[Uniques v2] New approvals structure

[jsidorenko]

Idea:

In the V1 of Uniques pallet, the user can grant permission to a 3rd party to move an NFT on his behalf.
If we check the Ethereum ERC-721 spec, there is a similar pattern, but they allow to store a list of approvals instead of a single one.

At the same time, there is a security concern since they don’t reset the approvals on the NFT transfer. That allows an attacker to pre-approve his second wallet, then sell the asset and claim it back.

I’m proposing to go the ERC721 way and have an array of approvals as well, plus reset that list on every item transfer.

Technical implementation:

#[pallet::config]
pub trait Config: frame_system::Config {
	/// The maximum amount of approvals an item could have.
	#[pallet::constant]
	type ApprovalsLimit: Get<u32>;
}

pub type ApprovalsOf<T> = BoundedBTreeMap<
	<T as frame_system::Config>::AccountId,
	Option<<T as frame_system::Config>::BlockNumber>,
	<T as Config>::ApprovalsLimit,
>;

// ----

pub fn approve_transfer(
	origin: OriginFor<T>,
	collection_id: T::CollectionId,
	item_id: T::ItemId,
	delegate: <T::Lookup as StaticLookup>::Source,
	deadline: Option<T::BlockNumber>,
) -> DispatchResult {
	// ----
}

pub fn remove_transfer_approval(
	origin: OriginFor<T>,
	collection_id: T::CollectionId,
	item_id: T::ItemId,
	delegate: <T::Lookup as StaticLookup>::Source,
) -> DispatchResult {
	// ----
}

pub fn clear_all_transfer_approvals(
	origin: OriginFor<T>,
	collection_id: T::CollectionId,
	item_id: T::ItemId,
) -> DispatchResult {
	// ----
}

where an optional BlockNumber would act as a deadline until the approval is valid