Re: Add a way to invoke redefined (overridden) GDB commands

[Pedro Alves <pedro@palves.net>]

On 10/19/20 6:41 PM, Marco Barisione wrote:
> On 12 Oct 2020, at 12:50, Pedro Alves <pedro@palves.net> wrote:
>> On 9/14/20 10:39 AM, Marco Barisione wrote:
>>> Currently, when a GDB command is redefined, the original implementation is not
>>> available any more.  This makes it difficult to build features on top of
>>> existing commands.
>>>
>>> Last year I submitted a patch to fix this but I ran out of time to address the
>>> review comments (the original patch was sent on the 28th of October 2019).
>>> These patches restart that work and should address all the comments I got last
>>> time.  As the patchea are very different and a long time passed, I'm
>>> submitting as a new series.
>>>
>>> My patches add a new "uplevel" command and a new gdb.Command.invoke_uplevel
>>> method inspired by TCL (as initially suggested by Andrew Burgess) so you can
>>> do this:
>>>
>>>    (gdb) define run
>>>    echo Will run!\n
>>>    uplevel 0 run
>>>    end
>>>    (gdb) run
>>>    Will run!
>>>    [... normal output of run ...]
>>>
>>>
>>> There are a couple of other things which could be added to make the "uplevel"
>>> command more helpful, but I think they are out of scope and my patches are
>>> already useful as they are.
>>
>> So I'm looking at this afresh, and really questioning this "uplevel N"
>> design.  This it not really like TCL's "uplevel".  With TCL's uplevel,
>> you are accessing a different scope or frame, not a previous implementation
>> of the function that was overwritten.  To me, the naming choice is
>> confusing, from that angle.  If someone extends GDB's CLI to gain support
>> for local variables, then a really-TCL-like uplevel is likely handy, and
>> then calling that feature "uplevel" would be good.
>>
>> I also question whether "uplevel N" with "N>0" is really usable, since
>> in general you don't know what other scripts may have overridden.  E.g.,
>> you never know what "uplevel 3 cmd" will run, since you don't know how
>> many scripts redefined/overridden cmd.
>>
>> If we stick with the TCL inspiration, I think a better approach would
>> be to add support for renaming commands, like TCL's rename command:
>>
>>  https://www.tcl.tk/man/tcl8.4/TclCmd/rename.htm
>>
>> So a user would do:
>>
>> (gdb) rename run org_run
>> (gdb) define run
>>> echo Will run!\n
>>> org_run
>>> end
>> (gdb) run
>>  Will run!
>>  [... normal output of run ...]
>> (gdb) org_run
>>  [... normal output of run ...]
>>
>> (You can find many examples of TCL's rename in use in GDB's testsuite.)
> 
> I will try implementing this then, including the semantics for deleting
> a command (copied from the TCL command).
> 
> A problem with GDB is that commands can have spaces so:
>     (gdb) rename foo bar
> Is ambiguous. Do you want to delete the "foo bar" command or rename the
> "foo" command to "bar"?
> Even without deleting command, this would be ambiguous:
>     (gdb) rename foo bar baz
> 
> Unless somebody proposes something different I will use "--" to split
> the two command names.

ISTM that (optional) quotes would be the natural thing here:

    (gdb) rename "foo bar" "baz"
    (gdb) rename "foo" "bar baz"

I'd rather leave "--" for the typical splitting of options from
other arguments.  Commands get that for free if they use
the cli/cli-option.h framework.  So "--" would be used like in other
commands, to unambiguously split options from commands that may
start with "-", like:

    (gdb) help - 
    Scroll window backward.
    Usage: - [N] [WIN]
    Scroll window WIN N lines backwards.  Both WIN and N are optional, N
    defaults to 1, and WIN defaults to the currently focused window.
    (gdb) 

    (gdb) rename -someoption -- - scroll-backward

Thanks,
Pedro Alves

> 
> For instance:
>     (gdb) # Rename "foo" to “bar":
>     (gdb) rename foo -- bar
>     (gdb) # Rename "foo bar" to "baz":
>     (gdb) rename foo bar -- baz
>     (gdb) # Delete "foo bar":
>     (gdb) rename foo bar --
>     (gdb) # Invalid:
>     (gdb) rename --
>     (gdb) rename -- hello
> 
>