Re: [PATCH v3 12/12] hash: stop depending on `the_repository` in `null_oid()`

>
> The `null_oid()` function returns the object ID that only consists of
> zeroes. Naturally, this ID also depends on the hash algorithm used, as
> the number of zeroes is different between SHA1 and SHA256. Consequently,
> the function returns the hash-algorithm-specific null object ID.
>
> This is currently done by depending on `the_hash_algo`, which implicitly
> makes us depend on `the_repository`. Refactor the function to instead
> pass in the hash algorithm for which we want to retrieve the null object
> ID. Adapt callsites accordingly by passing in `the_repository`, thus
> bubbling up the dependency on that global variable by one layer.
>
> There are a couple of trivial exceptions for subsystems that already got
> rid of `the_repository`. These subsystems instead use the repository
> that is available via the calling context:
>
>   - "builtin/grep.c"
>   - "grep.c"
>   - "refs/debug.c"
>
> There are also two non-trivial exceptions:
>
>   - "diff-no-index.c": Here we know that we may not have a repository
>     initialized at all, so we cannot rely on `the_repository`. Instead,
>     we adapt `diff_no_index()` to get a `struct git_hash_algo` as
>     parameter. The only caller is located in "builtin/diff.c", where we
>     know to call `repo_set_hash_algo()` in case we're running outside of
>     a Git repository. Consequently, it is fine to continue passing
>     `the_repository->hash_algo` even in this case.
>
>   - "builtin/ls-files.c": There is an in-flight patch series that drops
>     `USE_THE_REPOSITORY_VARIABLE` in this file, which causes a semantic
>     conflict because we use `null_oid()` in `show_submodule()`. The
>     value is passed to `repo_submodule_init()`, which may use the object
>     ID to resolve a tree-ish in the superproject from which we want to
>     read the submodule config. As such, the object ID should refer to an
>     object in the superproject, and consequently we need to use its hash
>     algorithm.
>
> This means that we could in theory just not bother about this edge case
> at all and just use `the_repository` in "diff-no-index.c". But doing so
> would feel misdesigned.

> diff --git a/merge-ort.c b/merge-ort.c
> index 46e78c3ffa6..eb3a834652a 100644
> --- a/merge-ort.c
> +++ b/merge-ort.c
> @@ -1817,7 +1817,7 @@ static int merge_submodule(struct merge_options *opt,
>                 BUG("submodule deleted on one side; this should be handled outside of merge_submodule()");
>
>         if ((sub_not_initialized = repo_submodule_init(&subrepo,
> -               opt->repo, path, null_oid()))) {
> +               opt->repo, path, null_oid(the_hash_algo)))) {
>                 path_msg(opt, CONFLICT_SUBMODULE_NOT_INITIALIZED, 0,
>                          path, NULL, NULL, NULL,
>                          _("Failed to merge submodule %s (not checked out)"),
> @@ -2199,7 +2199,7 @@ static int handle_content_merge(struct merge_options *opt,
>                 two_way = ((S_IFMT & o->mode) != (S_IFMT & a->mode));
>
>                 merge_status = merge_3way(opt, path,
> -                                         two_way ? null_oid() : &o->oid,
> +                                         two_way ? null_oid(the_hash_algo) : &o->oid,
>                                           &a->oid, &b->oid,
>                                           pathnames, extra_marker_size,
>                                           &result_buf);
> @@ -2231,7 +2231,7 @@ static int handle_content_merge(struct merge_options *opt,
>         } else if (S_ISGITLINK(a->mode)) {
>                 int two_way = ((S_IFMT & o->mode) != (S_IFMT & a->mode));
>                 clean = merge_submodule(opt, pathnames[0],
> -                                       two_way ? null_oid() : &o->oid,
> +                                       two_way ? null_oid(the_hash_algo) : &o->oid,
>                                         &a->oid, &b->oid, &result->oid);
