support transfering multiple assets(now only one) in orml-xtokens

[yrong]

related issues and prs:

paritytech/cumulus#827
paritytech/cumulus#806
paritytech/polkadot#4470

wrap xcm as following when send statemine asset back:
https://github.com/bifrost-finance/bifrost/blob/c7eb1bef7abfdeb6562aeda29ab76805308b77f5/pallets/salp/src/lib.rs?_pjax=%23js-repo-pjax-container%2C%20div%5Bitemtype%3D%22http%3A%2F%2Fschema.org%2FSoftwareSourceCode%22%5D%20main%2C%20%5Bdata-pjax-container%5D#L1012-L1031

[girazoki]

Just wanted to cross-post some a thought I had in bifrost-finance/bifrost#420 here too

HI! Just curious about the same problem (sending statemine assets back to statemine) and I am wondering a few things about this implementation:

• In transfer_statemine_assets, this PR is doing initateReserveWithdraw of both the statemine asset and the relay token (to be able to pay in for fees in statemine). This therefore implies that the sovereign account in Statemine holds both tokens (to be able to withdraw them?).

• If the above is true, wouldnt we have two sovereign accounts (the one in statemint, and the one in the relay) holding the relay token? And since both match to the same token in the bifrost parachain, wouldnt this imply that we could face the situation where a user transfers to the relay chain more tokens than what the sovereign account in the relay holds?

[xlc]

The reserve chain of KSM is Kusama, not Statemine, so we will simply reject direct KSM transfer from Statemine.

[girazoki]

How are you going to handle sending statrmine assets back to statemine then? given that statemine fees can only be paid in KSM, I am guessing the sovereign account in statemine will need to have KSM and the transferred asset somehow.

[xlc]

Yeah XCM basically doesn't offer much ways to manage fees on sovereign account (e.g. you can't really track how much fee is charged). So the simple solution is just topup the sovereign account on Statemine somehow. It can be done manually, or should be possible to do a reserve transfer of KSM to Kusama and teleport it to Statemine parachain sovereign account.

[yrong]

resolved in #658