bug report: git checkout deletes worktree file even though it is excluded by sparse-checkout

Hi all

I have a bug report: git checkout deletes a worktree file even though it is excluded by sparse-checkout, even if it is dirty.

Watch this (starting in an empty directory):

$ git init
Initialised empty Git repository in /mnt/amd/home/trenouf/amd/tmp/git/.git/
$ echo file1 >file1; echo file2 >file2
$ git add file1 file2
$ git commit -m"commit 1"
[master (root-commit) 4f7035d] commit 1
 2 files changed, 2 insertions(+)
 create mode 100644 file1
 create mode 100644 file2
$ git rm file2
rm 'file2'
$ git commit -m"rm file2"
[master d025c99] rm file2
 1 file changed, 1 deletion(-)
 delete mode 100644 file2
$ git checkout master~1
HEAD is now at 4f7035d commit 1
$ git sparse-checkout set /file1
$ ls
file1
$ echo dirty >file2
$ ls
file1  file2
$ git checkout master
Previous HEAD position was 4f7035d commit 1
Switched to branch 'master'
$ ls
file1

I set up sparse-checkout to include only file1, not file2. file2 is now not in the worktree, even though it is in the commit I am checked out at. Then I create file2 with arbitrary content. Then a git checkout switching to the commit where file2 is removed also deletes it from the worktree.

I assert that file2 should be left untouched by that checkout, because it is excluded by sparse-checkout. I guess file2 had its skip-worktree bit set before the checkout that removed it from the index; that should stop it being deleted in the worktree.

To be clear, I expect that last “ls” to still show “file1  file2”.

Thank you for your attention if you have got this far.

-tpr

[System Info] git version: git version 2.31.1 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.8.0-53-generic #60~20.04.1-Ubuntu SMP Thu May 6 09:52:46 UTC 2021 x86_64 compiler info: gnuc: 9.3 libc info: glibc: 2.31 $SHELL (typically, interactive shell): /bin/bash