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

@@ -14,6 +14,8 @@
 
 - **Breaking change** - Reduce exposed modules in cardano-api ([PR4546](https://github.com/input-output-hk/cardano-node/pull/4546))
 
+- 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

@@ -769,11 +769,6 @@ data TxBodyErrorAutoBalance =
        -- | One or more of the scripts were expected to fail validation, but none did.
      | TxBodyScriptBadScriptValidity
 
-       -- | The balance of the non-ada assets is not zero. The 'Value' here is
-       -- that residual non-zero balance. The 'makeTransactionBodyAutoBalance'
-       -- function only automatically balances ada, not other assets.
-     | TxBodyErrorAssetBalanceWrong Value
-
        -- | There is not enough ada to cover both the outputs and the fees.
        -- The transaction should be changed to provide more input ada, or
        -- otherwise adjusted to need less (e.g. outputs, script etc).
@@ -832,13 +827,6 @@ instance Error TxBodyErrorAutoBalance where
   displayError TxBodyScriptBadScriptValidity =
       "One or more of the scripts were expected to fail validation, but none did."
 
-  displayError (TxBodyErrorAssetBalanceWrong _value) =
-      "The transaction does not correctly balance in its non-ada assets. "
-   ++ "The balance between inputs and outputs should sum to zero. "
-   ++ "The actual balance is: "
-   ++ "TODO: move the Value renderer and parser from the CLI into the API and use them here"
-   -- TODO: do this ^^
-
   displayError (TxBodyErrorAdaBalanceNegative lovelace) =
       "The transaction does not balance in its use of ada. The net balance "
    ++ "of the transaction is negative: " ++ show lovelace ++ " lovelace. "
@@ -977,13 +965,30 @@ 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 mintedNonAda = case txMintValue txbodycontent1 of
+          TxMintNone -> mempty
+          TxMintValue _ v _ -> v
+    let nonAdaChange = mconcat
+          [ incomingNonAda
+          , mintedNonAda
+          , 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
                createAndValidateTransactionBody 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,
@@ -1015,13 +1020,7 @@ 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.
@@ -1147,7 +1146,7 @@ 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 =
@@ -1157,6 +1156,13 @@ 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