Re: [BUG] stash.index=true leaves a redundant stash entry after an autostash fast-forward

>
> Disclaimer, the following is written by an agent, but the bug is a
> real problem that I have.
>
>
> With stash.index=true, an autostash that is applied successfully is
> nevertheless stored as a stash entry, and deleting MERGE_AUTOSTASH
> fails.  A staged change at the time of the merge is required to
> trigger it.
>
> Reproduction (independent of the reporter's configuration):
>
>     #!/bin/sh
>     set -e
>     export GIT_AUTHOR_NAME=A GIT_AUTHOR_EMAIL=a@b \
>            GIT_COMMITTER_NAME=A GIT_COMMITTER_EMAIL=a@b
>     rm -rf /tmp/gitbug && mkdir /tmp/gitbug && cd /tmp/gitbug
>     git init -q -b main up
>     cd up && echo a >u && echo z >z && git add . &&
>         git commit -qm base && cd ..
>     git clone -q up dn
>     cd up && echo more >>u && git commit -qam up2 && cd ../dn
>     echo staged >>z && git add z          # a STAGED change is required
>     git fetch -q origin
>     git -c stash.index=true merge --ff-only --autostash origin/main
>     echo "--- git stash list:"; git stash list
>
> Actual output:
>
>     Updating 34a5e40..84ccd9d
>     Created autostash: 71d4617
>     Fast-forward
>      u | 1 +
>      1 file changed, 1 insertion(+)
>     Applied autostash.
>     error: cannot lock ref 'MERGE_AUTOSTASH': unable to resolve
> reference 'MERGE_AUTOSTASH'
>     --- git stash list:
>     stash@{0}: autostash
>
> Expected: the same without the error and with an empty stash list, as
> happens with stash.index=false (the only change to the script).

> Analysis
> --------
>
> Merge keeps its autostash in the MERGE_AUTOSTASH ref
> (builtin/merge.c:1675) and applies it from finish()
> (builtin/merge.c:540).  apply_save_autostash_ref() resolves the ref,
> applies it, and then deletes it (sequencer.c:4821-4848).
>
> The apply is a child process, `git stash apply <oid>`
> (sequencer.c:4737-4751).  stash.index turns that into an --index
> apply, which takes the index-restoring branch of do_apply_stash() and
> calls reset_head() (builtin/stash.c:684-691), i.e. a
> `git reset --quiet --refresh` child (builtin/stash.c:455-467).
>
> That reset has no pathspec, so it calls remove_branch_state()
> (builtin/reset.c:543) -> remove_merge_branch_state()
> (branch.c:829-838), whose last statement is
>
>     save_autostash_ref(r, "MERGE_AUTOSTASH");
>
> which stores the autostash into refs/stash and deletes the ref -- in
> the middle of the very apply that was about to consume it.  Control
> returns to apply_save_autostash_ref(), the apply reports success
> ("Applied autostash."), and its refs_delete_ref() then fails on a ref
> that is already gone, producing the error line.

> Possible directions, in case they are useful: remove_merge_branch_state()
> is about ending a merge, and `git stash apply --index` is not ending
> one -- having stash's reset_head() avoid the branch-state cleanup, or
> teaching an in-flight autostash apply to shield MERGE_AUTOSTASH, would
> both close it.  Making apply_save_autostash_ref() tolerate a missing
> ref would silence the error but leave the duplicate entry.