mining: add new getCoinbaseTx() returning a struct

Introduce a new method intended to replace getCoinbaseRawTx(), which
provides a struct with everything clients need to construct a coinbase.
This is safer than providing a raw dummy coinbase that clients then have
to manipulate.

The CoinbaseTx data is populated during the dummy transaction generation
and stored in struct CBlockTemplate.

Expand the interface_ipc.py functional test to document its usage
and ensure equivalence.

src/node/miner.cpp

@@ -158,18 +158,51 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock()
 
     // Create coinbase transaction.
     CMutableTransaction coinbaseTx;
+
+    // Construct coinbase transaction struct in parallel
+    CoinbaseTx& coinbase_tx{pblocktemplate->m_coinbase_tx};
+    coinbase_tx.version = coinbaseTx.version;
+
     coinbaseTx.vin.resize(1);
     coinbaseTx.vin[0].prevout.SetNull();
     coinbaseTx.vin[0].nSequence = CTxIn::MAX_SEQUENCE_NONFINAL; // Make sure timelock is enforced.
+    coinbase_tx.sequence = coinbaseTx.vin[0].nSequence;
+
+    // Add an output that spends the full coinbase reward.
     coinbaseTx.vout.resize(1);
     coinbaseTx.vout[0].scriptPubKey = m_options.coinbase_output_script;
-    coinbaseTx.vout[0].nValue = nFees + GetBlockSubsidy(nHeight, chainparams.GetConsensus());
+    // Block subsidy + fees
+    const CAmount block_reward{nFees + GetBlockSubsidy(nHeight, chainparams.GetConsensus())};
+    coinbaseTx.vout[0].nValue = block_reward;
+    coinbase_tx.block_reward_remaining = block_reward;
+
+    // Start the coinbase scriptSig with the block height as required by BIP34.
+    // The trailing OP_0 (historically an extranonce) is optional padding and
+    // could be removed without a consensus change. Mining clients are expected
+    // to append extra data to this prefix, so increasing its length would reduce
+    // the space they can use and may break existing clients.
     coinbaseTx.vin[0].scriptSig = CScript() << nHeight << OP_0;
+    coinbase_tx.script_sig_prefix = coinbaseTx.vin[0].scriptSig;
     Assert(nHeight > 0);
     coinbaseTx.nLockTime = static_cast<uint32_t>(nHeight - 1);
+    coinbase_tx.lock_time = coinbaseTx.nLockTime;
+
     pblock->vtx[0] = MakeTransactionRef(std::move(coinbaseTx));
     pblocktemplate->vchCoinbaseCommitment = m_chainstate.m_chainman.GenerateCoinbaseCommitment(*pblock, pindexPrev);
 
+    const CTransactionRef& final_coinbase{pblock->vtx[0]};
+    if (final_coinbase->HasWitness()) {
+        const auto& witness_stack{final_coinbase->vin[0].scriptWitness.stack};
+        // Consensus requires the coinbase witness stack to have exactly one
+        // element of 32 bytes.
+        Assert(witness_stack.size() == 1 && witness_stack[0].size() == 32);
+        coinbase_tx.witness = uint256(witness_stack[0]);
+    }
+    if (const int witness_index = GetWitnessCommitmentIndex(*pblock); witness_index != NO_WITNESS_COMMITMENT) {
+        Assert(witness_index >= 0 && static_cast<size_t>(witness_index) < final_coinbase->vout.size());
+        coinbase_tx.required_outputs.push_back(final_coinbase->vout[witness_index]);
+    }
+
     LogInfo("CreateNewBlock(): block weight: %u txs: %u fees: %ld sigops %d\n", GetBlockWeight(*pblock), nBlockTx, nFees, nBlockSigOpsCost);
 
     // Fill in header
@@ -440,4 +473,5 @@ std::optional<BlockRef> WaitTipChanged(ChainstateManager& chainman, KernelNotifi
     // avoid deadlocks.
     return GetTip(chainman);
 }
+
 } // namespace node