Re: ds/maintenance-part-3 (was Re: What's cooking in git.git (Nov 2020, #02; Mon, 9))

[Derrick Stolee <stolee@gmail.com>]

On 11/17/2020 2:34 PM, Junio C Hamano wrote:
> Derrick Stolee <stolee@gmail.com> writes:
>> Thank you for finding and reporting this bug.
>>
>> Can I at least have a short moment of griping about anyone putting
>> regex characters into their directory names? ;)
> 
> Sure, but the blame mostly lies in the one who thought using ERE was
> a good idea ('+' is very often used).

Yes, the example provided is not nearly as problematic as it could
be. I also appreciate finding this in 'next' instead of in a release!

>>>> 0. Quote the value_regex properly, instead of blindly using a value
>>>>    that comes from the environment.
...
>> The only place I see where we do that currently is in
>> builtin/sparse-checkout.c:escaped_pattern(). Please let me know if
>> you know of a more suitable way to escape regex characters.
> 
> If we wanted to go that route, yes, we need to prevent random input
> taken from the end user or the environment to be regexes, when they
> are literal strings.  But I think we should just bite the bullet and
> say "git config --unset --literal-value-pattern vari.able va+l+ue", etc.
> 
> This is not a suggestion for the option name, but a suggestion to do
> this with a new option and not with a special value-pattern syntax.
> 
> 	Side note.  It is tempting to declare that something like
> 
> 	    git config --unset vari.able "!!$end_user_value"
> 
> 	is the syntax to use literal/fixed pattern, and that way we
> 	do not have to touch the callchain from builtin/config.c
> 	leading down to git_config_set_multivar_in_file_gently().
> 	It is backward incompatible change that is unlikely hurt
> 	real people.  If a script is feeding "$end_user_value"
> 	without cleansing as the value_regex already, it is already
> 	broken (e.g. if $end_user_value happens to being with '!',
> 	this will unset everything that does not match the regexp)
> 	anyway.  And users already know to say '[!]some-pattern'
> 	when they mean the pattern begins with a literal '!' and not
> 	"does not match some-pattern", so reserving '!!' prefix does
> 	not sound too bad.

This is very clever. However, I fear it is _too_ clever.

>>>>> 1. Teach 'git config' to learn either which regex parser to use
>>>>> (including fixed), or at least to learn "value isn't a regex", or
>>>>>
>>>>> 2. Don't spin a child process in 'git maintenance [un]register' and
>>>>> instead just call the config API.
>>>> ...
>>>> My short-to-mid-term preference is to do #1 to allow a value to be
>>>> spelled literally (i.e. remove entry with _this_ value, and add this
>>>> one instead), and optionally do #2 as an optimization that is not
>>>> essential.  I do not offhand know how you can make #2 alone fly
>>>> without doing some form of #1, as I think the same value_regex that
>>>> ought to be ERE to specify entries to be replaced needs to be used
>>>> under the cover even if you use "config API" anyway.
>>>
>>> Ah, right you are - I had figured the regex parsing was done earlier,
>>> but it indeed looks to happen in
>>> config.c:git_config_set_multivar_in_file_gently. Thanks.
>>
>> So the "real fix" is to allow a command-line option to 'git config'
>> that makes the "value_regex" parameter a literal string? Of course,
>> this would either require wiring an option down into
>> git_config_set_multivar_in_file_gently() to treat the string as a
>> literal _or_ to escape the input string in builtin/config.c.
>>
>> Am I understanding the intended plan here?
> 
> Yup, if people cannot poke holes with the wishful thinking that the
> breaking of backward compatibility by using the "!!" prefix would
> not cause practical issues, then I am also fine with that, but
> inventing a flags word with a VALUE_PATTERN_FIXED bit in it and
> updating the callchain to pass it down from the command line option
> parser would be much less risky, I would think.

I will get started on this fix as a series on top of part-3.

Thanks,
-Stolee