavo@codesourcery.com> [2016-11-11 18:20:24 -0600]:
>>>>> @@ -151,8 +152,20 @@ do_set_command (const char *arg, int from_tty, struct cmd_list_element *c)
>>>>>  {
>>>>>    /* A flag to indicate the option is changed or not.  */
>>>>>    int option_changed = 0;
>>>>> +  /* Cleanups created to free the previous value of the setting.  */
>>>>> +  struct cleanup *cleanups;
>>>>> +  /* The previous value of the setting.  Restored if the set-hook function
>>>>> +     throws an error.  */
>>>>> +  union
>>>>> +  {
>>>>> +    enum auto_boolean b;
>>>>> +    int i;
>>>>> +    const char *s;
>>>>> +    unsigned int u;
>>>>> +  } prev_value;
>>>>
>>>> I wonder if we can share and potentially simplify the storage mechanism for
>>>> these settings with what struct cmd_list_element already has? As opposed to
>>>> declaring something new just for this particular function?
>>>
>>> I'm not sure how we'd do that.  The 'struct cmd_list_element' holds a
>>> 'void *' pointer to a variable of _some type_ the interpretation of
>>> that 'void *' depends on the 'var_type' field within the 'struct
>>> cmd_list_element'.  But we never actually hold the _value_ of the
>>> setting in the 'struct cmd_list_element'.
>>>
>>> So I think that we will always end up introducing _some_ new structure
>>> to hold the previous value.  I'm open to suggestions though, just
>>> because I can't see the solution doesn't mean it's not there...
>>>
>>
>> What i had in mind was not to declare values with explicit types like the
>> above union but reuse 'enum var_types' with a 'void *' pointer that is the
>> value.
>>
>> Something like:
>>
>> struct prev_value
>> {
>>   void *val;
>>   enum var_type type;
>> };
>>
>> That way we would be able to save the previous value as 'void *' at the top
>> of the function regardless of its type and register the cleanup only
>> once.
>
> I think that you're suggesting saving the _value_ of the setting into
> a 'void *', so a choice of an 'enum auto_boolean', an int (signed and
> unsigned) or a 'char *'.  I guess at the moment they'll all fit, but
> that seems like a fairly ugly bit of code.
>
> Plus we'd still have to have a switch as there's no guarantee that all
> those types are the same size, so when we dereference through the
> cmd_list_element to get the value we'd have to have different code for
> each var_type, otherwise we risk accessing outside the object type.
>
> So we'd end up with:
>
>   struct
>   {
>     void *val;
>     enum var_type type;
>   }  prev_value;
>
>   switch (c->var_type)
>     {
>     case var_string:
>     case var_string_noescape:
>     case var_filename:
>     case var_optional_filename:
>     case var_enum:
>       prev_value.val = (void *)(*(char **) c->var);
>       break;
>
>     case var_boolean:
>     case var_integer:
>     case var_zinteger:
>     case var_zuinteger_unlimited:
>       prev_value.val = (void *)(*(int *) c->var);
>       break;
>
>     case var_auto_boolean:
>       prev_value.val = (void *)(*(enum auto_boolean *) c->var);
>       break;
>
>     case var_uinteger:
>     case var_zuinteger:
>       prev_value.val = (void *)(*(unsigned int *) c->var);
>       break;
>     }
>
> I guess we could collapse the int/unsigned int cases as we know the
> sizes will match, and you'll still need similar code to copy the value
> back into place.
>
> I'm not seeing any particular code saving, nor any improvement in code
> quality there, and any time we use casts to place one item into
> another there's always scope for bugs to creep in....
>
>> Only when we get to the cleanup we decide to free it or not. Then we can use
>> the 'enum var_type' information to free whatever needs to be freed.
>>
>> That way we can get rid of all the type-specific assignments, like here:
>>
>> -	  xfree (*(char **) c->var);
>> +	  prev_value.s = *(const char **) c->var;
>> +	  make_cleanup (xfree, *(char **) c->var);
>>  	  *(char **) c->var = xstrdup (arg);
>>
>> The above gets repeated a few times along the function.
>
> The latest iteration removes the make_cleanup call here.  We now just
> backup the previous value, so that's save a little code.
>
>>
>> Hopefully i managed to make it a bit more clear?
>>
>>>> Since we're already touching this function, it may be worthwhile to get it
>>>> cleaned up a bit if that's not too much effort.
>>>
>>> I can do some clean up if you'd like, I can even make it part of this
>>> patch series, though obviously it'll be a new patch in this series.
>>> But you'll need to give some more specific indication of what you'd
>>> like to see changed....
>>>
>>>>
>>>> I also see a lot of repetition trying to save the old value. If we could get
>>>> it saved at the function's entry, that would save many lines of code, but i
>>>> think this is a nice-to-have and not required for this patch.
>>>
>>> I'm not sure I'd agree with "many", but in the new version I've tried
>>> to reduce the number of new lines added.  As I discuss above, given
>>> that the 'struct cmd_list_element' only has a 'void *' pointer,
>>> backing up the previous value has to be done on a case by case basis.
>>> Again, though, if you have a specific idea in mind that I'm not
>>> seeing, feel free to make a suggestion, I'm happy to change things.
>>>
>>
>> These two comments i've made had the same scope as the first. That is, to
>> use a 'void *' value plus its type and to save the previous value and
>> register the cleanup only once at the top of the function. It was not a
>> separate cleanup idea.
>>
>> In any case, this is just a suggestion.
>
> I've fixed the typos and other minor items you pointed out in your
> initial review (thank you for that), and the second iteration is
> possibly a little cleaner it its use of cleanups.
>
> I don't currently see the benefit / improvement that using a 'void *'
> to store the previous value would offer (over using an union), so I
> have no plan to rewrite in that direction.
>
> I'm always open to being convinced though, if you feel I'm still
> missing the point :)
>
> Thanks,
> Andrew
>

