create_scheduling_group / scheduling_group_key_create not exception safe when SG key data constructor throws

[piodul]

There are two cases where data for a scheduling group key can be constructed:

• When the SG key already exists and a new SG is created (create_scheduling_group)
• When a new SG key is being created (scheduling_group_key_create)

In both cases, the SG key data constructor can fail. This is an example test for the second case:

SEASTAR_THREAD_TEST_CASE(sg_key_constructor_exception_when_creating_new_key) {
    struct thrower {
        thrower() {
            throw std::runtime_error("oopsie daisy");
        }
        ~thrower() {
            // Shouldn't get here because the constructor shouldn't succeed
            BOOST_ASSERT(false);
        }
    };
    scheduling_group_key_config thrower_conf = make_scheduling_group_key_config<thrower>();
    BOOST_REQUIRE_THROW(scheduling_group_key_create(thrower_conf).get(), std::runtime_error);
}

Even though the constructor of thrower always throws, its destructor is called anyway:

scheduling_group_test: /home/piodul/code/seastar/tests/unit/scheduling_group_test.cc:392: sg_key_constructor_exception_when_creating_new_key::do_run_test_case()::thrower::~thrower(): Assertion `false' failed.
scheduling_group_test: /home/piodul/code/seastar/tests/unit/scheduling_group_test.cc:392: sg_key_constructor_exception_when_creating_new_key::do_run_test_case()::thrower::~thrower(): Assertion `false' failed.
scheduling_group_test: /home/piodul/code/seastar/tests/unit/scheduling_group_test.cc:392: sg_key_constructor_exception_when_creating_new_key::do_run_test_case()::thrower::~thrower(): Assertion `false' failed.
Aborted (core dumped)

It looks like both functions are not exception safe and cause issues later if the SG key data constructor fails with an exception. I believe that both operations should be able to roll back in case of an exception.