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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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31 changes: 29 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 +730,14 @@ 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: the fee is
-- considered /reasonable/ (and the current transaction returned) when
-- @minFee <= txBodyFee <= 2 * minFee@. The lower bound ensures the ledger
-- will accept the fee; the upper bound (twice the minimum) rules out a fee so
-- far above the minimum that it likely signals a calculation error rather than
-- ordinary oscillation. Only when these bounds are not met is
-- 'FeeCalculationDidNotConverge' returned.
calcMinFeeRecursive
:: forall era
. IsEra era
Expand Down Expand Up @@ -790,7 +797,27 @@ 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 "reasonable": at least the ledger-computed minimum and at most twice
-- that minimum (@minFee <= txBodyFee <= 2 * minFee@). The result is still
-- useful even though strict convergence was not confirmed.
--
-- The lower bound ensures the transaction will not be rejected by the ledger
-- for an insufficient fee. The upper bound (2 * minFee) rules out a fee that
-- is so far above the minimum that it likely indicates a calculation error
-- rather than ordinary oscillation. In practice the fee at this point is
-- a ledger-computed minimum from a prior iteration and will be within a tiny
-- fraction of the current minimum (oscillation stems from a few bytes of
-- CBOR encoding difference between iterations).
go 0 unSignTx@(UnsignedTx ledgerTx) utxo' pparams' _ _ _ nExtraWitnesses'
| txBodyFee >= minFee && txBodyFee <= 2 * minFee = do
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