I don't have any further feedback. Hopefully other's will chime in. :-)

Thanks,
Luis
  

Hi Andrew,

On 11/15/2016 11:02 PM, Andrew Burgess wrote:

> I've folded the whole restore or clean-up the previous value into a
> single cleanup function.  I'm not 100% convinced that the new code is
> better than the previous, but I don't think it's any worse.

I'm not sure the "revert if set throws" is the best design, but
I do prefer this version with a single cleanup over the first, because
it should be easier to convert the new cleanup to a RAII C++
class (we're trying to get rid of cleanups).

> 
> gdb: Restore a settings previous value on error during change
> 
> When changing the value of a setting in do_set_command, there are three
> phases:
> 
>  1. The old value is destroyed, and the new value is placed into the
>  settings variable.
> 
>  2. The set-hook, the 'func' field of the setting is called.
> 
>  3. A change notification is issued.
> 
> There are two problems that I try to address in this commit.
> 
>  1. If the set-hook does not like the new value then either the setting
>  is restored to a default value, or we have to have a complex system
>  within the set-hook to remember the previous value and restore it
>  manually when an invalid change is made.
> 
>  2. If the set-hook throws an error then the change notification is
>  skipped, even though the setting might still have changed (say, back to
>  some default value).
> 
> The solution I propose here is to delay destroying the old value of the
> setting until after the set-hook has been called.  If the set-hook runs
> successfully then the old value will be destroyed when do_set_command
> returns, the new value will be left in place, and the change
> notification will be sent sent out exactly as before.
> 
> However, if the set-hook throws an error this is now caught in
> do_set_command, the new value of the setting is removed, and the old
> value is restored.
> 
> After this change, it is no longer possible in a set-hook to change a
> setting to a default value _and_ throw an error.  If you throw an error
> you should assume that gdb will restore the previous value of the
> setting.  If you want to change a setting in the set-hook and inform the
> user, then a warning, or just a standard message should be fine.

I have a few questions on this design.

Take a look at solib.c:gdb_sysroot_changed, the "set sysroot"
set hook.  reload_shared_libraries does a lot of work, and it
can easily trip on some error trying to fetch DSOs out of the
new sysroot.  Should the sysroot setting change back to what it was before,
when such an error happens?  What happens to the shared libraries that might
have already been loaded in the new sysroot?  Should "set" hooks
handle cases like this themselves, by wrapping all but command variable
validation with TRY/CATCH?

