Disable asset transfers on Statemint (against Statemint release branch v6.0.1)

[KiChjang]

#834

[gavofyork]

This is safe, but I don't know if we want to disable the reserve asset transfer or just the teleporting?

[KiChjang]

Hmm ok, if we disable reserve asset transfers, that means no one would be able to transfer reserve assets via the reserve_transfer_assets extrinsic on the XCM pallet. However, this does not mean InitiateReserveWithdraw XCMs are going to be disabled, since they're processed on the source chain first, and then subsequently a WithdrawAsset instruction is sent to Statemint and then executed, and the XcmReserveTransferFilter is not used.

All this is to say that disabling XcmReserveTransferFilter only affects whether or not users are able to dispatch reserve_transfer_assets on the XCM pallet on Statemint, and other XCM operations dealing with reserve assets are not affected. I can see that it can be a prudent step for us to take to set XcmReserveTransferFilter to Nothing if we are being conservative here and do not want to have any funny business going on with people attempting to exploit reserve_transfer_assets.

[joepetrowski]

Unless we change paritytech/polkadot#4475 to allow reserve transfers to Statemint, I don't think it matters here as no DOT would arrive on Statemint (and thus asset creation isn't possible either because nobody could pay the deposit). I would rather restrict them so that opening them on the Relay Chain doesn't have the effect of making them bi-directional. That is, we allow teleports/reserve asset transfer on Statemint only after allowing them from the Relay Chain.