Re: [PATCH v3 06/10] diff-lib: define diff_get_merge_base()

From: Junio C Hamano <gitster@pobox.com>
Date: Sep 17, 2020, 17:16 UTC
Message-ID: <xmqqd02ks4qk.fsf@gitster.c.googlers.com>
In-Reply-To: <6aac57ca022963fb41d93905e41dff36dccd5969.1600328335.git.liu.denton@gmail.com>

Denton Liu <liu.denton@gmail.com> writes:

Show 13 quoted lines

> +void diff_get_merge_base(const struct rev_info *revs, struct object_id *mb)
> +{
> +	int i;
> +	struct commit *mb_child[2] = {0};
> +	struct commit_list *merge_bases;
> +
> +	for (i = 0; i < revs->pending.nr; i++) {
> +		struct object *obj = revs->pending.objects[i].item;
> +		if (obj->flags)
> +			die(_("--merge-base does not work with ranges"));
> +		if (obj->type != OBJ_COMMIT)
> +			die(_("--merge-base only works with commits"));
> +	}

This is the first use of die() in this file, that is designed to keep a set of reusable library functions so that the caller(s) can do their own die(). They may want to become

	return error(_(...));

The same comment applies to all die()s the patch adds.

Show 5 quoted lines

> +	/*
> +	 * This check must go after the for loop above because A...B
> +	 * ranges produce three pending commits, resulting in a
> +	 * misleading error message.
> +	 */

Should "git diff --merge-base A...B" be forbidden, or does it just ask the same thing twice and is not a die-worthy offence?

> +	if (revs->pending.nr > ARRAY_SIZE(mb_child))
> +		die(_("--merge-base does not work with more than two commits"));

Compare with hardcoded '2' in the condition. The requirement for mb_child[] is that it can hold at least 2 (changes in the future may find it convenient if its size gets increased to 3 to hold NULL sentinel to signal end-of-list, for example). Comparison with ARRAY_SIZE() may be appropriate in different situations but not here where the code knows it wants to reject more than two no matter how big mb_child[] is.

Show 7 quoted lines

> +	for (i = 0; i < revs->pending.nr; i++)
> +		mb_child[i] = lookup_commit_reference(the_repository, &revs->pending.objects[i].item->oid);
> +	if (revs->pending.nr < ARRAY_SIZE(mb_child)) {
> +		struct object_id oid;
> +
> +		if (revs->pending.nr != 1)
> +			BUG("unexpected revs->pending.nr: %d", revs->pending.nr);

This is an obviously impossible condition as we will not take more than 2.

Show 15 quoted lines

> +		if (get_oid("HEAD", &oid))
> +			die(_("unable to get HEAD"));
> +		mb_child[1] = lookup_commit_reference(the_repository, &oid);
> +	}
> +
> +	merge_bases = repo_get_merge_bases(the_repository, mb_child[0], mb_child[1]);
> +	if (!merge_bases)
> +		die(_("no merge base found"));
> +	if (merge_bases->next)
> +		die(_("multiple merge bases found"));
> +
> +	oidcpy(mb, &merge_bases->item->object.oid);
> +
> +	free_commit_list(merge_bases);
> +}

OK.

Show 12 quoted lines

>  int run_diff_index(struct rev_info *revs, unsigned int option)
>  {
>  	struct object_array_entry *ent;
> diff --git a/diff.h b/diff.h
> index 0cc874f2d5..ae2bb7001a 100644
> --- a/diff.h
> +++ b/diff.h
> @@ -580,6 +580,8 @@ void diff_warn_rename_limit(const char *varname, int needed, int degraded_cc);
>   */
>  const char *diff_aligned_abbrev(const struct object_id *sha1, int);
>  
> +void diff_get_merge_base(const struct rev_info *revs, struct object_id *mb);

Without an actual caller, we cannot judge if this interface is designed right, but we'll see soon in the next steps ;-)

Looking good so far.

Thanks.

> +
>  /* do not report anything on removed paths */
>  #define DIFF_SILENT_ON_REMOVED 01
>  /* report racily-clean paths as modified */