Extending whitespace checks

[Junio C Hamano <gitster@pobox.com>]

We have, via the attributes subsystem, a way to choose from a set of
predefined whitespace rules so that "git diff" can notice that you
are adding trailing whitespaces to your newly written lines, or you
are indenting a newly introduced line in a Python script with a HT.
This can be used, for example, in pre-commit hook to reject an
attempt to introduce whitespace-damaging changes to the codebase.

Which is great.

I am wondering what we can do to add a different kind of checks to
help file types with fixed format by extending the same mechanism,
or the checks I have in mind are too different from the whitespace
checks and shoehorning it into the existing mechanism does not make
sense.  The particular check I have an immediate need for is for a
filetype with lines, each has exactly 4 fields separated with HT in
between, so the check would ask "does each line have exactly 3 HT on
it?"  It would be extended to verify CSV files with fixed number of
fields (but the validator needs to be aware of the quoting rules for
comma in a value in fields).

I guess the best I could do (outside Git) is

 - write such a validator that can take one line of input and say
   "this line comforms to the rule".

 - add, via .gitattribute, my own attribute to allow me to mark
   the files that these rules apply.  Git does not do anything
   special for this attribute (remember, I said "outside Git").

 - in pre-commit hook, run "git diff ':(attr:myattr)'" to grab
   changes in these files with special formats, and have the
   line-by-line validator (above) check the new lines.

to make sure bad lines would not slip into the history, but it would
be really nice if I can trigger the check as part of "git diff --check",
which means it would be more ideal if we can do this "inside" Git.

Perhaps we could introduce a mechansim that allows me to do the
following:

 - An attribute, like whitespace=..., specifies what line-validation
   function to use to vet each new line introduced to a file with
   the attribute.

 - A line-validation function can be dynamically loaded/linked
   (here, we'd need ".gitattribute specifies the logical meaning,
   while .git/config and friends maps the 'logical meaning' to a
   specific implementation suitable for the platform" separation,
   similar to what we use for smudge/clean filters).  Perhaps this
   would be a good testbed for use of dll, written even in a foreign
   language like Rust?

 - In the diff machinery, where a '+' line is checked for whitespace
   anomalies in the existing code, add code to call the dynamically
   loaded line-validation function when applicable.

 - Profit?

Hmm?