[xcm-v5] implement RFC#122: InitiateTransfer can alias XCM original origin on destination

[acatangiu]

Built on top of #5876

Description

Currently, all XCM asset transfer instructions ultimately clear the origin in the remote XCM message by use of the ClearOrigin instruction. This is done for security considerations to ensure that subsequent (user-controlled) instructions cannot command the authority of the sending chain.

The problem with this approach is that it limits what can be achieved on remote chains through XCM. Most XCM operations require having an origin, and following any asset transfer the origin is lost, meaning not much can be done other than depositing the transferred assets to some local account or transferring them onward to another chain.

For example, we cannot transfer some funds for buying execution, then do a Transact (all in the same XCM message).

In the case of XCM programs going from source-chain directly to dest-chain without an intermediary hop, we can enable scenarios such as above by using the AliasOrigin instruction instead of the ClearOrigin instruction.

Instead of clearing the source-chain origin, the destination chain shall attempt to alias source-chain to "original origin" on the source chain. Most common such origin aliasing would be X1(Parachain(source-chain)) -> X2(Parachain(source-chain), AccountId32(origin-account)) for the case of a single hop transfer where the initiator is a (signed/pure/proxy) account origin-account on source-chain. This is equivalent to using the DescendOrigin instruction in this case, but also usable in the multi hop case.

This allows an actor on chain A to Transact on chain B without having to prefund its SA account on chain B, instead they can simply transfer the required fees in the same XCM program as the Transact.

As long as the asset transfer has the same XCM route/hops as the rest of the program, this pattern of usage can be composed across multiple hops, to ultimately Transact on the final hop using the original origin on the source chain, effectively abstracting away any intermediary hops.

XCM InitiateAssetsTransfer instruction changes

A new parameter preserve_origin to be added to the InitiateAssetsTransfer XCM instruction that specifies if the original origin should be preserved or cleared.

InitiateAssetsTransfer {
	destination: Location,
	assets: Vec<AssetTransferFilter>,
	remote_fees: Option<AssetTransferFilter>,
+	preserve_origin: bool,
	remote_xcm: Xcm<()>,
}

This parameter is explicitly necessary because the instruction should be usable between any two chains regardless of their origin-aliasing trust relationship. Preserving the origin requires some level of trust, while clearing it works regardless of that relationship.
Specifying preserve_origin: false will always work regardless of the configured alias filters of the
involved chains.

Testing

• e2e test: User on PenpalA registers foreign token (transacts) on PenpalB through XCM, while paying all fees using USDT (meaning XCM has to go through AssetHub) - AH carries over the original origin, effectively being a transparent proxy,
• e2e test: User/contract on Ethereum registers foreign token (transacts) on Polkadot-PenpalA through XCM (over bridge), while paying all fees using DOT (has to go through AssetHub) - AH carries over the original origin, effectively being a transparent proxy for Ethereum,

[franciscoaguirre]

bot fmt

[franciscoaguirre]

bot bench cumulus-assets --subcommand=xcm --runtime=asset-hub-westend --pallet=pallet_xcm_benchmarks::generic
bot bench cumulus-assets --subcommand=xcm --runtime=asset-hub-rococo --pallet=pallet_xcm_benchmarks::generic

bot bench cumulus-assets --subcommand=xcm --runtime=asset-hub-westend --pallet=pallet_xcm_benchmarks::fungible
bot bench cumulus-assets --subcommand=xcm --runtime=asset-hub-rococo --pallet=pallet_xcm_benchmarks::fungible

[acatangiu]

bot clean