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

From: Junio C Hamano <[email-masked]>
Date: Thu, 12 May 2022 08:39:34 -0700

Elijah Newren <[email-masked]> 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);
		}

but that save-then-restore triggers ONLY when there are multiple
strategies to try.  Unfortunately, octopus has no friends to fall
back on, so we do not do the save-restore dance on the calling side.

> But it clearly decides to leave the
> modifications it made to the index and working tree around, which just
> seems wrong to me.

If merge-recursive or merge-resolve is asked to merge a single
commit to the current branch without any other strategies to use as
a fallback, they leave the working tree and index into a state where
the end-user can conclude the conflict resolution and commit the
result.  In spirit, we are in the same situation, aren't we?

The user, if they want to proceed against octopus's opinion, would
resolve the current conflict, read-tree -m the next one, ..., to
conclude and commit the result. 

So I am not sure if it is a good idea to unconditionally "reset --merge"
in this situation.

> +# If octopus is inapplicable, make sure we undo any changes we made first
> +cannot_octopus() {
> +       git reset --merge
> +       exit 2
> +}