elements-cli decodepsbt doesn't set inputs.witness_utxo.asset field

[roconnor-blockstream]

Example:

$ VOUT=0

$ TXID=905e43dfb58c9d7d02512b17f118e44205d773fa5c414a9f8899ffc24f399751

$ PSET=$(result/bin/elements-cli utxoupdatepsbt $(result/bin/elements-cli createpsbt "[{\"txid\": \"$TXID\", \"vout\": $VOUT, \"sequence\": 4294967294}]" "[{\"tex1qyh6tyhspd9w8jhqee8a2uzvyk9lnwp2n8ssur0\": 0.000008, \"blinder_index\": 0}, {\"fee\": 0.000001}]"))

$ result/bin/elements-cli decodepsbt $PSET
{
  "global_xpubs": [
  ],
  "tx_version": 2,
  "fallback_locktime": 0,
  "input_count": 1,
  "output_count": 1,
  "psbt_version": 2,
  "proprietary": [
  ],
  "fees": {
    "bitcoin": 0.00000100
  },
  "unknown": {
  },
  "inputs": [
    {
      "witness_utxo": {
        "amount": 0.00000900,
        "scriptPubKey": {
          "asm": "0 25f4b25e01695c795c19c9faae0984b17f370553",
          "desc": "addr(tex1qyh6tyhspd9w8jhqee8a2uzvyk9lnwp2n8ssur0)#d4ydyt2f",
          "hex": "001425f4b25e01695c795c19c9faae0984b17f370553",
          "address": "tex1qyh6tyhspd9w8jhqee8a2uzvyk9lnwp2n8ssur0",
          "type": "witness_v0_keyhash"
        }
      },
      "previous_txid": "905e43dfb58c9d7d02512b17f118e44205d773fa5c414a9f8899ffc24f399751",
      "previous_vout": 0,
      "sequence": 4294967294
    }
  ],
  "outputs": [
    {
      "amount": 0.00000800,
      "script": {
        "asm": "0 25f4b25e01695c795c19c9faae0984b17f370553",
        "desc": "addr(tex1qyh6tyhspd9w8jhqee8a2uzvyk9lnwp2n8ssur0)#d4ydyt2f",
        "hex": "001425f4b25e01695c795c19c9faae0984b17f370553",
        "address": "tex1qyh6tyhspd9w8jhqee8a2uzvyk9lnwp2n8ssur0",
        "type": "witness_v0_keyhash"
      },
      "asset": "144c654344aa716d6f3abcc1ca90e5641e4e2a7f633bc09fe3baf64585819a49",
      "blinder_index": 0
    },
    {
      "amount": 0.00000100,
      "script": {
        "asm": "",
        "desc": "raw()#58lrscpx",
        "hex": "",
        "type": "fee"
      },
      "asset": "144c654344aa716d6f3abcc1ca90e5641e4e2a7f633bc09fe3baf64585819a49"
    }
  ]
}

Without the "asset" field from the utxo data, a taproot signature cannot be made.

Other PSET style commands such as walletcreatefundedpsbt appear to have the same issue.

I'm not really sure how/if taproot PSET even works at all.

[roconnor-blockstream]

cc @apoelstra

[apoelstra]

This is super weird. I don't see anywhere in the code that the asset would be being dropped. In the source code for utxoupdatepsbt you can see input.witness_utxo = coin.out; which copies the whole output out of the mempool and into the PSBT. So how does the asset go missing? I don't know.

The wallet RPCs appear not to set input.witness_utxo at all, so that's a bug.

[roconnor-blockstream]

I had an input.witness_utxo field why I tested walletcreatefundedpsbt.

[roconnor-blockstream]

It could be decodepsbt that is actually broken, which would explain a lot.

[apoelstra]

Ah... ok, de/serialization is done with SerializeToVector and DeserializeToVector which just pass to the consensus encoding of TxOut, which should include both the asset and the nonce. So that seems fine.

But the decodepsbt logic which converts to JSON explicitly pulls the amount, and not the asset or nonce, out of the output.

So this bug might merely be in the JSON display.

[roconnor-blockstream]

nonce shouldn't be considered part of the utxo, so let's not pull that. It is loaded up with some random uninitialized memory sometimes. ... OTOH, if it is encoded in the PSET data, it should reflect that. Maybe we need another bug report: nonces in psbt utxo's are f'd up.

[apoelstra]

Yeah -- I think we should probably change the serialization (and the spec) to define that they be nulled out.

Ideally the nonce ought to be there, but because of #1004 it's not practically possible. (See #1187 where Allen complained specifically that this results in nonces being f'd up in PSET.)

[roconnor-blockstream]

So we already have #1187 for the nonce issue. We can make this the decodepsbt issue for not showing the asset field. And I see I accidentally set the title of this issue correctly.