Re: Question about pre-merge and git merge octopus strategy

>
> On Thu, May 12, 2022 at 8:39 AM Junio C Hamano <gitster@pobox.com> wrote:
> >
> > Elijah Newren <newren@gmail.com> writes:
> >
> > >     Merge with strategy octopus failed.
> > >
> > > Also, if we check `git status`:
> > >
> > >     $ git status
> > >     On branch main
> > >     Unmerged paths:
> > >       (use "git restore --staged <file>..." to unstage)
> > >       (use "git add <file>..." to mark resolution)
> > >     both modified:   base
> > >
> > >     no changes added to commit (use "git add" and/or "git commit -a")
> > >
> > > And in git-merge-octopus.sh we see:
> > >
> > >     case "$OCTOPUS_FAILURE" in
> > >     1)
> > >     # We allow only last one to have a hand-resolvable
> > >     # conflicts.  Last round failed and we still had
> > >     # a head to merge.
> > >     gettextln "Automated merge did not work."
> > >     gettextln "Should not be doing an octopus."
> > >     exit 2
> > >     esac
> > >
> > > and in builtin/merge.c, we see:
> > >
> > >     /*
> > >      * The backend exits with 1 when conflicts are
> > >      * left to be resolved, with 2 when it does not
> > >      * handle the given merge at all.
> > >      */
> > >
> > > Which means git-merge-octopus.sh is claiming it can't handle this type
> > > of merge, and some other merge strategy should be tried, and
> > > implicitly that it didn't leave any conflicts to be resolved because
> > > it can't handle this merge.
> >
> > Correct.  Near the beginning of the loop you found the above
> > comment, there is this code:
> >
> >         if (use_strategies_nr == 1 ||
> >             /*
> >              * Stash away the local changes so that we can try more than one.
> >              */
> >             save_state(&stash))
> >                 oidclr(&stash);
> >
> >         for (i = 0; !merge_was_ok && i < use_strategies_nr; i++) {
> >                 int ret, cnt;
> >                 if (i) {
> >                         printf(_("Rewinding the tree to pristine...\n"));
> >                         restore_state(&head_commit->object.oid, &stash);
> >                 }
>
> Side-comment, which becomes important below: The save/restore code in
> builtin/merge.c appears to be broken to me.  As noted in the code
> above, stash will be set to null_oid() if save_state() returns
> non-zero (which happens when "stash create" has no output, which
> happens if there is _initially_ no state to save, i.e. if there are no
> local changes before the merge started).  restore_state() is a no-op
> whenever stash is the null_oid, meaning in that case it won't actually
> rewind the tree to a pristine state to undo the changes of the
> previous merge attempt.  So, if:
>
> * The user had no local changes before starting the merge
> * Multiple merge strategies are applicable
> * The first merge strategy makes index/working-tree changes, but
> returns with exit status 2
>
> Then the restore_state() called before the second merge strategy will
> do nothing, and the second merge strategy will be working on an index
> and working tree with garbage leftover from the first merge strategy.
> While this may have never been triggered (in what case do we have
> multiple merge strategies that all return an exit status of 2?), I
> suspect we want to fix this problem with something like this:
>
> diff --git a/builtin/merge.c b/builtin/merge.c
> index f178f5a3ee..7f3650fb09 100644
> --- a/builtin/merge.c
> +++ b/builtin/merge.c
> @@ -378,11 +378,11 @@ static void restore_state(const struct object_id *head,
>         struct strbuf sb = STRBUF_INIT;
>         const char *args[] = { "stash", "apply", NULL, NULL };
>
> +       reset_hard(head, 1);
> +
>         if (is_null_oid(stash))
>                 return;
>
> -       reset_hard(head, 1);
> -
>         args[2] = oid_to_hex(stash);
>
>         /*
>