fix(utxo): deserialize sapling root for PIVX block headers

[shamardy]

This PR resolves a deserialization failure for PIVX block headers that was causing an UnexpectedEnd error.

The root cause was that the BlockHeader deserializer did not account for the hash_final_sapling_root field, which is present in all current PIVX headers. This caused the stream reader to consume an incorrect number of bytes, leading to a misaligned stream for subsequent headers.

The fix introduces a PIVX variant to CoinVariant and updates the BlockHeader::deserialize function to correctly read the hash_final_sapling_root field when the coin is identified as PIVX, ensuring proper headers parsing.

fixes #2048 (comment)

[cipig]

thanks for the fix
calling recover_funds_of_swap with the same uuid that previously failed with "UnexpectedEnd" now shows
"error" : "rpc:198] RPC call failed: lp_swap:1519] saved_swap:130] maker_swap:1700] Too early to refund, wait until 1754357000" which is in 1h

[cipig]

oh, just saw that the swap is from 3 months ago and refund already failed with

      {
         "event" : {
            "data" : {
               "error" : "maker_swap:1315] !maker_coin.send_maker_refunds_payment: utxo_common:1958] HTLC spend fee 100000 is greater than transaction output 70000"
            },
            "type" : "MakerPaymentRefundFailed"
         },
         "timestamp" : 1747550467167
      },
      {
         "event" : {
            "type" : "Finished"
         },
         "timestamp" : 1747550467172
      }

so why does recover say i need to wait?
every time i call recover_funds_of_swap, the timestamp in the error increases

[shamardy]

every time i call recover_funds_of_swap, the timestamp in the error increases

Thanks for finding this. The deserializer assumes a Zcash-style field order, but PIVX places its hash_final_sapling_root field at a different position. This means the bytes fit and deserialization completes without errors, but the data assigned to the fields is corrupted. The bytes for the timestamp are incorrectly read as the sapling root, the bytes for the difficulty are read as the timestamp, and so on. Will fix it.

[shamardy]

should be fixed @cipig

[cipig]

it now refunded that tx that was supposed to be unrefundable because of HTLC spend fee 100000 is greater than transaction output 70000 3 months ago... that's likely due to other changes to kdf in the last 3 months

be42d8c0-127c-4df5-800b-4de8318e1bf5
{
   "result" : {
      "action" : "RefundedMyPayment",
      "coin" : "PIVX",
      "tx_hash" : "2d69c282ed1f59ade4f0d4f26ef2cdfa67aef29ad728835fddcf1424c27cd316",
      "tx_hex" : "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"
   }
}

on chain: https://chainz.cryptoid.info/pivx/tx.dws?2d69c282ed1f59ade4f0d4f26ef2cdfa67aef29ad728835fddcf1424c27cd316.htm
fees were higher than output, but ok, nice

[mariocynicys]

LGTM