Support XCM delivery fees in bridges

[svyatonik]

There's some support for XCM+bridges in https://github.com/paritytech/polkadot/tree/gav-xcm-v3 branch. Just making this issue to not to lose gained knowledge.

So in bridges+XCM:

1. dispatch fee is paid using BuyExecution XCVM instruction, so we don't need to pay it at the source chain. However I have some doubts about dispatch - if it'll be immediate dispatch, then we need to declare dispatch weight in advance (and relayer pays for it in advance). If it'll be just queueing - we'll need to pay for the queueing operation (single db write?). It must be decided before any changes are made;
2. XCM+bridge connector shall return message delivery fee from its SendXcm::validate() implementation;
3. right now the delivery fee is not actually paid when the XCM message is sent - Ok(price) needs to be handled properly here. But when it'll be fixed, we need to decide whether we need a separate relayer strategy? Like what happens with an XCM message when its pre-paid fee becomes lower than the actual cost of all involved transactions? For encoded calls, we have this increase_message_fee call. But for XCM messages, the fee is paid at the upper level (so I assume on messages pallet it'll be zero) => this method doesn't work. Do we need ordered delivery in this case at all? Can we just skip underpaid XCM? This is a bit larger than just fee-related question, but it needs an answer before this work starts. https://github.com/paritytech/polkadot/pull/4097/files#r854844093

[svyatonik]

So we need to pay rewards from treasury paritytech/polkadot#4097 (comment)

UPD1: all rewards in statemint are going to collators (?): https://github.com/paritytech/cumulus/blob/8eee3c37c4f08d48c4109271a13a1c44be565b67/polkadot-parachains/parachains-common/src/impls.rs#L37-L70 - i.e. there's no treasury pallet there . Shall we add treasury pallet to bridge hub parachain and transfer fees there (instead of burning)? Who'll be managing this treasury - relay chain governance? Or shall we just keep relayers reward account (as we did before) and call it 'treasury'?

[svyatonik]

Since we'll be deploying messaging pallet at the parachain, I suggest to defer this until we'll have some bridge-hub parachain template/prototype. Handling this at relay chain is just a waste of time now

[svyatonik]

Current state of this issue: we are not taking any fees (and are not rewarding relayers) at Rialto<>Millau and at RialtoParachain<>Millau bridges. The plan is to discuss it (by opening a PR) when we'll have dedicate chain(s) for RococoBridgeHub/WococoBridgeHub.

[svyatonik]

Solution for initial deployment:

1. no relayer rewards :/;
2. complex relayer running in altruistic mode (meaning on-demand finality);
3. price oracle at Statemine to help compute XCM message cost (bridge fee + dispatch fee at the DOT side).

Long term solution would probably be #2486

@acatangiu @bkontur Please correct me I'm wrong here