Untracked Cache and --untracked-files=all

Hi folks,

I'm hoping for a little help understanding *intent* around a particular code comment in dir.c, and reaching out to the whole list because someone (Junio?) said they consider any mail that *doesn't* copy the list to be spam anyway, and the original author of the comment in question (Duy Nguyen) is no longer active on git.

Context: I am trying to explore how to get "--untracked-files=all" to play nice with the untracked cache, so that windows users using tooling that sets "--untracked-files=all" can benefit from the same orders-of-magnitude git status performance improvements as commandline users.

There is a "naive" approach to this (store the untracked cache in the index file with whatever dir flags were specified/used in the recursive walk, and ignore/rewrite that cached data every time the flags change), and there is presumably a more "comprehensive" approach (store all the information required in the untracked cache to be able to satisfy requests with either set of flags - even if this is a little more expensive on first run).

The main disadvantage of the "naive" approach is that is every time you flip-flop between "git status" and "git status -u", the untracked cache is ignored, a recursive directory walk ensues, and the untracked cache is rewritten to the index file for the next time you rerun, hopefully with the same flags. However, I would think in most situations flip-flopping will be less common - more commonly you're using a tool or workflow that ends up running the same command(s) repeatedly... At least, that's my thesis. I would put this "store the untracked cache every time dir flags change" behavior behind a config switch, anyway.

This "naive" approach *is* rather easy to achieve - you just need to recreate a new "untracked" structure inside dir.c#validate_untracked_cache() if you find a mismatch of flags (and make other small fixes to store the correct flags in that new "untracked" structure).

The one thing that sticks out after making these changes is a code comment first introduced in 2015 by Duy Nguyen in ccad261f, explaining *why* we refuse to use the untracked cache with "-uall":

I've seen this comment many times over the past few months, and I've previously always interpreted it as a "correctness" concern.

Looking at the original code in question, however (as of ccad261f, dir.c#treat_directory(), "resolve_gitlink_ref" call), I don't understand how correctness could be impacted.

Fast-forward 6 years and all this code has been substantially overhauled by several folks over the years (most recently and majorly Elijah Newren), and the "resolve_gitlink_ref()" call is long-gone.

Does this look familiar to anyone? Is there any remaining obvious reason to be wary of storing the untracked cache structure produced with '-uall'?

Thanks, Tao