[AHM] migrate Teleports Checking account for DOT/KSM from Relay to AH

[acatangiu]

Polkadot runtimes use a special "checking" account to track oubound and inbound teleports.

This account is especially useful when tracking assets native to the local chain, as it enforces that amount teleported "back in" cannot surpass the amount teleported "out".

During/after AHM, we need to switch the behavior of this checking account for DOT/KSM on the Relay chain and Asset Hub:

Teleports Checking account tracking DOT/KSM:

AHM	Relay	AH
Before	OUT	IN
After	IN	OUT

The checking account balance also needs to change as it changes meaning between debit and credit.

[acatangiu]

Investigation

All chains use the same account as checking account. So we don't need to do any special mapping during migration.

On the Relay Chain, the checking account is used for DOT with MintLocal meaning DOT teleported out is locally tracked in the checking account by minting same amount to it.
This also allows the total_issuance for DOT on the Relay chain to reflect the actual total issuance.

On Asset Hub (and same for all other system parachains), DOT teleports are not tracked through checking account. This makes sense because it'd be superfluous, AH only gets DOT by teleporting IN and can never teleport OUT more than that.

On-chain state also confirms the above: Relay, Asset Hub, Bridge Hub, Coretime, Collectives, People.

Conclusions

1. Migration of funds within checking account falls under base case of generic account migration: DOTs in this acc on Relay will simply be teleported over to the same checking account on Asset Hub.
2. No special action needed on the account on AH.
3. AHM has no implications for the other System Chains' checking accounts.

But even though the balance of this account will be correctly migrated using implicit mechanism, the runtime configurations of Relay and AH need to change how they use this account before, during and after the migration. More details below.

DOT Teleports tracking in Checking Account	Relay	AH
Before	Yes, MintLocal	No Checking
During	No Checking	No Checking
After	No Checking	Yes, MintLocal

Action Items

• create multiple FungibleAdapters for Relay and AH according to the table above to be used before, during and after migration
• integrate them with ahm-coordination pallet so the right one is used as the AssetTransactor during the various stages of the migration

[acatangiu]

Migrating DOT total issuance tracking from RC to AH

Relay Chain:

• on migration start:
  • teleports IN or OUT are suspended/disallowed, teleport tracking is suspended, which is functionally equivalent to total issuance snapshot-ed
• on migration finish:
  • teleports are enabled both in and out,
  • no longer tracking teleports in checking account, the source of truth for DOT issuance has moved to AH

Asset Hub:

• on migration start:
  • teleports IN or OUT are suspended/disallowed,
  • existing total DOT issuance on AH is noted: ah_issuance_before
  • accounts are migrated, including checking account balance rc_checking_balance_before
  • AH checking balance is adjusted: ah_checking_balance_after = rc_checking_balance_before - ah_issuance_before
• on migration finish:
  • AH total issuance after now matches RC total issuance before
  • teleports are enabled both in and out, with tracking of IN/OUT to maintain total issuance

[karolk91]

@acatangiu ,

with regards to:

Asset Hub:

  * accounts are migrated, including checking account balance `rc_checking_balance_before`
  * AH checking balance is adjusted: `ah_checking_balance_after = rc_checking_balance_before - ah_issuance_before`

Shouldn't ah_checking_balance_after simply become rc_checking_balance_before (considering DOT only) ? so its ah_checking_balance_after = rc_checking_balance_before

Why would there be a need for adjustments ? As per my understanding, checking account on Relay as of today is the total DOT in the system (relay + system chains). Is there a possibility that current Asset Hub minted some DOTs without Relay Chain being aware of that ?

[karolk91]

checking account on Relay as of today is the total DOT in the system (relay + system chains)

sorry, confused myself, I meant "all balances (including checking account) = total DOT"

[acatangiu]

As per my understanding, checking account on Relay as of today is the total DOT in the system (relay + system chains).

No, checking account on Relay holds the total DOT in the other system chains. On Relay when you add all accounts (all local accounts + checking which is all dot teleported out) you get total DOT overall.

On AH post migration we want checking to hold total DOT in other system chains (and Relay):

Arithmetics explanation:

At this point, because checking account was completely migrated:
	`checking_balance` = ah_check_before + rc_check_before
	(0) rc_check_before = `checking_balance` - ah_check_before

Invariants:
	(1) rc_check_before = sum_total_before(ah, bh, collectives, coretime, people)
	(2) rc_check_before = sum_total_before(bh, collectives, coretime, people) + ah_total_before
Because teleports are disabled for RC and AH during migration, we can say:
	(3) sum_total_before(bh, collectives, coretime, people) = sum_total_after(bh, collectives, coretime, people)
Ergo use (3) in (2):
	(4) rc_check_before = sum_total_after(bh, collectives, coretime, people) + ah_total_before

We want:
	ah_check_after = sum_total_after(rc, bh, collectives, coretime, people)
	ah_check_after = sum_total_after(bh, collectives, coretime, people) + rc_balance_kept
Use (3):
	ah_check_after = sum_total_before(bh, collectives, coretime, people) + rc_balance_kept
	ah_check_after = sum_total_before(ah, bh, collectives, coretime, people) - ah_total_before + rc_balance_kept
Use (1):
	ah_check_after = rc_check_before - ah_total_before + rc_balance_kept
Use (0):
	ah_check_after = `checking_balance` - ah_check_before - ah_total_before + rc_balance_kept
	ah_check_after = `checking_balance` + rc_balance_kept - ah_total_before - ah_check_before

[acatangiu]

Fixed in polkadot-fellows/runtimes#607 and polkadot-fellows/runtimes#636