Backward incompatibility between XCM v0 and v1

[xlc]

polkadot/xcm/src/v1/order.rs

Line 149 in aeea9b7

fees: MultiAsset,

This is v1::MultiAsset which is no compatible with v0::MultiAsset.

On Karura 1011 (The current live runtime), we are using v0::MultiAsset::All and it failed to be decoded on Kusama 9100.

https://github.com/open-web3-stack/open-runtime-module-library/blob/4bd9eed87814e74aa94720f38af0b7328aed2771/xtokens/src/lib.rs#L217

[KiChjang]

IIRC we specifically did not want to allow MultiAsset::All to be used in the fees field in BuyExecution, the justification here being that it is a potential hazard for XCM executors to use up all the transferred assets for fee payment. Thus, starting from V1, we need to always specify the exact amount of assets that we are willing to set aside just for fees, so that we limit the amount of spending that can arise from XCM execution.

[gavofyork]

See #2815 for information on incompatibilities and #3578 for the fix in Polkadot.