I'm also concerned about ctrl-c/QUIT while inside the set hook.
E.g., if the user presses ctrl-c while the set hook is working,
and a QUIT is thrown out of the set hook, should the setting's value
be reverted or not?  Maybe the simplest to imagine is if the set hook
prints something that paginates, and the user presses ctrl-c to
abort the pagination.  Maybe whether to revert depends on whether
the setting's new value was already validated or not?

I'm wondering whether a new "validate" hook, called before
the "set" hook is called, would be a better design.  Maybe not.
I'd like to have the design around those points clarified, so
that we have some agreed direction on how to handle such scenarios.

> +
> +  /* Should we revert the setting to the previous value?  If not then we
> +     should clean up the previous value.  */
> +  int revert_p;

"bool".

> -  c->func (c, NULL, from_tty);
> +
> +  TRY
> +    {
> +      c->func (c, NULL, from_tty);
> +    }
> +  CATCH (ex, RETURN_MASK_ERROR)
> +    {
> +      /* Set the REVERT_P flag based on OPTION_CHANGED.  If this leaves
> +	 REVERT_P as false then the cleanup SET_CMD_CLEANUP_OR_REVERT will
> +	 try to cleanup the previous value, but as OPTION_CHANGED was
> +	 false, there is no previous value, and so nothing is done.  If
> +	 REVERT_P is changed to TRUE here then there was a previous value,
> +	 and so the SET_CMD_CLEANUP_OR_REVERT will restore the previous
> +	 value, and cleanup the new value that we no longer need.  */
> +      set_cmd_cleanup_data.revert_p = option_changed;
> +      throw_exception (ex);
> +    }
> +  END_CATCH

Why wrap with TRY/CATCH instead of:

      set_cmd_cleanup_data.revert_p = option_changed;
      c->func (c, NULL, from_tty);
      set_cmd_cleanup_data.revert_p = false;

Note how ctrl-c inside the "set" hook ends up not reverting,
of RETURN_MASK_ERROR instead of RETURN_MASK_ALL.
If that was the reason for the TRY/CATCH, then I'd expect the
comment to mention it.



> +gdb_exit
> +gdb_start
> +gdb_reinitialize_dir $srcdir/$subdir
> +
> +# Read the current dcache line-size, check default is still 64.
> +set default_line_size 0
> +set testname "read default value for dcache line-size"
> +send_gdb "show dcache line-size\n"
> +gdb_expect {

Use gdb_test_multiple.

> +    -re "Dcache line size is (\[0-9\]+\)\.\r\n$gdb_prompt $" {
> +	set default_line_size $expect_out(1,string)
> +	pass $testname
> +    }
> +    -re ".*$gdb_prompt $"   { fail $testname }
> +    timeout	            { fail "$testname (timeout)" }
> +}
> +
> +if { $default_line_size == 0 } then {
> +    unsupported "dcache-line-set-error"

untested.

> +# Try to change to some invalid (non power of 2 value)

Please write complete sentences in comments.  I.e., add a period.

> +set invalid_line_size [expr $default_line_size - 1]
> +gdb_test "set dcache line-size $invalid_line_size" \
> +    "Invalid dcache line size: $invalid_line_size .*" \
> +    "First attempt to set invalid dcache line size"

Lowercase test messages:

    "first attempt to set invalid dcache line size"

(throughout) 

> +
> +# Check the default is still 64

Period.

> +gdb_test "show dcache line-size" \
> +    "Dcache line size is $new_line_size\." \
> +    "Check that the new value took"

"took effect" ?  Or is that idiomatic English perhaps? (I had never
seen it).

Thanks,
Pedro Alves
  

* Pedro Alves <palves@redhat.com> [2016-11-24 23:52:03 +0000]:

> Hi Andrew,

Pedro,

Thanks for reviewing this patch series, and apologies that it's taken
so long for me to look at this again.

