git filter bug

["Udoff, Marc" <Marc.Udoff@deshaw.com>]

Hi,

I believe there is a bug in git status that happens when a file changes but the filtered version of the file does not. Correctly, git diff does not show anything as different and git commit believes there is nothing to commit.

Reproducer:
$ git init
$ touch bar
$ git add bar
$ git commit -am 'Bar'
[main (root-commit) dd12b3e] Bar
1 file changed, 0 insertions(+), 0 deletions(-)
create mode 100644 bar
$ echo -en '\n[filter "noat"]\n     clean = grep -v "@"\n' >> .git/config
$ cat .git/config 
[core]
        repositoryformatversion = 0
        filemode = true
        bare = false
        logallrefupdates = true

[filter "noat"]
     clean = grep -v "@"
$ echo -en 'abc\n@def\nghi\n' > bar 
$ cat bar 
abc
@def
ghi
$ echo "* filter=noat" > .gitattributes
$ git commit -am 'No at bar'
[main e81ee3b] No at bar
2 files changed, 3 insertions(+)
create mode 100644 .gitattributes
$ git show HEAD:bar
abc
ghi
$ echo "@another line" >> bar # Add another @ which will be filtered. touch doesn't cause this bug
$ git status --porcelain
M bar
$ git diff # no output as there is no diff
$ git commit -am "I did not update"
On branch main
nothing to commit, working tree clean


While this reproducer is a bit contrived, the real world examples are with Jupyter notebooks filtering output, so I expect this is a somewhat common occurrence. I think it also may be the same as https://stackoverflow.com/questions/62641222/how-to-make-git-status-consider-the-clean-filter.

Git version: 2.35.1
OS: Linux


Note: I also logged this here, but I believe this mailing list is the correct place to raise the issue: https://github.com/gitgitgadget/git/issues/1256

Thanks,
Marc