mining: drop unused -nFees and sigops from CBlockTemplate

For the coinbase vTxFees used a dummy value of -nFees. This
value was never returned by the RPC or used in a test.

Similarly the fist vTxSigOpsCost entry was calculated from
the dummy coinbase transaction.

Drop both and add code comments to prevent confusion.

Author: Sjors

src/interfaces/mining.h

@@ -34,9 +34,12 @@ class BlockTemplate
     virtual ~BlockTemplate() = default;
 
     virtual CBlockHeader getBlockHeader() = 0;
+    // Block contains a dummy coinbase transaction that should not be used.
     virtual CBlock getBlock() = 0;
 
+    // Fees per transaction, not including coinbase transaction.
     virtual std::vector<CAmount> getTxFees() = 0;
+    // Sigop cost per transaction, not including coinbase transaction.
     virtual std::vector<int64_t> getTxSigops() = 0;
 
     virtual CTransactionRef getCoinbaseTx() = 0;

src/node/miner.cpp

@@ -124,10 +124,9 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock()
     pblocktemplate.reset(new CBlockTemplate());
     CBlock* const pblock = &pblocktemplate->block; // pointer for convenience
 
-    // Add dummy coinbase tx as first transaction
+    // Add dummy coinbase tx as first transaction. It is skipped by the
+    // getblocktemplate RPC and mining interface consumers must not use it.
     pblock->vtx.emplace_back();
-    pblocktemplate->vTxFees.push_back(-1); // updated at end
-    pblocktemplate->vTxSigOpsCost.push_back(-1); // updated at end
 
     LOCK(::cs_main);
     CBlockIndex* pindexPrev = m_chainstate.m_chain.Tip();
@@ -165,7 +164,6 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock()
     coinbaseTx.vin[0].scriptSig = CScript() << nHeight << OP_0;
     pblock->vtx[0] = MakeTransactionRef(std::move(coinbaseTx));
     pblocktemplate->vchCoinbaseCommitment = m_chainstate.m_chainman.GenerateCoinbaseCommitment(*pblock, pindexPrev);
-    pblocktemplate->vTxFees[0] = -nFees;
 
     LogPrintf("CreateNewBlock(): block weight: %u txs: %u fees: %ld sigops %d\n", GetBlockWeight(*pblock), nBlockTx, nFees, nBlockSigOpsCost);
 
@@ -174,7 +172,6 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock()
     UpdateTime(pblock, chainparams.GetConsensus(), pindexPrev);
     pblock->nBits          = GetNextWorkRequired(pindexPrev, pblock, chainparams.GetConsensus());
     pblock->nNonce         = 0;
-    pblocktemplate->vTxSigOpsCost[0] = WITNESS_SCALE_FACTOR * GetLegacySigOpCount(*pblock->vtx[0]);
 
     BlockValidationState state;
     if (m_options.test_block_validity && !TestBlockValidity(state, chainparams, m_chainstate, *pblock, pindexPrev,

src/node/miner.h

@@ -37,7 +37,9 @@ static const bool DEFAULT_PRINT_MODIFIED_FEE = false;
 struct CBlockTemplate
 {
     CBlock block;
+    // Fees per transaction, not including coinbase transaction (unlike CBlock::vtx).
     std::vector<CAmount> vTxFees;
+    // Sigops per transaction, not including coinbase transaction (unlike CBlock::vtx).
     std::vector<int64_t> vTxSigOpsCost;
     std::vector<unsigned char> vchCoinbaseCommitment;
     /* A vector of package fee rates, ordered by the sequence in which

src/rpc/mining.cpp

@@ -893,7 +893,7 @@ static RPCHelpMan getblocktemplate()
         }
         entry.pushKV("depends", std::move(deps));
 
-        int index_in_template = i - 1;
+        int index_in_template = i - 2;
         entry.pushKV("fee", tx_fees.at(index_in_template));
         int64_t nTxSigOps{tx_sigops.at(index_in_template)};
         if (fPreSegWit) {