Sv2NewTemplate: add coinbase_witness_reserved_value

Adding coinbase_witness_reserved_value to the Sv2NewTemplate message
so that it is automatically propagated to other roles in the Stratum
v2 ecosystem, rather than assumed to be 0x00...00.

This ensures that if this value ever gets consensus meaning, miners
only need to upgrade their node software, not the rest of the mining
stack.

This is a breaking change requiring an updated Stratum v2 spec as
well as support on the SRI side.

Author: Sjors

src/sv2/messages.cpp

@@ -11,6 +11,13 @@ node::Sv2NewTemplateMsg::Sv2NewTemplateMsg(const CBlockHeader& header, const CTr
 
     m_coinbase_tx_version = coinbase_tx->CURRENT_VERSION;
     m_coinbase_prefix = coinbase_tx->vin[0].scriptSig;
+    if (coinbase_tx->HasWitness()) {
+        const auto& witness_stack{coinbase_tx->vin[0].scriptWitness.stack};
+        Assert(witness_stack.size() == 1 || witness_stack[0].size() == 32);
+        m_coinbase_witness_reserved_value = uint256(witness_stack[0]);
+    } else {
+        m_coinbase_witness_reserved_value = uint256(0);
+    }
     m_coinbase_tx_input_sequence = coinbase_tx->vin[0].nSequence;
 
     // The coinbase nValue already contains the nFee + the Block Subsidy when built using CreateBlock().

src/sv2/messages.h

@@ -282,6 +282,12 @@ struct Sv2NewTemplateMsg
      */
     CScript m_coinbase_prefix;
 
+    /**
+     * BIP 141 witness reserved value. If there is no segwit commitment in
+     * m_coinbase_tx_outputs this value must be ignored.
+     */
+    uint256 m_coinbase_witness_reserved_value;
+
     /**
      * The coinbase transaction input’s nSequence field.
      */
@@ -324,6 +330,7 @@ struct Sv2NewTemplateMsg
           << m_version
           << m_coinbase_tx_version
           << m_coinbase_prefix
+          << m_coinbase_witness_reserved_value
           << m_coinbase_tx_input_sequence
           << m_coinbase_tx_value_remaining
           << m_coinbase_tx_outputs_count;

src/test/sv2_messages_tests.cpp

@@ -110,6 +110,7 @@ BOOST_AUTO_TEST_CASE(Sv2NewTemplate_test)
     // U32              02000000            coinbase tx version
     // B0_255           04                  coinbase_prefix len
     //                  03012100            coinbase prefix
+    // U256             0000000000000000000000000000000000000000000000000000000000000000 - witness reserved value
     // U32              ffffffff            coinbase tx input sequence
     // U64              0040075af0750700    coinbase tx value remaining
     // U32              01000000            coinbase tx outputs count
@@ -119,7 +120,7 @@ BOOST_AUTO_TEST_CASE(Sv2NewTemplate_test)
     // U32              dbc80d00            coinbase lock time (height 903,387)
     // SEQ0_255[U256]   01                  merkle path length
     //                  1a6240823de4c8d6aaf826851bdf2b0e8d5acf7c31e8578cff4c394b5a32bd4e - merkle path
-    std::string expected{"01000000000000000000000030020000000403012100ffffffff0040075af0750700010000000c000100000000000000036a012adbc80d00011a6240823de4c8d6aaf826851bdf2b0e8d5acf7c31e8578cff4c394b5a32bd4e"};
+    std::string expected{"010000000000000000000000300200000004030121000000000000000000000000000000000000000000000000000000000000000000ffffffff0040075af0750700010000000c000100000000000000036a012adbc80d00011a6240823de4c8d6aaf826851bdf2b0e8d5acf7c31e8578cff4c394b5a32bd4e"};
 
     node::Sv2NewTemplateMsg new_template;
     new_template.m_template_id = 1;
@@ -131,6 +132,8 @@ BOOST_AUTO_TEST_CASE(Sv2NewTemplate_test)
     CScript prefix(coinbase_prefix.begin(), coinbase_prefix.end());
     new_template.m_coinbase_prefix = prefix;
 
+    new_template.m_coinbase_witness_reserved_value = uint256(0);
+
     new_template.m_coinbase_tx_input_sequence = 4294967295;
     new_template.m_coinbase_tx_value_remaining = MAX_MONEY;
 

src/test/sv2_template_provider_tests.cpp

@@ -18,7 +18,7 @@
 
 BOOST_FIXTURE_TEST_SUITE(sv2_template_provider_tests, TestChain100Setup)
 
-static constexpr size_t SV2_NEW_TEMPLATE_MESSAGE_SIZE{91};
+static constexpr size_t SV2_NEW_TEMPLATE_MESSAGE_SIZE{91 + 32};
 
 /**
   * A class for testing the Template Provider. Each TPTester encapsulates a