Re: [PATCH v5 2/2] gc: introduce `gc.recentObjectsHook`

> +gc.recentObjectsHook::
> +	When considering whether or not to remove an object (either when
> +	generating a cruft pack or storing unreachable objects as
> +	loose), use the shell to execute the specified command(s).
> +	Interpret their output as object IDs which Git will consider as
> +	"recent", regardless of their age. By treating their mtimes as
> +	"now", any objects (and their descendants) mentioned in the
> +	output will be kept regardless of their true age.

> +static int run_one_gc_recent_objects_hook(struct oidset *set,
> +					    const char *args)
> +{
> +	struct child_process cmd = CHILD_PROCESS_INIT;
> +	struct strbuf buf = STRBUF_INIT;
> +	FILE *out;
> +	int ret = 0;
> +
> +	cmd.use_shell = 1;
> +	cmd.out = -1;
> +
> +	strvec_push(&cmd.args, args);
> +
> +	if (start_command(&cmd))
> +		return -1;
> +
> +	out = xfdopen(cmd.out, "r");
> +	while (strbuf_getline(&buf, out) != EOF) {
> +		struct object_id oid;
> +		const char *rest;
> +
> +		if (parse_oid_hex(buf.buf, &oid, &rest) || *rest) {
> +			ret = error(_("invalid extra cruft tip: '%s'"), buf.buf);

> +static void load_gc_recent_objects(struct recent_data *data)
> +{
> +	const struct string_list *programs;
> +	int ret = 0;
> +	size_t i;
> +
> +	data->extra_recent_oids_loaded = 1;
> +
> +	if (git_config_get_string_multi("gc.recentobjectshook", &programs))
> +		return;
> +
> +	for (i = 0; i < programs->nr; i++) {
> +		ret = run_one_gc_recent_objects_hook(&data->extra_recent_oids,
> +						       programs->items[i].string);
> +		if (ret)
> +			die(_("unable to enumerate additional recent objects"));

> +test_expect_success 'gc.recentObjectsHook' '
> +	git init repo &&
> +	test_when_finished "rm -fr repo" &&
> +	(
> +		cd repo &&
> +
> +		# Create a handful of objects.
> +		#
> +		#   - one reachable commit, "base", designated for the reachable
> +		#     pack
> +		#   - one unreachable commit, "cruft.discard", which is marked
> +		#     for deletion
> +		#   - one unreachable commit, "cruft.old", which would be marked
> +		#     for deletion, but is rescued as an extra cruft tip

> +test_expect_success 'multi-valued gc.recentObjectsHook' '
> +	git init repo &&
> +	test_when_finished "rm -fr repo" &&
> +	(
> +		cd repo &&
> +
> +		test_commit base &&
> +		git branch -M main &&
> +
> +		git checkout --orphan cruft.a &&
> +		git rm -fr . &&
> +		test_commit --no-tag cruft.a &&
> +		cruft_a="$(git rev-parse HEAD)" &&
> +
> +		git checkout --orphan cruft.b &&
> +		git rm -fr . &&
> +		test_commit --no-tag cruft.b &&
> +		cruft_b="$(git rev-parse HEAD)" &&
> +
> +		git checkout main &&
> +		git branch -D cruft.a cruft.b &&
> +		git reflog expire --all --expire=all &&
> +
> +		echo "echo $cruft_a" | write_script extra-tips.a &&
> +		echo "echo $cruft_b" | write_script extra-tips.b &&
> +		echo "false" | write_script extra-tips.c &&
> +
> +		git rev-list --objects --no-object-names $cruft_a $cruft_b \
> +			>cruft.raw &&
> +		sort cruft.raw >cruft.expect &&
> +
> +		# ensure that each extra cruft tip is saved by its