Cardano-wallet: [BUG] Generated an unbalanced tx! Too much left for fees

[Mercurial]

Context

Hi Guys, I'm trying to estimate fees for my transaction using the cardano-wallet-byron REST API

But I'm getting this error

Generated an unbalanced tx! Too much left for fees : fee (raw) = 169356 : fee (dangling) = 169312 , diff = 169444
selection = inputs: - 1st 53dc6b97 (~ 1000000 @ 82d81858...f4c18d36)
outputs: - 800000 @ 82d81858...b51caf97
change: [30556]

CallStack (from HasCallStack):
  error, called at src/Cardano/Wallet/Primitive/Fee.hs:330:18 in cardano-wallet-core-2020.4.7-J7ISR43lhLPDUNes6kwzqg:Cardano.Wallet.Primitive.Fee
  remainingFee, called at src/Cardano/Wallet/Primitive/Fee.hs:207:22 in cardano-wallet-core-2020.4.7-J7ISR43lhLPDUNes6kwzqg:Cardano.Wallet.Primitive.Fee

| Information | - |
| --- | --- |
| Version | 2020.4.7 (git revision: cff0e391985887f60b9076aace207674bef6a2b4) |
| Platform | Ubuntu 18.04 WSL |

Steps to Reproduce

I have a wallet with balance 1 ADA

I send this payload into the
/payment-fees endpoint

this is my payload

{
  "payments": [
    {
      "address": "Ae2tdPwUPEZLeGE96wfn8ro2ZsSYj8SFbaNXkfPexXV7sgJwUyLfJq3A7Nv",
      "amount": {
        "quantity": 800000,
        "unit": "lovelace"
      }
    }
  ]
}

Expected behavior

Return the estimated fee

{
  "amount": {
    "quantity": 42000000,
    "unit": "lovelace"
  }
}

Actual behavior

Logs

Generated an unbalanced tx! Too much left for fees : fee (raw) = 169356 : fee (dangling) = 169312 , diff = 169444
selection = inputs: - 1st 53dc6b97 (~ 1000000 @ 82d81858...f4c18d36)
outputs: - 800000 @ 82d81858...b51caf97
change: [30556]

CallStack (from HasCallStack):
  error, called at src/Cardano/Wallet/Primitive/Fee.hs:330:18 in cardano-wallet-core-2020.4.7-J7ISR43lhLPDUNes6kwzqg:Cardano.Wallet.Primitive.Fee
  remainingFee, called at src/Cardano/Wallet/Primitive/Fee.hs:207:22 in cardano-wallet-core-2020.4.7-J7ISR43lhLPDUNes6kwzqg:Cardano.Wallet.Primitive.Fee

Endpoint Response

"Something went wrong"

Resolution

• Rework fee balancing so it doesn't actually need an invariant and can chose to generate slightly unbalanced transaction if the node supports it and, if it benefits the users ==> #1578

---

QA

• Added a regression test capturing the example above ==> https://github.com/input-output-hk/cardano-wallet/blob/master/lib/byron/test/unit/Cardano/Wallet/Byron/TransactionSpec.hs#L135-L164

• Added also a property for balancing coin selection and showing that it always eventually resolve. Passed locally with 1M cases after several round of bug and fixes ==> https://github.com/input-output-hk/cardano-wallet/blob/master/lib/byron/test/unit/Cardano/Wallet/Byron/TransactionSpec.hs#L166-L169

[Mercurial]

edit: I think its when I send a very low amount transaction

[KtorZ]

Awe crap. Yes, this is an important one.

TL; DR; We need to allow _slightly_ unbalanced transaction for cardano-node.

A bit of context:

cardano-wallet is now integrated with two nodes, the cardano-node and jormungandr. They can have different behaviors on some validations they do. Jörmungandr is actually more strict with regards to fees and require a transaction to be _exactly balanced_. This isn't a big issue with jörmungandr because the underlying binary encoding is relatively simply.

cardano-node on the contrary are more lenient on the fee validation and would allow a transaction to leave more that the minimal amount of fee necessary. This is, partly, to cope with the more complex binary encoding on cardano-node where the length of the underlying data may vary based on the _value_ of its constituants (and no their type! e.g. 14 is encoded over 1 byte, but 137536 over 3 bytes ...). Our estimator is doing its best and, we do actually know what's are error margin here: up to 10 bytes per change output. We need to take this into account and make this invariant a bit more lenient, for cardano-node at least.

[Mercurial]

Awesome, I'm glad to help you guys out!

[piotr-iohk]

Indeed, I can reproduce with 1 ada balance and try estimate payment (or send tx) with 800000 lovelace.

[piotr-iohk]

LGTM.

Verified additionally on testnet for random and sequential (icarus, trezor and ledger) wallets.
In all cases transaction (and fee estimation) is successful with balance = 1 ADA and amount to be sent = 800000 lovelaces.