Re: [PATCH 3/3] builtin/pack-objects.c: respect 'pack.preferBitmapTips'

> When writing a new pack with a bitmap, it is sometimes convenient to
> indicate some reference prefixes which should receive priority when
> selecting which commits to receive bitmaps.
> 
> A truly motivated caller could accomplish this by setting
> 'pack.islandCore', (since all commits in the core island are similarly
> marked as preferred) but this requires callers to opt into using delta
> islands, which they may or may not want to do.
> 
> Introduce a new multi-valued configuration, 'pack.preferBitmapTips' to
> allow callers to specify a list of reference prefixes. All references
> which have a prefix contained in 'pack.preferBitmapTips' will mark their
> tips as "preferred" in the same way as commits are marked as preferred
> for selection by 'pack.islandCore'.

> +static int mark_bitmap_preferred_tip(const char *refname,
> +				     const struct object_id *oid, int flags,
> +				     void *_data)
> +{
> +	struct object_id peeled;
> +	struct object *object;
> +
> +	if (!peel_iterated_oid(oid, &peeled))
> +		oid = &peeled;
> +
> +	object = parse_object_or_die(oid, refname);
> +	if (object->type == OBJ_COMMIT)
> +		object->flags |= NEEDS_BITMAP;
> +
> +	return 0;
> +}

> +static void mark_bitmap_preferred_tips(void)
> +{
> +	struct string_list_item *item;
> +	const struct string_list *preferred_tips;
> +
> +	preferred_tips = bitmap_preferred_tips(the_repository);
> +	if (!preferred_tips)
> +		return;
> +
> +	for_each_string_list_item(item, preferred_tips) {
> +		for_each_ref_in(item->string, mark_bitmap_preferred_tip, NULL);
> +	}
> +}

> +test_expect_success 'pack.preferBitmapTips' '
> +	git init repo &&
> +	test_when_finished "rm -fr repo" &&
> +	(
> +		cd repo &&
> +
> +		# create enough commits that not all are receive bitmap
> +		# coverage even if they are all at the tip of some reference.
> +		test_commit_bulk --message="%s" 103 &&
> +
> +		git rev-list HEAD >commits.raw &&
> +		sort <commits.raw >commits &&
> +
> +		git log --format="create refs/tags/%s %H" HEAD >refs &&
> +		git update-ref --stdin <refs &&
> +
> +		git repack -adb &&
> +		test-tool bitmap list-commits | sort >bitmaps &&
> +
> +		# remember which commits did not receive bitmaps
> +		comm -13 bitmaps commits >before &&
> +		test_file_not_empty before &&
> +
> +		# mark the commits which did not receive bitmaps as preferred,
> +		# and generate the bitmap again
> +		perl -pe "s{^}{create refs/tags/include/$. }" <before |
> +			git update-ref --stdin &&
> +		git -c pack.preferBitmapTips=refs/tags/include repack -adb &&