Problem when several fueled UTxOs

[pgrange]

In an experiment, my node was unable to initiate a head with a not enough funds error.
However I did have a fueled UTxO with enough found.
Maybe the problem came from the fact that I had a second fueled UTxO without enough funds.

I'm wondering if that might come from the selection of the fueled UTxO choosing the too small one.

Context & versions

Using the latest version from October 19th.

Having the following UTxO available in my wallet:

                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
043b9fcb61006b42967714135607b632a9f0d50efbf710822365d214b237a504     0        89832651 lovelace + TxOutDatumNone
043b9fcb61006b42967714135607b632a9f0d50efbf710822365d214b237a504     1        10000000 lovelace + TxOutDatumHash ScriptDataInBabbageEra "a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3"
13400285bb2db7179ac470f525fe341b0b2f91f579da265a350fa1db6829bf7f     0        10000000 lovelace + TxOutDatumNone
e9c45dd07cdafd96a399b52c9c73f5d9886bbd283b09b7eae054092e5b926b58     1        3654400 lovelace + TxOutDatumHash ScriptDataInBabbageEra "a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3"

Steps to reproduce

1. Setup a small fueled UTxO (around 3654400 Lovelace)
2. Setup a big enough fueled UTxO (around 10000000 lovelace)
3. run hydra tui and press I to try open a head

Actual behavior

The initialization fails. You should observe an error regarding not enough funds.

Expected behavior

The initialization succeeds

[ghost]

Should we do something about this? It seems that when we retrieve the fuel UTxO we sort by value and take the smallest one:

findFuelUTxO :: Map TxIn TxOut -> Maybe (TxIn, TxOut)
findFuelUTxO utxo =
  let utxosWithDatum = Map.toList $ Map.filter hasMarkerDatum utxo
      sortingCriteria (_, Babbage.BabbageTxOut _ v _ _) = fst' (gettriples' v)
      sortedByValue = sortOn sortingCriteria utxosWithDatum
   in case sortedByValue of
        [] -> Nothing
        as -> Just (List.last as)

A "simple" fix would be to put a lower-bound on the selection, an reasonable approximation of fees?

[ch1bo]

This be addressed by #924