Merge #18826: Expose txinwitness for coinbase in JSON form from RPC

34645c4 Test txinwitness is accessible on coinbase vin (Rod Vagg)
3e44210 Expose txinwitness for coinbase in JSON form (Rod Vagg)

Pull request description:

  ## Rationale

  The CLI can provide you with everything about transactions and blocks that you need to reconstruct the block structure and raw block itself **except** for the witness commitment nonce which is stored in the `scriptWitness` of the coinbase and is not printed. You could manually parse the raw `"hex"` fields for transactions if you really wanted to, but this seems to defeat the point of having a JSONification of the raw block/transaction data.

  Without the nonce you can't:

  1. calculate and validate the witness commitment yourself, you can generate the witness tx merkle root but you don't have the nonce to combine it with
  2. reconstruct the raw block form because you don't have `scriptWitness` stack associated with the coinbase (although you know how big it will be and can guess the common case of `[0x000...000]`)

  I'm building some archiving tooling for block data and being able to do a validated two-way conversion is very helpful.

  ## What

  This PR simply makes the `txinwitness` field not dependent on whether we are working with the coinbase or not. So you get it for the coinbase as well as the rest.

  ## Examples

  Common case of a `[0x000...000]` nonce: 00000000000000000000140a7289f3aada855dfd23b0bb13bb5502b0ca60cdd7

  ```json
        "vin": [
          {
            "coinbase": "0368890904c1fe8d5e2f706f6f6c696e2e636f6d2ffabe6d6d5565843a681160cf7b08b1b74ac90a719e6d6ab28c16d336b924f0dc2fcabdc6010000000000000051bf2ad74af345dbe642154b2658931612a70d195e007add0100ffffffff",
            "txinwitness": [
              "0000000000000000000000000000000000000000000000000000000000000000"
            ],
            "sequence": 4294967295
          }
        ],
  ...
  ```

  Novel nonce value: 000000000000000000008c31945b2012258366cc600a3e9a3ee0598e8f797731

  ```json
        "vin": [
          {
            "coinbase": "031862082cfabe6d6d80c099b5e21f4c186d54eb292e17026932e52b1b807fa1380574c5adc1c843450200000000000000",
            "txinwitness": [
              "5b5032506f6f6c5d5b5032506f6f6c5d5b5032506f6f6c5d5b5032506f6f6c5d"
            ],
            "sequence": 4294967295
          }
        ],
  ...
  ```

  ## Alternatives

  This field could be renamed for the coinbase, `"witnessnonce"` perhaps. It could also be omitted when null/zero (`0x000...000`).

  ## Tests

  This didn't break any tests and I couldn't find an obvious way to include a test for this. If this is desired I'd apreicate some pointers.

ACKs for top commit:
  MarcoFalke:
    ACK 34645c4

Tree-SHA512: b192facc1dfd210a5ec3f0d5d1ac6d0cae81eb35be15eaa71f60009a538dd6a79ab396f218434e7e998563f7f0df2c396cc925cb91619f6841c5a67806148c85

Author: MarcoFalke

src/core_write.cpp

@@ -198,13 +198,13 @@ void TxToUniv(const CTransaction& tx, const uint256& hashBlock, UniValue& entry,
             o.pushKV("asm", ScriptToAsmStr(txin.scriptSig, true));
             o.pushKV("hex", HexStr(txin.scriptSig.begin(), txin.scriptSig.end()));
             in.pushKV("scriptSig", o);
-            if (!tx.vin[i].scriptWitness.IsNull()) {
-                UniValue txinwitness(UniValue::VARR);
-                for (const auto& item : tx.vin[i].scriptWitness.stack) {
-                    txinwitness.push_back(HexStr(item.begin(), item.end()));
-                }
-                in.pushKV("txinwitness", txinwitness);
+        }
+        if (!tx.vin[i].scriptWitness.IsNull()) {
+            UniValue txinwitness(UniValue::VARR);
+            for (const auto& item : tx.vin[i].scriptWitness.stack) {
+                txinwitness.push_back(HexStr(item.begin(), item.end()));
             }
+            in.pushKV("txinwitness", txinwitness);
         }
         in.pushKV("sequence", (int64_t)txin.nSequence);
         vin.push_back(in);

test/functional/feature_segwit.py

@@ -20,6 +20,7 @@
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
+    assert_is_hex_string,
     assert_raises_rpc_error,
     connect_nodes,
     hex_str_to_bytes,
@@ -188,6 +189,14 @@ def run_test(self):
             assert self.nodes[1].getrawtransaction(tx_id, False, blockhash) == self.nodes[2].gettransaction(tx_id)["hex"]
             assert self.nodes[0].getrawtransaction(tx_id, False, blockhash) == tx.serialize_without_witness().hex()
 
+        # Coinbase contains the witness commitment nonce, check that RPC shows us
+        coinbase_txid = self.nodes[2].getblock(blockhash)['tx'][0]
+        coinbase_tx = self.nodes[2].gettransaction(txid=coinbase_txid, verbose=True)
+        witnesses = coinbase_tx["decoded"]["vin"][0]["txinwitness"]
+        assert_equal(len(witnesses), 1)
+        assert_is_hex_string(witnesses[0])
+        assert_equal(witnesses[0], '00'*32)
+
         self.log.info("Verify witness txs without witness data are invalid after the fork")
         self.fail_accept(self.nodes[2], 'non-mandatory-script-verify-flag (Witness program hash mismatch)', wit_ids[NODE_2][P2WPKH][2], sign=False)
         self.fail_accept(self.nodes[2], 'non-mandatory-script-verify-flag (Witness program was passed an empty witness)', wit_ids[NODE_2][P2WSH][2], sign=False)