>                 if (clean < 0)
>                         return -1;
> @@ -2739,7 +2739,7 @@ static void apply_directory_rename_modifications(struct merge_options *opt,
>                 assert(!new_ci->match_mask);
>                 new_ci->dirmask = 0;
>                 new_ci->stages[1].mode = 0;
> -               oidcpy(&new_ci->stages[1].oid, null_oid());
> +               oidcpy(&new_ci->stages[1].oid, null_oid(the_hash_algo));
>
>                 /*
>                  * Now that we have the file information in new_ci, make sure
> @@ -2752,7 +2752,7 @@ static void apply_directory_rename_modifications(struct merge_options *opt,
>                                 continue;
>                         /* zero out any entries related to files */
>                         ci->stages[i].mode = 0;
> -                       oidcpy(&ci->stages[i].oid, null_oid());
> +                       oidcpy(&ci->stages[i].oid, null_oid(the_hash_algo));
>                 }
>
>                 /* Now we want to focus on new_ci, so reassign ci to it. */
> @@ -3122,7 +3122,7 @@ static int process_renames(struct merge_options *opt,
>                         if (type_changed) {
>                                 /* rename vs. typechange */
>                                 /* Mark the original as resolved by removal */
> -                               memcpy(&oldinfo->stages[0].oid, null_oid(),
> +                               memcpy(&oldinfo->stages[0].oid, null_oid(the_hash_algo),
>                                        sizeof(oldinfo->stages[0].oid));
>                                 oldinfo->stages[0].mode = 0;
>                                 oldinfo->filemask &= 0x06;
> @@ -3994,7 +3994,7 @@ static int process_entry(struct merge_options *opt,
>                         if (ci->filemask & (1 << i))
>                                 continue;
>                         ci->stages[i].mode = 0;
> -                       oidcpy(&ci->stages[i].oid, null_oid());
> +                       oidcpy(&ci->stages[i].oid, null_oid(the_hash_algo));
>                 }
>         } else if (ci->df_conflict && ci->merged.result.mode != 0) {
>                 /*
> @@ -4041,7 +4041,7 @@ static int process_entry(struct merge_options *opt,
>                                 continue;
>                         /* zero out any entries related to directories */
>                         new_ci->stages[i].mode = 0;
> -                       oidcpy(&new_ci->stages[i].oid, null_oid());
> +                       oidcpy(&new_ci->stages[i].oid, null_oid(the_hash_algo));
>                 }
>
>                 /*
> @@ -4163,11 +4163,11 @@ static int process_entry(struct merge_options *opt,
>                         new_ci->merged.result.mode = ci->stages[2].mode;
>                         oidcpy(&new_ci->merged.result.oid, &ci->stages[2].oid);
>                         new_ci->stages[1].mode = 0;
> -                       oidcpy(&new_ci->stages[1].oid, null_oid());
> +                       oidcpy(&new_ci->stages[1].oid, null_oid(the_hash_algo));
>                         new_ci->filemask = 5;
>                         if ((S_IFMT & b_mode) != (S_IFMT & o_mode)) {
>                                 new_ci->stages[0].mode = 0;
> -                               oidcpy(&new_ci->stages[0].oid, null_oid());
> +                               oidcpy(&new_ci->stages[0].oid, null_oid(the_hash_algo));
>                                 new_ci->filemask = 4;
>                         }
>
> @@ -4175,11 +4175,11 @@ static int process_entry(struct merge_options *opt,
>                         ci->merged.result.mode = ci->stages[1].mode;
>                         oidcpy(&ci->merged.result.oid, &ci->stages[1].oid);
>                         ci->stages[2].mode = 0;
> -                       oidcpy(&ci->stages[2].oid, null_oid());
> +                       oidcpy(&ci->stages[2].oid, null_oid(the_hash_algo));
>                         ci->filemask = 3;
