rpc: add coinbase_tx field to getblock

This adds a "coinbase_tx" field to the getblock RPC result, starting
at verbosity level 1. It contains only fields guaranteed to be small,
i.e. not the outputs.

Author: Sjors

doc/release-notes-34512.md

@@ -0,0 +1,8 @@
+Updated RPCs
+------------
+
+- The `getblock` RPC now returns a `coinbase_tx` object at verbosity levels 1, 2,
+  and 3. It contains `version`, `locktime`, `sequence`, `coinbase` and
+  `witness`. This allows for efficiently querying coinbase
+  transaction properties without fetching the full transaction data at
+  verbosity 2+. (#xxxxx)

src/rpc/blockchain.cpp

@@ -181,13 +181,35 @@ UniValue blockheaderToJSON(const CBlockIndex& tip, const CBlockIndex& blockindex
     return result;
 }
 
+/** Serialize coinbase transaction metadata */
+UniValue coinbaseTxToJSON(const CTransaction& coinbase_tx)
+{
+    CHECK_NONFATAL(!coinbase_tx.vin.empty());
+    const CTxIn& vin_0{coinbase_tx.vin[0]};
+    UniValue coinbase_tx_obj(UniValue::VOBJ);
+    coinbase_tx_obj.pushKV("version", coinbase_tx.version);
+    coinbase_tx_obj.pushKV("locktime", coinbase_tx.nLockTime);
+    coinbase_tx_obj.pushKV("sequence", vin_0.nSequence);
+    coinbase_tx_obj.pushKV("coinbase", HexStr(vin_0.scriptSig));
+    const auto& witness_stack{vin_0.scriptWitness.stack};
+    if (!witness_stack.empty()) {
+        CHECK_NONFATAL(witness_stack.size() == 1);
+        coinbase_tx_obj.pushKV("witness", HexStr(witness_stack[0]));
+    }
+    return coinbase_tx_obj;
+}
+
 UniValue blockToJSON(BlockManager& blockman, const CBlock& block, const CBlockIndex& tip, const CBlockIndex& blockindex, TxVerbosity verbosity, const uint256 pow_limit)
 {
     UniValue result = blockheaderToJSON(tip, blockindex, pow_limit);
 
     result.pushKV("strippedsize", ::GetSerializeSize(TX_NO_WITNESS(block)));
     result.pushKV("size", ::GetSerializeSize(TX_WITH_WITNESS(block)));
     result.pushKV("weight", ::GetBlockWeight(block));
+
+    CHECK_NONFATAL(!block.vtx.empty());
+    result.pushKV("coinbase_tx", coinbaseTxToJSON(*block.vtx[0]));
+
     UniValue txs(UniValue::VARR);
     txs.reserve(block.vtx.size());
 
@@ -760,6 +782,14 @@ static RPCHelpMan getblock()
                     {RPCResult::Type::NUM, "size", "The block size"},
                     {RPCResult::Type::NUM, "strippedsize", "The block size excluding witness data"},
                     {RPCResult::Type::NUM, "weight", "The block weight as defined in BIP 141"},
+                    {RPCResult::Type::OBJ, "coinbase_tx", "Coinbase transaction metadata",
+                    {
+                        {RPCResult::Type::NUM, "version", "The coinbase transaction version"},
+                        {RPCResult::Type::NUM, "locktime", "The coinbase transaction's locktime (nLockTime)"},
+                        {RPCResult::Type::NUM, "sequence", "The coinbase input's sequence number (nSequence)"},
+                        {RPCResult::Type::STR_HEX, "coinbase", "The coinbase input's script"},
+                        {RPCResult::Type::STR_HEX, "witness", /*optional=*/true, "The coinbase input's first (and only) witness stack element, if present"},
+                    }},
                     {RPCResult::Type::NUM, "height", "The block height or index"},
                     {RPCResult::Type::NUM, "version", "The block version"},
                     {RPCResult::Type::STR_HEX, "versionHex", "The block version formatted in hexadecimal"},

test/functional/rpc_blockchain.py

@@ -646,6 +646,26 @@ def _test_getblock(self):
         self.wallet.send_self_transfer(fee_rate=fee_per_kb, from_node=node)
         blockhash = self.generate(node, 1)[0]
 
+        def assert_coinbase_metadata(hash, verbosity):
+            block = node.getblock(hash, verbosity)
+            coinbase_tx = node.getblock(hash, 2)["tx"][0]
+
+            expected_keys = {"version", "locktime", "sequence", "coinbase"}
+            if "txinwitness" in coinbase_tx["vin"][0]:
+                expected_keys.add("witness")
+            assert_equal(set(block["coinbase_tx"].keys()), expected_keys)
+
+            assert_equal(block["coinbase_tx"]["version"], coinbase_tx["version"])
+            assert_equal(block["coinbase_tx"]["locktime"], coinbase_tx["locktime"])
+            assert_equal(block["coinbase_tx"]["sequence"], coinbase_tx["vin"][0]["sequence"])
+            assert_equal(block["coinbase_tx"]["coinbase"], coinbase_tx["vin"][0]["coinbase"])
+
+            witness_stack = coinbase_tx["vin"][0].get("txinwitness")
+            if witness_stack is None:
+                assert "witness" not in block["coinbase_tx"]
+            else:
+                assert_equal(block["coinbase_tx"]["witness"], witness_stack[0])
+
         def assert_hexblock_hashes(verbosity):
             block = node.getblock(blockhash, verbosity)
             assert_equal(blockhash, hash256(bytes.fromhex(block[:160]))[::-1].hex())
@@ -692,6 +712,9 @@ def assert_vin_does_not_contain_prevout(hash, verbosity):
         assert_fee_not_in_block(blockhash, 1)
         assert_fee_not_in_block(blockhash, True)
 
+        self.log.info("Test getblock coinbase metadata fields")
+        assert_coinbase_metadata(blockhash, 1)
+
         self.log.info('Test that getblock with verbosity 2 and 3 includes expected fee')
         assert_fee_in_block(blockhash, 2)
         assert_fee_in_block(blockhash, 3)