Re: [RFC PATCH 4/7] merge-recursive: fix assumption that head tree being merged is HEAD

[Junio C Hamano <gitster@pobox.com>]

Elijah Newren <newren@gmail.com> writes:

> `git merge-recursive` does a three-way merge between user-specified trees
> base, head, and remote.  Since the user is allowed to specify head, we can
> not necesarily assume that head == HEAD.
>
> We modify index_has_changes() to take an extra argument specifying the
> tree to compare the index to.  If NULL, it will compare to HEAD.  We then
> use this from merge-recursive to make sure we compare to the
> user-specified head.
>
> Signed-off-by: Elijah Newren <newren@gmail.com>
> ---
>
> I'm really unsure where the index_has_changes() declaration should go;
> I stuck it in tree.h, but is there a better spot?

I think I saw you tried to lift an assumption that we're always
working on the_index in a separate patch recently.  Should that
logic apply also to this part of the codebase?  IOW, shouldn't
index_has_changes() take a pointer to istate (as opposed to a
function that uses the implicit the_index that should be named as
"cache_has_changes()" or something?)

I tend to think this function as part of the larger read-cache.c
family whose definitions are in cache.h and accompanied by macros
that are protected by NO_THE_INDEX_COMPATIBILITY_MACROS so if we
were to move it elsewhere, I'd keep the header part as-is and
implementation to read-cache.c to keep it together with the family,
but I do not see a huge issue with the current placement, either.

> diff --git a/cache.h b/cache.h
> index 89a107a7f7..439b9d9f6e 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -634,14 +634,6 @@ extern int discard_index(struct index_state *);
>  extern void move_index_extensions(struct index_state *dst, struct index_state *src);
>  extern int unmerged_index(const struct index_state *);
>  
> -/**
> - * Returns 1 if the index differs from HEAD, 0 otherwise. When on an unborn
> - * branch, returns 1 if there are entries in the index, 0 otherwise. If an
> - * strbuf is provided, the space-separated list of files that differ will be
> - * appended to it.
> - */
> -extern int index_has_changes(struct strbuf *sb);
> -
>  extern int verify_path(const char *path, unsigned mode);
>  extern int strcmp_offset(const char *s1, const char *s2, size_t *first_change);
>  extern int index_dir_exists(struct index_state *istate, const char *name, int namelen);
> diff --git a/merge-recursive.c b/merge-recursive.c
> index b3deb7b182..762aa087d0 100644
> --- a/merge-recursive.c
> +++ b/merge-recursive.c
> @@ -3263,7 +3263,7 @@ int merge_trees(struct merge_options *o,
>  	if (oid_eq(&common->object.oid, &merge->object.oid)) {
>  		struct strbuf sb = STRBUF_INIT;
>  
> -		if (!o->call_depth && index_has_changes(&sb)) {
> +		if (!o->call_depth && index_has_changes(&sb, head)) {
>  			err(o, _("Your local changes to the following files would be overwritten by merge:\n  %s"),
>  			    sb.buf);
>  			return -1;
> diff --git a/merge.c b/merge.c
> index 0783858739..965d287646 100644
> --- a/merge.c
> +++ b/merge.c
> @@ -14,19 +14,21 @@ static const char *merge_argument(struct commit *commit)
>  	return oid_to_hex(commit ? &commit->object.oid : the_hash_algo->empty_tree);
>  }
>  
> -int index_has_changes(struct strbuf *sb)
> +int index_has_changes(struct strbuf *sb, struct tree *tree)
>  {
> -	struct object_id head;
> +	struct object_id cmp;
>  	int i;
>  
> -	if (!get_oid_tree("HEAD", &head)) {
> +	if (tree)
> +		cmp = tree->object.oid;
> +	if (tree || !get_oid_tree("HEAD", &cmp)) {
>  		struct diff_options opt;
>  
>  		diff_setup(&opt);
>  		opt.flags.exit_with_status = 1;
>  		if (!sb)
>  			opt.flags.quick = 1;
> -		do_diff_cache(&head, &opt);
> +		do_diff_cache(&cmp, &opt);
>  		diffcore_std(&opt);
>  		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
>  			if (i)
> diff --git a/t/t6044-merge-unrelated-index-changes.sh b/t/t6044-merge-unrelated-index-changes.sh
> index 3876cfa4fa..1d43712c52 100755
> --- a/t/t6044-merge-unrelated-index-changes.sh
> +++ b/t/t6044-merge-unrelated-index-changes.sh
> @@ -126,7 +126,7 @@ test_expect_success 'recursive, when merge branch matches merge base' '
>  	test_path_is_missing .git/MERGE_HEAD
>  '
>  
> -test_expect_failure 'merge-recursive, when index==head but head!=HEAD' '
> +test_expect_success 'merge-recursive, when index==head but head!=HEAD' '
>  	git reset --hard &&
>  	git checkout C^0 &&
>  
> diff --git a/tree.h b/tree.h
> index e2a80be4ef..2e1d8ea720 100644
> --- a/tree.h
> +++ b/tree.h
> @@ -37,4 +37,12 @@ extern int read_tree_recursive(struct tree *tree,
>  extern int read_tree(struct tree *tree, int stage, struct pathspec *pathspec,
>  		     struct index_state *istate);
>  
> +/**
> + * Returns 1 if the index differs from tree, 0 otherwise.  If tree is NULL,
> + * compares to HEAD.  If tree is NULL and on an unborn branch, returns 1 if
> + * there are entries in the index, 0 otherwise. If an strbuf is provided,
> + * the space-separated list of files that differ will be appended to it.
> + */
> +extern int index_has_changes(struct strbuf *sb, struct tree *tree);
> +
>  #endif /* TREE_H */