Introduce XcmFeesToAccount fee manager

[KiChjang]

Combination of paritytech/polkadot#7005, its addon PR paritytech/polkadot#7585 and its companion paritytech/cumulus#2433.

This PR introduces a new XcmFeesToAccount struct which implements the FeeManager trait, and assigns this struct as the FeeManager in the XCM config for all runtimes.

The struct simply deposits all fees handled by the XCM executor to a specified account. In all runtimes, the specified account is configured as the treasury account.

XCM delivery fees are now being introduced (unless the root origin is sending a message to a system parachain on behalf of the originating chain).

Note for reviewers

Most file changes are tests that had to be modified to account for the new fees.
Main changes are in:

• cumulus/pallets/xcmp-queue/src/lib.rs <- To make it track the delivery fees exponential factor
• polkadot/xcm/xcm-builder/src/fee_handling.rs <- Added. Has the FeeManager implementation
• All runtime xcm_config files <- To add the FeeManager to the XCM configuration

Important note

After this change, instructions that create and send a new XCM (Query*, Report*, ExportMessage, InitiateReserveWithdraw, InitiateTeleport, DepositReserveAsset, TransferReserveAsset, LockAsset and RequestUnlock) will require the corresponding origin account in the origin register to pay for transport delivery fees, and the onward message will fail to be sent if the origin account does not have the required amount. This delivery fee is in addition to what we already collect as tx fees in pallet-xcm and XCM BuyExecution fees!

Wallet UIs that want to expose the new delivery fee can do so using the formula:

delivery_fee_factor * (base_fee + encoded_msg_len * per_byte_fee)

where the delivery fee factor can be obtained from the corresponding pallet based on which transport you are using (UMP, HRMP or bridges), the base fee is a constant, the encoded message length from the message itself and the per byte fee is the same as the configured per byte fee for txs (i.e. TransactionByteFee).

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/frame-monthly-update/4628/1

[albertov19]

My two cents here.

Although I understand why XCM delivery fees are needed, telling ecosystem developers to calculate the fee with the following formula:

delivery_fee_factor * (base_fee + encoded_msg_len * per_byte_fee)

It is probably the WORST developer experience ever. All chains (through an SDK) should have one standard way of requesting the chain: "How many tokens do I need to be able to send this call successfully?" and the answer should be "XYY amount of tokens."

This new delivery fee is obscured to the paymentInfo endpoint of the Polkadot.js SDK, meaning that dApps should now go and figure it out for themselves to calculate this delivery fee, which is just a terrible developer experience. Moreover, in certain edge cases, this can result in assets being locked in Parachain's sovereign account because the source wallet had enough tokens for the transaction fee but not enough for the delivery fee. For example:

https://assethub-kusama.subscan.io/extrinsic/6284005-2
https://assethub-kusama.subscan.io/extrinsic/6289790-2

This happens because the XCM delivery fee exceeds the account's balance, but this is not checked before executing the call.

[KiChjang]

@albertov19 Appreciate your comment here, and I completely hear you. Certainly a single API that users can query to get delivery fees in the currency you specify is a really nice feature to have, and the XCM team is going to be putting it on the roadmap for this half of the year. I surmise that we would only be able to get the delivery fees part of it right in the beginning; we're going to have to need a bit more time to look further into the feasibility of listing just every single fee you need to pay for sending XCMs.

I do value the community's opinion on XCM related matters, and I do understand that there's currently no single source of information dissemination nor a hub to collect community feedback. We do have the Polkadot forums, but XCM news can easily get drowned with all other happenings in the ecosystem. Thus, on the first half of this year, we will be pushing for creating a sort of XCM info page where we can update you all on the stuff that the XCM team is working on, such as new features or important fee updates like this one. That way, we'll be able to hear from you much sooner and we'd be able to be more responsive to your concerns.

Finally, I'd like to take this brief moment to explain my position on DevEx. While I do see it as an important factor for adoption, we are unfortunately in a resource-constrained context in terms of engineering hours. As such, between better DevEx, optimized solutions and security concerns, I have to always prioritize security and functionality of the blockchain. I always follow the mantra of "first make it work; then make it fast; then make it pretty".

I hope I didn't come across as giving excuses for poor DevEx, but rather help you understand my circumstances.

[xlc]

we implement security feature to prevent people from losing money, not the other way around 💁‍♂️
and speaking about responsive, I raised this issue 10 months ago #690

[albertov19]

@KiChjang Thanks for your response.

I agree security must come first, there is no doubt about that. But to quote you on something: "We are unfortunately in a resource-constrained context in terms of engineering hours."

Parachain teams are in the same position, and current and ongoing problems with XCM and developer experience around it are creating an engineering nightmare in terms of overhead! We are dedicating our engineering hours to ongoing fixes and adaptation to non-documented changes.

https://kusama.subscan.io/extrinsic/21573927-7
#3070