> 
> On 11/15/2016 11:02 PM, Andrew Burgess wrote:
> 
> > I've folded the whole restore or clean-up the previous value into a
> > single cleanup function.  I'm not 100% convinced that the new code is
> > better than the previous, but I don't think it's any worse.
> 
> I'm not sure the "revert if set throws" is the best design, but
> I do prefer this version with a single cleanup over the first, because
> it should be easier to convert the new cleanup to a RAII C++
> class (we're trying to get rid of cleanups).

I'll take a look through some recent commits and move to an RAII style
for a final revision of this patch, I might as well get it "right
first time".

As far as the design, goes then hopefully we can find something that
you are happy with.

My motivation is best expressed in the second patch (which you already
looked at) trying to unify all of the roll-back code into one place.
I often find myself (in out of tree ports) adding new settings that
all duplicate the roll-back mechanism.

> 
> > 
> > gdb: Restore a settings previous value on error during change
> > 
> > When changing the value of a setting in do_set_command, there are three
> > phases:
> > 
> >  1. The old value is destroyed, and the new value is placed into the
> >  settings variable.
> > 
> >  2. The set-hook, the 'func' field of the setting is called.
> > 
> >  3. A change notification is issued.
> > 
> > There are two problems that I try to address in this commit.
> > 
> >  1. If the set-hook does not like the new value then either the setting
> >  is restored to a default value, or we have to have a complex system
> >  within the set-hook to remember the previous value and restore it
> >  manually when an invalid change is made.
> > 
> >  2. If the set-hook throws an error then the change notification is
> >  skipped, even though the setting might still have changed (say, back to
> >  some default value).
> > 
> > The solution I propose here is to delay destroying the old value of the
> > setting until after the set-hook has been called.  If the set-hook runs
> > successfully then the old value will be destroyed when do_set_command
> > returns, the new value will be left in place, and the change
> > notification will be sent sent out exactly as before.
> > 
> > However, if the set-hook throws an error this is now caught in
> > do_set_command, the new value of the setting is removed, and the old
> > value is restored.
> > 
> > After this change, it is no longer possible in a set-hook to change a
> > setting to a default value _and_ throw an error.  If you throw an error
> > you should assume that gdb will restore the previous value of the
> > setting.  If you want to change a setting in the set-hook and inform the
> > user, then a warning, or just a standard message should be fine.
> 
> I have a few questions on this design.
> 
> Take a look at solib.c:gdb_sysroot_changed, the "set sysroot"
> set hook.  reload_shared_libraries does a lot of work, and it
> can easily trip on some error trying to fetch DSOs out of the
> new sysroot.  Should the sysroot setting change back to what it was before,
> when such an error happens?  What happens to the shared libraries that might
> have already been loaded in the new sysroot?  Should "set" hooks
> handle cases like this themselves, by wrapping all but command variable
> validation with TRY/CATCH?

I've been thinking about this on and off since your review, and I
think that if the set-hook does not already catch and handle thrown
errors (except for "incorrect value" type errors) then we _might_
already have bugs.

If the set-hook is made from 3 steps, and step 2 throws an error, what
does that mean right now?  We'll have done some of the steps to switch
to the new value, but not all, and we'll have not sent out a
notification that the setting changed.  It feels like we'll not be in
a consistent state as it is.... though I agree that switching the
setting back doesn't make this better (and maybe makes it worse).

> I'm also concerned about ctrl-c/QUIT while inside the set hook.
> E.g., if the user presses ctrl-c while the set hook is working,
> and a QUIT is thrown out of the set hook, should the setting's value
> be reverted or not?  Maybe the simplest to imagine is if the set hook
> prints something that paginates, and the user presses ctrl-c to
> abort the pagination.  Maybe whether to revert depends on whether
> the setting's new value was already validated or not?

Again for the same reasons as above this currently (possibly) leaves
GDB in an inconsistent state, right?

> I'm wondering whether a new "validate" hook, called before
> the "set" hook is called, would be a better design.  Maybe not.
> I'd like to have the design around those points clarified, so
> that we have some agreed direction on how to handle such scenarios.

