[BUG] - Incorrect change output computation inside makeTransactionBodyAutoBalance

[sourabhxyz]

External

Area
Tx balancing.

Summary

makeTransactionBodyAutoBalance tries to compute change output by considering only inputs, minted value & outputs here but ada source could also come from stake/drep credential unregistration as well. This can cause the makeUnsignedTx function called shortly afterwards to fail as this computed change could be negative. This was not the case for cardano-api-9.1.0.0 where such a conversion was not being performed at least before the ada value in change output was set to maximum word limit.

Also, the utxo parameter expected by makeTransactionBodyAutoBalance is suppose to include reference inputs, collateral inputs as well and not just spending inputs but totalValueAtSpendableUTxO seems to suggest that it's only spending inputs. This is not a big deal if our goal is just to overestimate but thought I'd also mention this misnomer.

Version
Using tag cardano-api-10.1.0.0.

[sourabhxyz]

Fwiw, I did this change upon cardano-api-10.1.0.0 and it now again works with our tx building tool, Atlas.

[carbolymer]

Thank you for the detailed analysis and a proposed fix! I confirm that GHC exception is being thrown at the beginning of makeTransactionBodyAutoBalance when the calculated change is negative.