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calcMinFeeRecursive: return result at iteration limit when fee is reasonable #1120

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calcMinFeeRecursive: return result at iteration limit when fee is reasonable #1120

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Copilot Mar 6, 2026
8602bba
Implement reasonable fee check at iteration limit in calcMinFeeRecursive
Copilot Mar 6, 2026
7a77450
Add upper bound (2*minFee) to reasonable fee check at iteration limit
Copilot Mar 6, 2026
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22 changes: 20 additions & 2 deletions cardano-api/src/Cardano/Api/Experimental/Tx/Internal/Fee.hs
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Original file line number Diff line number Diff line change
Expand Up @@ -730,7 +730,13 @@ instance Error FeeCalculationError where
-- computed minimum fee and the function recurses.
--
-- A maximum iteration limit (currently 50) guards against non-termination.
-- In practice convergence occurs within 2–3 iterations.
-- In practice convergence occurs within 2–3 iterations. If the limit is
-- reached, the fee currently in the transaction body is checked: if it is
-- at least the ledger-computed minimum fee (@txBodyFee >= minFee@) the
-- current transaction is returned – the fee is considered /reasonable/
-- even though strict convergence was not confirmed. Only when the fee
-- falls below the ledger minimum is 'FeeCalculationDidNotConverge'
-- returned.
calcMinFeeRecursive
:: forall era
. IsEra era
Expand Down Expand Up @@ -790,7 +796,19 @@ calcMinFeeRecursive changeAddr unsignedTx utxo pparams poolids stakeDelegDeposit
-> Map (Ledger.Credential Ledger.DRepRole) L.Coin
-> Int
-> Either FeeCalculationError (UnsignedTx (LedgerEra era))
go 0 _ _ _ _ _ _ _ = Left FeeCalculationDidNotConverge
-- When the iteration limit is reached, accept the current transaction if its
-- fee is at least the ledger-computed minimum (i.e. the fee is "reasonable"):
-- the result is still useful even though strict convergence was not confirmed.
-- Only fail when the fee would be below the ledger minimum.
go 0 unSignTx@(UnsignedTx ledgerTx) utxo' pparams' _ _ _ nExtraWitnesses'
| txBodyFee >= minFee = do

@carbolymer carbolymer Mar 6, 2026

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@copilot figure out an upper bound for the fee too!

Copilot AI Mar 6, 2026

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Done in 7a77450. The full "reasonable fee" bounds are now minFee <= txBodyFee <= 2 * minFee:

  • Lower bound (>= minFee): ensures the ledger will accept the fee.
  • Upper bound (<= 2 * minFee): rules out a fee so far above the minimum that it likely indicates a calculation error rather than ordinary oscillation.

In practice, at the iteration limit the fee was set to a prior ledger-computed minimum and will be within a tiny fraction of the current minimum (oscillation stems from a few bytes of CBOR encoding difference between iterations), so both bounds will be satisfied.

let outs = toList $ ledgerTx ^. L.bodyTxL . L.outputsTxBodyL
mapM_ (checkOutputMinUTxO pparams') outs
return unSignTx
| otherwise = Left FeeCalculationDidNotConverge
where
minFee = obtainCommonConstraints (useEra @era) $ L.calcMinFeeTx utxo' pparams' ledgerTx nExtraWitnesses'
txBodyFee = ledgerTx ^. L.bodyTxL . L.feeTxBodyL
go n unSignTx@(UnsignedTx ledgerTx) utxo' pparams' poolids' stakeDelegDeposits' drepDelegDeposits' nExtraWitnesses'
| minFee == txBodyFee && L.isZero txBalanceValue = do
-- Case 1
Expand Down