file loss by git stash push --staged --keep-index

file loss by git stash push --staged --keep-index

[Maximilian Reichel]

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Thank you for filling out a Git bug report!
Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)
Running the following script:
```
#!/bin/bash
git init workDir
cd workDir
git commit -m first --allow-empty
echo b > b
git add b
GIT_TRACE=TRUE git stash push --staged --keep-index
git stash pop
cat b
```

What did you expect to happen? (Expected behavior)
I expect file `b` to be present after running the script.
If I run the script with only `git stash push --staged` or 
`git stash push --keep-index`, the file is present afterward.

What happened instead? (Actual behavior)
The file is not present after running the script.
`cat b` will report that the file does not exist.

[-- Attachment #2: output--staged.txt --]
[-- Type: text/plain, Size: 1007 bytes --]

Initialized empty Git repository in /test/workDir/.git/
[master (root-commit) 5ebd5d0] first
17:28:10.379881 git.c:459               trace: built-in: git stash push --staged
17:28:10.381318 run-command.c:654       trace: run_command: git diff-tree -p -U1 HEAD 6be660545b31f61a82a87d2b1915f0b88bb9f16f --
17:28:10.382374 git.c:459               trace: built-in: git diff-tree -p -U1 HEAD 6be660545b31f61a82a87d2b1915f0b88bb9f16f --
17:28:10.383488 run-command.c:654       trace: run_command: git apply -R
Saved working directory and index state WIP on master: 5ebd5d0 first
17:28:10.384517 git.c:459               trace: built-in: git apply -R
17:28:10.384942 run-command.c:654       trace: run_command: git reset -q --refresh --
17:28:10.386018 git.c:459               trace: built-in: git reset -q --refresh --
On branch master
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	new file:   b

Dropped refs/stash@{0} (546ccd88cdcd416cd0bfa5b0c63600b64011da53)
b

[-- Attachment #3: output--keep-index.txt --]
[-- Type: text/plain, Size: 1211 bytes --]

Initialized empty Git repository in /test/workDir/.git/
[master (root-commit) e972d48] first
17:26:07.021096 git.c:459               trace: built-in: git stash push --keep-index
17:26:07.022721 run-command.c:654       trace: run_command: GIT_INDEX_FILE=.git/index.stash.101 git update-index --ignore-skip-worktree-entries -z --add --remove --stdin
17:26:07.023845 git.c:459               trace: built-in: git update-index --ignore-skip-worktree-entries -z --add --remove --stdin
17:26:07.024747 run-command.c:654       trace: run_command: git reset --hard -q --no-recurse-submodules
Saved working directory and index state WIP on master: e972d48 first
17:26:07.025846 git.c:459               trace: built-in: git reset --hard -q --no-recurse-submodules
17:26:07.026947 run-command.c:654       trace: run_command: git checkout --no-overlay 6be660545b31f61a82a87d2b1915f0b88bb9f16f -- :/
17:26:07.028043 git.c:459               trace: built-in: git checkout --no-overlay 6be660545b31f61a82a87d2b1915f0b88bb9f16f -- :/
On branch master
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	new file:   b

Dropped refs/stash@{0} (3ed20e369087861f1dd3089342a656217e537d34)
b

[-- Attachment #4: output--keep-index--staged.txt --]
[-- Type: text/plain, Size: 1069 bytes --]

Initialized empty Git repository in /test/workDir/.git/
[master (root-commit) b3f6a1b] first
17:26:28.802913 git.c:459               trace: built-in: git stash push --keep-index --staged
17:26:28.804521 run-command.c:654       trace: run_command: git diff-tree -p -U1 HEAD 6be660545b31f61a82a87d2b1915f0b88bb9f16f --
17:26:28.805602 git.c:459               trace: built-in: git diff-tree -p -U1 HEAD 6be660545b31f61a82a87d2b1915f0b88bb9f16f --
17:26:28.806705 run-command.c:654       trace: run_command: git apply -R
Saved working directory and index state WIP on master: b3f6a1b first
17:26:28.807796 git.c:459               trace: built-in: git apply -R
On branch master
Changes to be committed:
  (use "git restore --staged <file>..." to unstage)
	new file:   b

Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	deleted:    b

Dropped refs/stash@{0} (0314f565bf7ae2738cece5d41045fd4dcf32aac2)
cat: b: No such file or directory

[-- Attachment #5: Type: text/plain, Size: 909 bytes --]


`git status` reports a staged change with the created file and 
an unstaged change that the file is deleted.

What's different between what you expected and what actually happened?
git stash pop does not recreate the file when stashing with
`--staged` and `--keep-index`.

Anything else you want to add:
I got the same behavior on git 2.35.1 and the latest master.
I attached to output of the script for all three variants:
stash push with `--staged --keep-index`, `--staged` and `--keep-index`.

[System Info]
git version:
git version 2.36.0.rc0
cpu: x86_64
built from commit: faa21c10d44184f616d391c158dcbb13b9c72ef3
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.10.25-linuxkit #1 SMP Tue Mar 23 09:27:39 UTC 2021 x86_64
compiler info: gnuc: 10.2
libc info: glibc: 2.31
$SHELL (typically, interactive shell): <unset>


[Enabled Hooks]
not run from a git repository - no hooks to show