[FVM] Short Circuit Transactions with insufficient balance

[janezpodhostnik]

Context

Fee deduction currently happens as the last step of the transaction execution. This means that the users transaction is fully executed before failing with insufficient balance on the payers account. This means it is possible to send computationally heavy transactions without having the funds to cover them.

This however also allows the use case where the payer does not have enough funds to pay for the transaction, but withdraws those funds from a different source (withdraw staking rewards) during the transaction, so by the end of the transaction its balance is OK.

Solution

Main point: Transactions where the payer has insufficient balance should fail earlier.

Define "earlier"

Ideally the payers balance would be checked even before the transaction gets included in a collection, so that it doesn't cost any resources on the execution/verification side. This is the next step and not in the scope of this issue.

Even if the transaction's payer's balance is checked before hitting the execution node, the execution node still has to re-check each transaction.

The current transaction execution steps:

• check all signatures
• check sequence number
• regular path:
  • invoke users transactions
  • deduct transaction fees
  • check storage
• failure path (in case of error during the regular path):
  • deduct transaction fees

The transaction execution steps would Ideally look like this:

• Phase 1.
  • check payers signature
  • check sequence number
• Phase 2.
  • regular path
    • check payers balance
    • check other signatures
    • invoke users transactions
    • deduct transaction fees
    • check storage
  • failure path (in case of error during the regular path):
    • deduct transaction fees

A failure in the Phase 1. would mean we cannot charge the payer at all, because this isn't a validly signed transaction and someone might be impersonating the payer or re-sending old transactions in order to drain the payers balance.

A failure in the regular path of Phase 2. means fees will be deducted from the payer even if the payer is below the minimum account balance. This might lead to the payer eventually dropping to 0 balance if the payer keeps sending transactions.

For this issue just the first building block will be added which is the check payers balance step, and the transaction execution steps will look like this:

• check all signatures
• check sequence number
• regular path:
  • check payers balance
  • invoke users transactions
  • deduct transaction fees
  • check storage
• failure path (in case of error during the regular path):
  • deduct transaction fees

Define "insufficient balance"

Insufficient balance means the payers balance is less then:

• Minimum account balance (MAB)
• PLUS transaction inclusion fees
• PLUS maximum transaction execution fees (in respect to the max execution effort (gas) defined by the transaction itself)

Alternatives considered:

• balance is less than inclusion fees => insufficient condition, execution costs could potentially not be charged
• balance is less than inclusion fees + maximum transaction execution fees => if this is lower then the MAB the transaction will fail anyway
• MAB => fees will always be > 0 so transaction will fail if the payers balance is MAB before the transaction

Related changes

There is another connected issues this might tie into. Currently the regular path fee deduction happens before the storage check. The reason why we currently cannot switch to this order is that the payers storage capacity depends on the payers balance. so fees must be deducted first. This also means that we cannot meter and charge fees for the storage check, which is something the user has control over, since touching more account means the storage check runs for longer.

However if we change the check for storage so that we can tell it that the payers balance is actually lower by the fee amount, the ordering can be switched.If we then also say that the payers balance is actually lower by the maximum fee amount during the storage check, then we can meter and charge fees for the storage check.

Definition of Done

• Transactions where payer cannot pay, fail before the transaction body is invoked