>                         if ((S_IFMT & a_mode) != (S_IFMT & o_mode)) {
>                                 ci->stages[0].mode = 0;
> -                               oidcpy(&ci->stages[0].oid, null_oid());
> +                               oidcpy(&ci->stages[0].oid, null_oid(the_hash_algo));
>                                 ci->filemask = 2;
>                         }
>
> @@ -4304,7 +4304,7 @@ static int process_entry(struct merge_options *opt,
>                 /* Deleted on both sides */
>                 ci->merged.is_null = 1;
>                 ci->merged.result.mode = 0;
> -               oidcpy(&ci->merged.result.oid, null_oid());
> +               oidcpy(&ci->merged.result.oid, null_oid(the_hash_algo));
>                 assert(!ci->df_conflict);
>                 ci->merged.clean = !ci->path_conflict;
>         }

> diff --git a/merge-recursive.c b/merge-recursive.c
> index 5dfaf32b2c1..d88e61b07a9 100644
> --- a/merge-recursive.c
> +++ b/merge-recursive.c
> @@ -502,7 +502,7 @@ static int get_tree_entry_if_blob(struct repository *r,
>
>         ret = get_tree_entry(r, tree, path, &dfs->oid, &dfs->mode);
>         if (S_ISDIR(dfs->mode)) {
> -               oidcpy(&dfs->oid, null_oid());
> +               oidcpy(&dfs->oid, null_oid(the_hash_algo));
>                 dfs->mode = 0;
>         }
>         return ret;
> @@ -1238,7 +1238,7 @@ static int merge_submodule(struct merge_options *opt,
>         if (is_null_oid(b))
>                 return 0;
>
> -       if (repo_submodule_init(&subrepo, opt->repo, path, null_oid())) {
> +       if (repo_submodule_init(&subrepo, opt->repo, path, null_oid(the_hash_algo))) {
>                 output(opt, 1, _("Failed to merge submodule %s (not checked out)"), path);
>                 return 0;
>         }
> @@ -1698,7 +1698,7 @@ static int handle_file_collision(struct merge_options *opt,
>
>         /* Store things in diff_filespecs for functions that need it */
>         null.path = (char *)collide_path;
> -       oidcpy(&null.oid, null_oid());
> +       oidcpy(&null.oid, null_oid(the_hash_algo));
>         null.mode = 0;
>
>         if (merge_mode_and_contents(opt, &null, a, b, collide_path,
> @@ -2898,14 +2898,14 @@ static int process_renames(struct merge_options *opt,
>                         dst_other.mode = ren1->dst_entry->stages[other_stage].mode;
>                         try_merge = 0;
>
> -                       if (oideq(&src_other.oid, null_oid()) &&
> +                       if (oideq(&src_other.oid, null_oid(the_hash_algo)) &&
>                             ren1->dir_rename_original_type == 'A') {
>                                 setup_rename_conflict_info(RENAME_VIA_DIR,
>                                                            opt, ren1, NULL);
>                         } else if (renamed_to_self) {
>                                 setup_rename_conflict_info(RENAME_NORMAL,
>                                                            opt, ren1, NULL);
> -                       } else if (oideq(&src_other.oid, null_oid())) {
> +                       } else if (oideq(&src_other.oid, null_oid(the_hash_algo))) {
>                                 setup_rename_conflict_info(RENAME_DELETE,
>                                                            opt, ren1, NULL);
>                         } else if ((dst_other.mode == ren1->pair->two->mode) &&
> @@ -2924,7 +2924,7 @@ static int process_renames(struct merge_options *opt,
>                                                       1, /* update_cache */
>                                                       0  /* update_wd    */))
>                                         clean_merge = -1;
> -                       } else if (!oideq(&dst_other.oid, null_oid())) {
> +                       } else if (!oideq(&dst_other.oid, null_oid(the_hash_algo))) {
>                                 /*
>                                  * Probably not a clean merge, but it's
>                                  * premature to set clean_merge to 0 here,