I'm happy to do the work to switch over to a validate-hook/set-hook
split.  And for what I want to achieve this would solve my problems,
while leaving the above issues untouched.

Given the large amount of change this would be I'd ideally like some
level of agreement that it's the right direction before I started.

The current patch simply ignores the issues above; we're currently on
iffy ground if the set-hook throws an error anyway, so after my change
it's not clear if we're really worse off (w.r.t. the above issues) or
if things are just iffy in a different way.... My suspicion is that
resolving that above issues might be something that needs to be done
on a case-by-case basis in the set-hooks.

Again, my goal here is to unify the rollback code we have spread
throughout GDB, and I'm not tied to any implementation.  I'd welcome
any feedback for what might be the best direction to take.

Thanks,
Andrew



> 
> > +
> > +  /* Should we revert the setting to the previous value?  If not then we
> > +     should clean up the previous value.  */
> > +  int revert_p;
> 
> "bool".
> 
> > -  c->func (c, NULL, from_tty);
> > +
> > +  TRY
> > +    {
> > +      c->func (c, NULL, from_tty);
> > +    }
> > +  CATCH (ex, RETURN_MASK_ERROR)
> > +    {
> > +      /* Set the REVERT_P flag based on OPTION_CHANGED.  If this leaves
> > +	 REVERT_P as false then the cleanup SET_CMD_CLEANUP_OR_REVERT will
> > +	 try to cleanup the previous value, but as OPTION_CHANGED was
> > +	 false, there is no previous value, and so nothing is done.  If
> > +	 REVERT_P is changed to TRUE here then there was a previous value,
> > +	 and so the SET_CMD_CLEANUP_OR_REVERT will restore the previous
> > +	 value, and cleanup the new value that we no longer need.  */
> > +      set_cmd_cleanup_data.revert_p = option_changed;
> > +      throw_exception (ex);
> > +    }
> > +  END_CATCH
> 
> Why wrap with TRY/CATCH instead of:
> 
>       set_cmd_cleanup_data.revert_p = option_changed;
>       c->func (c, NULL, from_tty);
>       set_cmd_cleanup_data.revert_p = false;
> 
> Note how ctrl-c inside the "set" hook ends up not reverting,
> of RETURN_MASK_ERROR instead of RETURN_MASK_ALL.
> If that was the reason for the TRY/CATCH, then I'd expect the
> comment to mention it.
> 
> 
> 
> > +gdb_exit
> > +gdb_start
> > +gdb_reinitialize_dir $srcdir/$subdir
> > +
> > +# Read the current dcache line-size, check default is still 64.
> > +set default_line_size 0
> > +set testname "read default value for dcache line-size"
> > +send_gdb "show dcache line-size\n"
> > +gdb_expect {
> 
> Use gdb_test_multiple.
> 
> > +    -re "Dcache line size is (\[0-9\]+\)\.\r\n$gdb_prompt $" {
> > +	set default_line_size $expect_out(1,string)
> > +	pass $testname
> > +    }
> > +    -re ".*$gdb_prompt $"   { fail $testname }
> > +    timeout	            { fail "$testname (timeout)" }
> > +}
> > +
> > +if { $default_line_size == 0 } then {
> > +    unsupported "dcache-line-set-error"
> 
> untested.
> 
> > +# Try to change to some invalid (non power of 2 value)
> 
> Please write complete sentences in comments.  I.e., add a period.
> 
> > +set invalid_line_size [expr $default_line_size - 1]
> > +gdb_test "set dcache line-size $invalid_line_size" \
> > +    "Invalid dcache line size: $invalid_line_size .*" \
> > +    "First attempt to set invalid dcache line size"
> 
> Lowercase test messages:
> 
>     "first attempt to set invalid dcache line size"
> 
> (throughout) 
> 
> > +
> > +# Check the default is still 64
> 
> Period.
> 
> > +gdb_test "show dcache line-size" \
> > +    "Dcache line size is $new_line_size\." \
> > +    "Check that the new value took"
> 
> "took effect" ?  Or is that idiomatic English perhaps? (I had never
> seen it).
> 
> Thanks,
> Pedro Alves
>