consensus: Use Coin span in CheckTxInputs

Move the responsibility of retrieving coins from CheckTxInputs
to its caller. The missing inputs check will be moved in an upcoming
commit.

This is a part of a series of commits for removing access to the
CCoinsViewCache in consensus verification functions. The goal is to
allow calling verification functions with pre-fetched, or a user-defined
set of coins.

Define two explicit template specializations for both a span of
references to coins and a span of coins. This allows using it for both
Coin entries referenced from the CCoinsViewCache, and from contiguous
memory, like the vector in CBlockUndo.

src/consensus/tx_verify.cpp

@@ -186,7 +186,8 @@ template int64_t GetTransactionSigOpCost<std::span<const Coin>>(
 template int64_t GetTransactionSigOpCost<std::span<std::reference_wrapper<const Coin>>>(
     const CTransaction& tx, const std::span<std::reference_wrapper<const Coin>> coins, uint32_t flags);
 
-bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state, const CCoinsViewCache& inputs, int nSpendHeight, CAmount& txfee)
+template <typename T>
+bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state, const CCoinsViewCache& inputs, const T coins, int nSpendHeight, CAmount& txfee)
 {
     // are the actual inputs available?
     if (!inputs.HaveInputs(tx)) {
@@ -195,15 +196,16 @@ bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state,
     }
 
     CAmount nValueIn = 0;
-    for (unsigned int i = 0; i < tx.vin.size(); ++i) {
-        const COutPoint &prevout = tx.vin[i].prevout;
-        const Coin& coin = inputs.AccessCoin(prevout);
+    Assert(coins.size() == tx.vin.size());
+    auto input_it = tx.vin.begin();
+    for (auto it = coins.begin(); it != coins.end(); ++it, ++input_it) {
+        const Coin& coin{GetCoin(*it)};
         assert(!coin.IsSpent());
 
         // If prev is coinbase, check that it's matured
         if (coin.IsCoinBase() && nSpendHeight - coin.nHeight < COINBASE_MATURITY) {
             return state.Invalid(TxValidationResult::TX_PREMATURE_SPEND, "bad-txns-premature-spend-of-coinbase",
-                strprintf("tried to spend coinbase at depth %d", nSpendHeight - coin.nHeight));
+                strprintf("tried to spend coinbase at depth %d", static_cast<int>(nSpendHeight - coin.nHeight)));
         }
 
         // Check for negative or overflow input values
@@ -228,3 +230,9 @@ bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state,
     txfee = txfee_aux;
     return true;
 }
+
+template bool Consensus::CheckTxInputs<std::span<const Coin>>(
+    const CTransaction& tx, TxValidationState& state, const CCoinsViewCache& inputs, const std::span<const Coin> coins, int nSpendHeight, CAmount& txfee);
+
+template bool Consensus::CheckTxInputs<std::span<std::reference_wrapper<const Coin>>>(
+    const CTransaction& tx, TxValidationState& state, const CCoinsViewCache& inputs, const std::span<std::reference_wrapper<const Coin>> coins, int nSpendHeight, CAmount& txfee);

src/consensus/tx_verify.h

@@ -25,7 +25,8 @@ namespace Consensus {
  * @param[out] txfee Set to the transaction fee if successful.
  * Preconditions: tx.IsCoinBase() is false.
  */
-[[nodiscard]] bool CheckTxInputs(const CTransaction& tx, TxValidationState& state, const CCoinsViewCache& inputs, int nSpendHeight, CAmount& txfee);
+template <typename T>
+[[nodiscard]] bool CheckTxInputs(const CTransaction& tx, TxValidationState& state, const CCoinsViewCache& inputs, const T coins, int nSpendHeight, CAmount& txfee);
 } // namespace Consensus
 
 /** Auxiliary functions for transaction validation (ideally should not be exposed) */

src/test/fuzz/coins_view.cpp

@@ -255,7 +255,10 @@ FUZZ_TARGET(coins_view, .init = initialize_coins_view)
                     // It is not allowed to call CheckTxInputs if CheckTransaction failed
                     return;
                 }
-                if (Consensus::CheckTxInputs(transaction, state, coins_view_cache, fuzzed_data_provider.ConsumeIntegralInRange<int>(0, std::numeric_limits<int>::max()), tx_fee_out)) {
+                // are the actual inputs available?
+                if (!coins_view_cache.HaveInputs(transaction)) return;
+                auto coins{coins_view_cache.AccessCoins(transaction)};
+                if (Consensus::CheckTxInputs(transaction, state, coins_view_cache, std::span{coins}, fuzzed_data_provider.ConsumeIntegralInRange<int>(0, std::numeric_limits<int>::max()), tx_fee_out)) {
                     assert(MoneyRange(tx_fee_out));
                 }
             },

src/txmempool.cpp

@@ -777,7 +777,8 @@ void CTxMemPool::check(const CCoinsViewCache& active_coins_tip, int64_t spendhei
         TxValidationState dummy_state; // Not used. CheckTxInputs() should always pass
         CAmount txfee = 0;
         assert(!tx.IsCoinBase());
-        assert(Consensus::CheckTxInputs(tx, dummy_state, mempoolDuplicate, spendheight, txfee));
+        auto coins{mempoolDuplicate.AccessCoins(tx)};
+        assert(Consensus::CheckTxInputs(tx, dummy_state, mempoolDuplicate, std::span{coins}, spendheight, txfee));
         for (const auto& input: tx.vin) mempoolDuplicate.SpendCoin(input.prevout);
         AddCoins(mempoolDuplicate, tx, std::numeric_limits<int>::max());
     }

src/validation.cpp

@@ -874,7 +874,8 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     }
 
     // The mempool holds txs for the next block, so pass height+1 to CheckTxInputs
-    if (!Consensus::CheckTxInputs(tx, state, m_view, m_active_chainstate.m_chain.Height() + 1, ws.m_base_fees)) {
+    auto coins{m_view.AccessCoins(tx)};
+    if (!Consensus::CheckTxInputs(tx, state, m_view, std::span{coins}, m_active_chainstate.m_chain.Height() + 1, ws.m_base_fees)) {
         return false; // state filled in by CheckTxInputs
     }
 
@@ -887,7 +888,6 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
         return state.Invalid(TxValidationResult::TX_WITNESS_MUTATED, "bad-witness-nonstandard");
     }
 
-    auto coins{m_view.AccessCoins(tx)};
     int64_t nSigOpsCost = GetTransactionSigOpCost(tx, std::span{coins}, STANDARD_SCRIPT_VERIFY_FLAGS);
 
     // Keep track of transactions that spend a coinbase, which we re-scan
@@ -2648,14 +2648,15 @@ bool Chainstate::ConnectBlock(const CBlock& block, BlockValidationState& state,
     {
         if (!state.IsValid()) break;
         const CTransaction &tx = *(block.vtx[i]);
-
         nInputs += tx.vin.size();
 
         if (!tx.IsCoinBase())
         {
             CAmount txfee = 0;
             TxValidationState tx_state;
-            if (!Consensus::CheckTxInputs(tx, tx_state, view, pindex->nHeight, txfee)) {
+
+            auto coins{view.AccessCoins(tx)};
+            if (!Consensus::CheckTxInputs(tx, tx_state, view, std::span{coins}, pindex->nHeight, txfee)) {
                 // Any transaction validation failure in ConnectBlock is a block consensus failure
                 state.Invalid(BlockValidationResult::BLOCK_CONSENSUS,
                               tx_state.GetRejectReason(),