Have createNewBlock return BlockTemplate interface

An external program that uses the Mining interface may need quick access to some information in the block template, while it can wait a bit longer for the full raw transaction data.

This would be the case for a Stratum v2 Template Provider which needs to send a NewTemplate message (which doesn't include transactions) as quickly as possible.

src/node/interfaces.cpp

@@ -67,6 +67,7 @@
 
 #include <boost/signals2/signal.hpp>
 
+using interfaces::BlockTemplate;
 using interfaces::BlockTip;
 using interfaces::Chain;
 using interfaces::FoundBlock;
@@ -863,6 +864,52 @@ public:
     NodeContext& m_node;
 };
 
+class BlockTemplateImpl : public BlockTemplate
+{
+public:
+    explicit BlockTemplateImpl(std::unique_ptr<CBlockTemplate> block_template) : m_block_template(std::move(block_template))
+    {
+        assert(m_block_template);
+    }
+
+    CBlockHeader getBlockHeader() override
+    {
+        return m_block_template->block;
+    }
+
+    CBlock getBlock() override
+    {
+        return m_block_template->block;
+    }
+
+    std::vector<CAmount> getTxFees() override
+    {
+        return m_block_template->vTxFees;
+    }
+
+    std::vector<int64_t> getTxSigops() override
+    {
+        return m_block_template->vTxSigOpsCost;
+    }
+
+    CTransactionRef getCoinbaseTx() override
+    {
+        return m_block_template->block.vtx[0];
+    }
+
+    std::vector<unsigned char> getCoinbaseCommitment() override
+    {
+        return m_block_template->vchCoinbaseCommitment;
+    }
+
+    int getWitnessCommitmentIndex() override
+    {
+        return GetWitnessCommitmentIndex(m_block_template->block);
+    }
+
+    const std::unique_ptr<CBlockTemplate> m_block_template;
+};
+
 class MinerImpl : public Mining
 {
 public:
@@ -909,11 +956,11 @@ public:
         return TestBlockValidity(state, chainman().GetParams(), chainman().ActiveChainstate(), block, tip, /*fCheckPOW=*/false, check_merkle_root);
     }
 
-    std::unique_ptr<CBlockTemplate> createNewBlock(const CScript& script_pub_key, const BlockCreateOptions& options) override
+    std::unique_ptr<BlockTemplate> createNewBlock(const CScript& script_pub_key, const BlockCreateOptions& options) override
     {
         BlockAssembler::Options assemble_options{options};
         ApplyArgsManOptions(*Assert(m_node.args), assemble_options);
-        return BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(script_pub_key);
+        return std::make_unique<BlockTemplateImpl>(BlockAssembler{chainman().ActiveChainstate(), context()->mempool.get(), assemble_options}.CreateNewBlock(script_pub_key));
     }
 
     NodeContext* context() override { return &m_node; }