Auto-balance multiasset transactions

Previously we gave up when the non-Ada part of a transaction wasn't
balanced. We now balance the transaction and correctly update the fee
accordingly (since the fee will be higher). We also return an error in
the case where the is non-Ada change, but not at least minUTxO
change (e.g. in the case where the Ada is already balanced).

Resolves: #3068

Author: Robert 'Probie' Offner

cardano-api/ChangeLog.md

@@ -6,6 +6,8 @@
 
 - Append, not prepend change output when balancing a transaction ([PR 4343](https://github.com/input-output-hk/cardano-node/pull/4343))
 
+- Auto-balance multi asset transactions ([PR 4450](https://github.com/input-output-hk/cardano-node/pull/4450))
+
 ### Bugs
 
 - Allow reading text envelopes from pipes ([PR 4384](https://github.com/input-output-hk/cardano-node/pull/4384))

cardano-api/src/Cardano/Api/Fees.hs

@@ -971,13 +971,23 @@ makeTransactionBodyAutoBalance eraInMode systemstart history pparams
     -- output and fee. Yes this means this current code will only work for
     -- final fee of less than around 4000 ada (2^32-1 lovelace) and change output
     -- of less than around 18 trillion ada  (2^64-1 lovelace).
+    -- However, since at this point we know how much non-Ada change to give
+    -- we can use the true values for that.
+
+    let outgoingNonAda = mconcat [filterValue isNotAda v | (TxOut _ (TxOutValue _ v) _ _) <- txOuts txbodycontent]
+    let incomingNonAda = mconcat [filterValue isNotAda v | (TxOut _ (TxOutValue _ v) _ _) <- Map.elems $ unUTxO utxo]
+    let nonAdaChange = incomingNonAda <> negateValue outgoingNonAda
+
+    let changeTxOut = case multiAssetSupportedInEra cardanoEra of
+          Left _ -> lovelaceToTxOutValue $ Lovelace (2^(64 :: Integer)) - 1
+          Right multiAsset -> TxOutValue multiAsset (lovelaceToValue (Lovelace (2^(64 :: Integer)) -1) <> nonAdaChange)
 
     let (dummyCollRet, dummyTotColl) = maybeDummyTotalCollAndCollReturnOutput txbodycontent changeaddr
     txbody1 <- first TxBodyError $ -- TODO: impossible to fail now
                makeTransactionBody txbodycontent1 {
                  txFee  = TxFeeExplicit explicitTxFees $ Lovelace (2^(32 :: Integer) - 1),
                  txOuts = TxOut changeaddr
-                                (lovelaceToTxOutValue $ Lovelace (2^(64 :: Integer)) - 1)
+                                changeTxOut
                                 TxOutDatumNone ReferenceScriptNone
                         : txOuts txbodycontent,
                  txReturnCollateral = dummyCollRet,
@@ -1009,13 +1019,7 @@ makeTransactionBodyAutoBalance eraInMode systemstart history pparams
 
     -- check if the balance is positive or negative
     -- in one case we can produce change, in the other the inputs are insufficient
-    case balance of
-      TxOutAdaOnly _ _ -> balanceCheck balance
-      TxOutValue _ v   ->
-        case valueToLovelace v of
-          Nothing -> Left $ TxBodyErrorNonAdaAssetsUnbalanced v
-          Just _ -> balanceCheck balance
-
+    balanceCheck balance
 
     --TODO: we could add the extra fee for the CBOR encoding of the change,
     -- now that we know the magnitude of the change: i.e. 1-8 bytes extra.
@@ -1135,7 +1139,7 @@ makeTransactionBodyAutoBalance eraInMode systemstart history pparams
 
    balanceCheck :: TxOutValue era -> Either TxBodyErrorAutoBalance ()
    balanceCheck balance
-    | txOutValueToLovelace balance == 0 = return ()
+    | txOutValueToLovelace balance == 0 && onlyAda (txOutValueToValue balance) = return ()
     | txOutValueToLovelace balance < 0 =
         Left . TxBodyErrorAdaBalanceNegative $ txOutValueToLovelace balance
     | otherwise =
@@ -1145,6 +1149,13 @@ makeTransactionBodyAutoBalance eraInMode systemstart history pparams
           Left err -> Left err
           Right _ -> Right ()
 
+   isNotAda :: AssetId -> Bool
+   isNotAda AdaAssetId = False
+   isNotAda _ = True
+
+   onlyAda :: Value -> Bool
+   onlyAda = null . valueToList . filterValue isNotAda
+
    checkMinUTxOValue
      :: TxOut CtxTx era
      -> ProtocolParameters