Re: [PATCH] read-cache: avoid sparse-index expansion for unborn HEAD

>
> repo_index_has_changes() normally checks whether the index differs from
> a tree by passing that tree to the diff machinery. When no tree is
> passed, it tries to use HEAD for that comparison.
>
> If HEAD does not resolve, as on an unborn branch, the function falls
> back to walking the index directly. With a sparse index, however, sparse
> directory entries may stand in for many paths, so the fallback first
> expands the index before reporting the changed paths.
>
> That expansion is unnecessary. An unborn HEAD is equivalent for this
> check to comparing the index against the empty tree: every index entry
> is new relative to that tree.
>
> Use the empty tree when HEAD cannot be resolved. This keeps the
> unborn-branch case on the same diff code path as the normal
> tree-comparison case, avoiding the sparse-index expansion while still
> letting callers see paths inside sparse directories.
>
> Teach test-tool read-cache to exercise repo_index_has_changes(), and
> add a t1092 check that the unborn-branch case reports paths inside a
> sparse directory without expanding the index.

> @@ -12,6 +13,24 @@ int cmd__read_cache(int argc, const char **argv)
>         int i, cnt = 1;
>         const char *name = NULL;
>
> +       if (argc == 2 && !strcmp(argv[1], "--index-has-changes")) {
> +               struct strbuf sb = STRBUF_INIT;
> +               int ret;
> +
> +               setup_git_directory(the_repository);
> +               repo_config(the_repository, git_default_config, NULL);
> +               prepare_repo_settings(the_repository);
> +               the_repository->settings.command_requires_full_index = 0;
> +
> +               repo_read_index(the_repository);
> +               ret = repo_index_has_changes(the_repository, NULL, &sb);
> +               printf("has_changes=%d\n", ret);
> +               if (sb.len)
> +                       printf("dirty=%s\n", sb.buf);