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xcm-executor: take transport fee from transferred assets if necessary #4834

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xcm-executor: take transport fee from transferred assets if necessary #4834

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Commits on Dec 2, 2024

  1. Sending XCM messages to other chains requires paying a "transport fee". 

    This can be paid either:
- from `origin` local account if `jit_withdraw = true`,
- taken from Holding register otherwise.

This currently works for following hops/scenarios:
1. On destination no transport fee needed (only sending costs, not receiving),
2. Local/originating chain: just set JIT=true and fee will be paid from signed account,
3. Intermediary hops - only if intermediary is acting as reserve between two untrusted
chains (aka only for `DepositReserveAsset` instruction) - this was fixed in
paritytech#3142

But now we're seeing more complex asset transfers that are mixing reserve transfers
with teleports depending on the involved chains.

E.g. transferring DOT between Relay and parachain, but through AH (using AH instead
of the Relay chain as parachain's DOT reserve).

In the `Parachain --1--> AssetHub --2--> Relay` scenario, DOT has to be reserve-withdrawn
in leg `1`, then teleported in leg `2`.
On the intermediary hop (AssetHub), `InitiateTeleport` fails to send onward message because
of missing transport fees. We also can't rely on `jit_withdraw` because the original origin
is lost on the way, and even if it weren't we can't rely on the user having funded accounts
on each hop along the way.

- Charge the transport fee in the executor from the transferred assets (if available),
- Only charge from transferred assets if JIT_WITHDRAW was not set,
- Only charge from transferred assets if Holding doesn't already contain enough (other)
assets to pay for the transport fee.

Added regression tests in emulated transfers.

Fixes paritytech#4832

Signed-off-by: Adrian Catangiu <[email protected]>
    @acatangiu
    acatangiu committed Dec 2, 2024
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