Allow to specify ChangeAddress in BalanceTX API manually

[oskin1]

Area

Plutus Application Framework Related to the Plutus application backend (PAB), emulator, Plutus libraries

Describe the feature you'd like

It would be infinitely useful if PAB's BalanceTX API allowed to specify change address manually. https://github.com/input-output-hk/plutus-apps/blob/d637b1916522e4ec20b719487a8a2e066937aceb/plutus-contract/src/Plutus/Contract/Request.hs#L705

Probably the changeAddress field should be added to this structure https://github.com/input-output-hk/plutus-apps/blob/ac3f39e1c4384ce44fdb8c321985ac447e85e411/plutus-ledger/src/Ledger/Constraints/OffChain.hs#L170

This would allow to use PAB as a transaction assembly service for trusted external applications which usually require setting a change address depending on a transaction.

[Yasuke]

Hello @oskin1 for a change like this, work needs to be done in deciding on the proper api, and implementing this not only in the PAB but also in cardano-wallet. We are unlikely to add this in the short term but it has been added to the roadmap. Thank you for your suggestion!

[etiennejf]

A possible workaround to this issue is to perform the following steps

• call balanceTx
• transform its outputs back to TxOuts using getCardanoTxOutRefs
• replaced the change output produced during balancing with the expected change address
• convert the updated list of txouts back to Cardano TxOut using CardanoAPI.toCardanoTxOut
• produced an updated CardanoTx by providing a new Alonzo.TxBody instance
• submit the updated balanced Tx.

However, to be able to perform the following steps, you will have to pass the networkId and protocol parameters to your off-chain code. You will also need to make sure that the only additional outputs that will be added during balancing will concern the change address selected by cardano-wallet (which may not be the primary payment address in the wallet). To do so, you must ensure that you have specify the necessary mustPayToXXX constraints in your offchain code. Note also that the change address must contain both the pubkey and stake credentials so as to target the expect bech32 payment address in the wallet. So there is also a need to pass the stake public key hash of the invoking wallet to your off-chain code.