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?
