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

Ok, here we go.

On Thu, Sep 17, 2026 at 5:24 AM Phillip Wood [off-list ref] wrote:

Hi Ben

On 16/09/2026 15:30, Ben Knoble wrote:

quoted

quoted

Le 16 sept. 2026 à 09:35, Phillip Wood [off-list ref] a écrit :
Taking a step back, this code applies the stashed index changes
into the current index, writes the result to a tree and then resets
the index to HEAD.

I noted that we save the current index (?) into c_tree with
write_index_as_tree(), then feed diff_tree_binary() into git apply
--cache, aka applying the stashed index changes.

[from my notes]
    - that is, diff H..I in the diagram from the manual:

       A stash entry is represented as a commit whose tree records the state
       of the working directory, and its first parent is the commit at HEAD
       when the entry was created. The tree of the second parent records the
       state of the index when the entry is made, and it is made a child of
       the HEAD commit. The ancestry graph looks like this:

                  .----W
                 /    /
           -----H----I

       where H is the HEAD commit, I is a commit that records the state of the
       index, and W is a commit that records the state of the working tree.

We then have a discard_index()/repo_read_index() pair, which I assume
is refreshing the in-memory index? Followed by
write_index_as_tree(&index_tree)… so that must be the "save the
results of applying the stashed changes" part.

What I totally misunderstood was "reset the index to HEAD"---of course
that's what "git reset" does! But I didn't understand why until…

It is a bit confusing the way it updates the index, then resets it only
to update it again at the end. I don't think we can avoid that though if
we want to error out when there are conflicts merging the index.

…which now makes (some) sense. That also explains why my attempts to
use reset_working_tree() in various forms could never work :) I had
the wrong idea entirely. But it seemed in my debugging like something
was touching the working tree, so I wish I had kept better notes.

Just finishing up the flow:

- we then refresh the in-memory index again (we just reset the on-disk
index to HEAD)
- we continue on with a "normal" stash apply merge of c_tree (old
index), w_tree (W), and b_tree (which I assume is H?); this applies
the stashed working tree changes on the current state?
- [skipping ahead] in the index case, we reset_tree(&index_tree, 0,
0), restoring the stashed index changes. Sans doc comments for
unpack_trees() beyond "N-way merge len trees […] resulting index […]",
I haven't puzzled out what's going on, but it _looks_ like we merge a
single tree, possibly with the original index (opts.src_index) and
write to a destination which is the repository's index.

It's extra unclear what the reset bit is applied to in

It looks like stash has its own unpack_trees() wrapper, so I think the
simplest fix is to replace reset_head() with

        reset_tree(&c_tree, 0, 1);

I'm not sure if that is a pre-merge reset, post-merge reset, or
something in between.
Unfortunately, a whole bunch of tests fail (6 files) with this suggestion :/

Summary of Failures:

 339/1062 git:t3904-stash-patch                              ERROR
      0.45s   exit status 1
 503/1062 git:t3903-stash                                    ERROR
      4.40s   exit status 1
 763/1062 git:t6424-merge-unrelated-index-changes            ERROR
      0.90s   exit status 1
 778/1062 git:t6402-merge-rename                             ERROR
      2.06s   exit status 1
 864/1062 git:t1092-sparse-checkout-compatibility            ERROR
     26.19s   exit status 1
 868/1062 git:t7611-merge-abort                              ERROR
      0.45s   exit status 1

It _also_ doesn't make the bug go away, hm.

quoted

quoted

We could avoid touching the index at all if we
used merge_incore_nonrecursive() to cherry pick the index changes
instead. That way we'd get a proper three-way merge and avoid
spawning subprocesses for "git diff-tree", "git apply --cached",
and "git reset". We're already using merge_ort_nonrecursive() to
merge the working tree changes in that function so we have nearly
everything we need already set up to merge the index changes as
well. Essentially, when merging the index, we just need to call
merge_incore_nonrecursive() instead of merge_ort_nonrecursive()
and use info->i_tree instead of info->w_tree.

If I'm following this, the suggestion is to replace (parts of) the
early "if (index)" block with a merge_incore_nonrecursive() to merge
index changes, reporting conflicts as we do today, and saving the tree
for later… and this would not touch the real index, so we wouldn't
have to reset at all? Interesting!

The attached patch [Gmail headaches, sorry], which needs some
polishing [*], passes tests and fixes the bug! Yahoo. I'll send a
series later, tomorrow probably.
(It won't apply directly, because it's on top of the experimental
reset_tree() version, but resolving conflicts should be easy.)

[*] namely, the log message, some tiny first cleanups, and removing
now-unused functions

-- 
D. Ben Knoble