How to spend a multisig descriptor wallet transaction with bitcoin-cli?

Question

I am looking for the correct way to spend multisig outputs with descriptor wallet bitcoin-cli.

In regtest, I successfully created a funding transaction tx1 with the following outputs:

out:0 sh(multi(1,KEY1,KEY2))

out:1 wsh(multi(1,KEY3,KEY4,KEY5))

I used bitcoin-cli -regtest deriveaddresses to get P2SH_ADDRESS and P2WSH_ADDRESS from these descriptors and then

bitcoin-cli -regtest -rpcwallet="$WALLET" createrawtransaction '''
  [
    {
      "txid": "'$prev_tx'",
      "vout": 0
    }
  ]
''' '''
  {
    "'$P2SH_ADDRESS'": 3,
    "'$P2WSH_ADDRESS'": 2,
    "'$CHANGE_ADDRESS'": 18
  }
'''

Then I broadcast and mined this transaction.

Now I want to create a spending transaction, that spends both of these inputs, using

PSBT=$( bitcoin-cli -regtest -named createpsbt inputs='''[
  {
    "txid": "tx1",
    "vout": 0
  },
  {
    "txid": "tx1",
    "vout": 1
  },
]''' outputs='''[
  {
    "'$ADDRESS1'": 1
  },
  {
    "'$ADDRESS2'": 1
  },
  {
    "$CHANGE_ADDRESS": 4
  }
]''')

My main question:

• how do I add input information to this PSBT (i.e. redeemScript for inp:0 and witnessScript for inp:1)? What is the modern correct way to get a redeemScript and a witnessScript given output descriptors?

My second question:

• say KEY1,KEY3 belong to wallet1, and KEY2,KEY4,KEY5 belong to wallet2. I want to spend out:0 using KEY1 and out:1 using KEY4. I tried to simply sign with both wallets: bitcoin-cli -regtest -rpcwallet=walletN walletprocesspsbt $PSBT but is it possible to select the specific key (i.e. wallet in this case) between KEY1 and KEY2 to spend the sh(multi(1,KEY1,KEY2)) vout:0, these keys are from the different wallets? And is it even correct to use walletprocesspsbt in this case?

Answer 1

Found an answer to my main question after trial and error:

One needs to first create a watch-only wallet and import descriptors into it, like so:

bitcoin-cli -regtest createwallet "multisig_watch" true true "" false true 
                                                                   
bitcoin-cli -regtest -rpcwallet=multisig_watch importdescriptors "[
  {\"desc\":\"$DESCRIPTOR_PREVOUT0\", \"timestamp\":\"now\"},    
  {\"desc\":\"$DESCRIPTOR_PREVOUT1\", \"timestamp\":\"now\"}    
]"

Then update PSBT with that watch-only wallet:

# Update PSBT with watch-only wallet                                             
PSBT=$(bitcoin-cli -regtest -rpcwallet=multisig_watch walletprocesspsbt "$PSBT")

After that the raw PSBT created with createpsbt will gain the required redeemScript and witnessScript data.
Then, sign as usual with all wallets that contain corresponding private keys for redemption:

for wallet in "${signing_wallets[@]}"; do 
  PSBT=$( bitcoin-cli -regtest -rpcwallet=$wallet walletprocesspsbt $(jq -r .psbt <<< $PSBT))
done

I think these multisig operations should be better documented in Bitcoin Core.

What remains unclear is if I have sh(multi(1,KEY1_FROM_WALLET1,KEY2_FROM_WALLET2,KEY3_FROM_WALLET2,...)) if/how I can explicitly choose between KEY2_FROM_WALLET2 and KEY3_FROM_WALLET2?