Re: icase pathspec magic support in ls-tree

> On Sun, Oct 2, 2022 at 9:07 PM brian m. carlson
> <sandals@crustytoothpaste.net> wrote:
> >
> > On 2022-09-30 at 13:53:16, Ævar Arnfjörð Bjarmason wrote:
> > > You might find ASCII-only sufficient, but note that even if you get this
> > > working you won't catch the more complex Unicode normalization rules
> > > various filesystems perform, see the fsck code we carefully crafted to
> > > make sure we don't get something those FS's will mistake for a ".git"
> > > directory in-tree.
> >
> > What's even worse is that different OSes case-fold differently and the
> > behaviour differs based on the version of the OS that formatted the file
> > system (which is of course not exposed to userspace), so in general it's
> > impossible to know exactly how case folding works on a particular
> > system.
> >
> > It might be possible to implement some general rules that are
> > overzealous (in that they will catch patterns that will case-fold on
> > _some_ system), but in general this is very difficult.  The rules will
> > also almost certainly change with newer versions of Unicode.
> >
> > I'll also point out that there is no locale-independent way to correctly
> > case-fold Unicode text.  Correct case-folding is sensitive to the
> > language, script, and region.
>
> Thanks for the feedback!
>
> If I'm understanding correctly, both of these responses were targeted
> specifically at my motivation/usecase (preventing the submission of
> case-insensitively duplicate files into a repository) rather than the
> question of whether anyone has worked or is working on anything
> relevant to adding icase pathspec magic support to ls-tree.
>
> I understand that case-folding is a complex topic, and doing it
> correctly in some universal sense is undoubtedly beyond me - but "my"
> context certainly does not require a high standard of correctness:
> There's a repo shared by some 1000 engineers, 200k files, lots of
> activity, three different OSes of which two default to
> case-insensitive filesystems, and every once in a while a user on
> linux creates a case-insensitive-duplicate file with differing
> content, which causes git on case-insensitive filesystems to lose the
> plot (you end up with one file's content looking like changes to the
> other file's content - "ghost changes" that appear as soon as you
> check out, that prevent you from doing a simple "pull", and that you
> just can't reset).
>
> I don't imagine I can make a perfectly correct and universal fix to
> this, but with case-insensitive matching on ls-tree in an update hook
> I believe I could reduce the frequency of this already-infrequent
> issue by at least 1000X, which would suit my purposes just fine. In my
> case filenames are mostly ansi-based, and I don't expect we've ever
> had Turkish filenames (turkish "i" being the most famous case-folding
> gotcha I think?).
>
> Coming at this from another angle, I guess we could teach git on
> case-insensitive filesystems to detect this situation (where two files
> in the index, with different contents, are pointing to the exact same
> filesystem file) and more explicitly warn the user of what's wrong,
> giving them clear help on how to fix it? And temporarily exclude those
> two files from its change reconciliation processes altogether to avoid
> ghost changes interfering with recovery actions like "pull"? Certainly
> that would be better than the current "ghost changes" behavior... but
> it would still be far less convenient than preventing (the vast
> majority of) these issues altogether, be that with a custom hook or a
> core option prohibiting clearly case-insensitive-duplicate files from
> being pushed.
>
> By the time a case-insensitive-FS-user encounters this issue in their
> repo as they checkout or clone, it's likely that the problem is in
> master/main and others are already affected, and both the cycle time
> to fixing the issue, and the communication impact in the current state
> ("please wait, the issue is being addressed, once the remote branch is
> fixed here's what you'll do to 'pull' successfully in spite of the
> local repo thinking there are filesystem changes that really don't
> exist and can't be reset") are... suboptimal.
>
> It feels like adding case-insensitivity pathspec magic support to
> ls-tree (however reliable or universal the subsequent
> duplicate-detection is or isn't) *should* be much simpler than what it
> would have taken to support it in ls-files in the first place - but at
> a glance, I see the official pathspec-supporting function
> "match_pathspec()" is deep in index-land, with an "index_state"
> structure being passed around all over the place. If it really was
> easy, someone would already have done it I guess? :)
>
> I don't see this being something I can take on in my spare time, so
> for now I suspect I'll have to do a full-tree duplicate-file-search on
> every ref update, and simply accept the 1-second update hook
> processing time/delay per pushed ref :(
>
> I'm assuming the "ghost changes" behavior I allude to here (where two
> different files in the index, with different contents, point to the
> same single file in the case-insensitive filesystem, and one or the
> other index file appears modified / the working tree looks "dirty") is
> a known issue, but if there's any value in my opening a thread more
> clearly/explicitly about this behavior, please let me know.
>
> Thanks,
> Tao