Do not allow upgrading to the same code

[pepyakin]

paritytech/polkadot#4457 will impose an additional constraint on upgrading: the parachain won't be able to upgrade to the same code. While this is silly, if attempted, this will most likely break the parachain. Thus parachain-system should protect the users from doing that.

[bkchr]

Why will that break the parachain? Why isn't that just rejected by the relay chain if it doesn't want this? Just send the signal that the upgrade was rejected?

[pepyakin]

Yeah, this is exactly the problem: the relay-chain will reject such an upgrade. Parachain will break because it won't receive a GoAhead::Abort signal. The relay-chain does not the abort signal because of technical reasons: right now the code structured in such a way, that signals set in schedule_code_upgrade will be removed in note_new_head. I pitched an idea to reverse their order but was told that it is too big of a change.

We can try to work on this issue more, but I figured it would be just easier to forbid this.

[bkchr]

Okay, it will not break, more being stuck in a wait loop for the signal.

[pepyakin]

Ah, that's right, stall.

Which makes me think that we should add some recovery functions for that as well.