consensus: Use Coin span in GetP2SHSigOpCount

Move the responsibility of retrieving coins from GetP2SHSigOpCount to
its caller.

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/coins.cpp

@@ -160,6 +160,16 @@ const Coin& CCoinsViewCache::AccessCoin(const COutPoint &outpoint) const {
     }
 }
 
+std::vector<std::reference_wrapper<const Coin>> CCoinsViewCache::AccessCoins(const CTransaction& tx) const
+{
+    std::vector<std::reference_wrapper<const Coin>> coins;
+    coins.reserve(tx.vin.size());
+    for (const CTxIn& input : tx.vin) {
+        coins.emplace_back(AccessCoin(input.prevout));
+    }
+    return coins;
+}
+
 bool CCoinsViewCache::HaveCoin(const COutPoint &outpoint) const {
     CCoinsMap::const_iterator it = FetchCoin(outpoint);
     return (it != cacheCoins.end() && !it->second.coin.IsSpent());

src/coins.h

@@ -415,6 +415,15 @@ public:
      */
     const Coin& AccessCoin(const COutPoint &output) const;
 
+    /**
+     * Return a vector of references to Coins in the cache, or coinEmpty if
+     * the Coin is not found.
+     *
+     * Generally, this should only be held for a short scope. The coins should
+     * generally not be held through any other calls to this cache.
+     */
+    std::vector<std::reference_wrapper<const Coin>> AccessCoins(const CTransaction& tx) const;
+
     /**
      * Add a coin. Set possible_overwrite to true if an unspent version may
      * already exist in the cache.

src/consensus/tx_verify.cpp

@@ -14,6 +14,16 @@
 #include <util/check.h>
 #include <util/moneystr.h>
 
+template <typename T>
+static constexpr const Coin& GetCoin(const T& item)
+{
+    if constexpr (std::is_same_v<T, std::reference_wrapper<const Coin>>) {
+        return item.get();
+    } else {
+        return item;
+    }
+}
+
 bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime)
 {
     if (tx.nLockTime == 0)
@@ -123,23 +133,31 @@ unsigned int GetLegacySigOpCount(const CTransaction& tx)
     return nSigOps;
 }
 
-unsigned int GetP2SHSigOpCount(const CTransaction& tx, const CCoinsViewCache& inputs)
+template <typename T>
+unsigned int GetP2SHSigOpCount(const CTransaction& tx, const T coins)
 {
     if (tx.IsCoinBase())
         return 0;
 
     unsigned int nSigOps = 0;
-    for (unsigned int i = 0; i < tx.vin.size(); i++)
-    {
-        const Coin& coin = inputs.AccessCoin(tx.vin[i].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());
         const CTxOut &prevout = coin.out;
         if (prevout.scriptPubKey.IsPayToScriptHash())
-            nSigOps += prevout.scriptPubKey.GetSigOpCount(tx.vin[i].scriptSig);
+            nSigOps += prevout.scriptPubKey.GetSigOpCount(input_it->scriptSig);
     }
     return nSigOps;
 }
 
+template unsigned int GetP2SHSigOpCount<std::span<const Coin>>(
+    const CTransaction& tx, const std::span<const Coin>);
+
+template unsigned int GetP2SHSigOpCount<std::span<std::reference_wrapper<const Coin>>>(
+    const CTransaction& tx, const std::span<std::reference_wrapper<const Coin>>);
+
 int64_t GetTransactionSigOpCost(const CTransaction& tx, const CCoinsViewCache& inputs, uint32_t flags)
 {
     int64_t nSigOps = GetLegacySigOpCount(tx) * WITNESS_SCALE_FACTOR;
@@ -148,7 +166,8 @@ int64_t GetTransactionSigOpCost(const CTransaction& tx, const CCoinsViewCache& i
         return nSigOps;
 
     if (flags & SCRIPT_VERIFY_P2SH) {
-        nSigOps += GetP2SHSigOpCount(tx, inputs) * WITNESS_SCALE_FACTOR;
+        auto coins{inputs.AccessCoins(tx)};
+        nSigOps += GetP2SHSigOpCount(tx, std::span{coins}) * WITNESS_SCALE_FACTOR;
     }
 
     for (unsigned int i = 0; i < tx.vin.size(); i++)

src/consensus/tx_verify.h

@@ -5,6 +5,7 @@
 #ifndef BITCOIN_CONSENSUS_TX_VERIFY_H
 #define BITCOIN_CONSENSUS_TX_VERIFY_H
 
+#include <coins.h>
 #include <consensus/amount.h>
 
 #include <stdint.h>
@@ -39,11 +40,12 @@ unsigned int GetLegacySigOpCount(const CTransaction& tx);
 /**
  * Count ECDSA signature operations in pay-to-script-hash inputs.
  *
- * @param[in] mapInputs Map of previous transactions that have outputs we're spending
+ * @param[in] coins Sorted span of Coins containing previous transaction outputs we're spending
  * @return maximum number of sigops required to validate this transaction's inputs
  * @see CTransaction::FetchInputs
  */
-unsigned int GetP2SHSigOpCount(const CTransaction& tx, const CCoinsViewCache& mapInputs);
+template <typename T>
+unsigned int GetP2SHSigOpCount(const CTransaction& tx, const T coins);
 
 /**
  * Compute total signature operation cost of a transaction.

src/test/fuzz/coins_view.cpp

@@ -266,7 +266,8 @@ FUZZ_TARGET(coins_view, .init = initialize_coins_view)
                     // consensus/tx_verify.cpp:130: unsigned int GetP2SHSigOpCount(const CTransaction &, const CCoinsViewCache &): Assertion `!coin.IsSpent()' failed.
                     return;
                 }
-                (void)GetP2SHSigOpCount(transaction, coins_view_cache);
+                auto coins{coins_view_cache.AccessCoins(transaction)};
+                (void)GetP2SHSigOpCount(transaction, std::span{coins});
             },
             [&] {
                 const CTransaction transaction{random_mutable_transaction};

src/test/script_p2sh_tests.cpp

@@ -362,7 +362,8 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
 
     BOOST_CHECK(::AreInputsStandard(CTransaction(txTo), coins));
     // 22 P2SH sigops for all inputs (1 for vin[0], 6 for vin[3], 15 for vin[4]
-    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txTo), coins), 22U);
+    auto coinsTxTo{coins.AccessCoins(CTransaction(txTo))};
+    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txTo), std::span{coinsTxTo}), 22U);
 
     CMutableTransaction txToNonStd1;
     txToNonStd1.vout.resize(1);
@@ -374,7 +375,8 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
     txToNonStd1.vin[0].scriptSig << std::vector<unsigned char>(sixteenSigops.begin(), sixteenSigops.end());
 
     BOOST_CHECK(!::AreInputsStandard(CTransaction(txToNonStd1), coins));
-    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txToNonStd1), coins), 16U);
+    auto coinsTxToNonStd1{coins.AccessCoins(CTransaction(txToNonStd1))};
+    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txToNonStd1), std::span{coinsTxToNonStd1}), 16U);
 
     CMutableTransaction txToNonStd2;
     txToNonStd2.vout.resize(1);
@@ -386,7 +388,8 @@ BOOST_AUTO_TEST_CASE(AreInputsStandard)
     txToNonStd2.vin[0].scriptSig << std::vector<unsigned char>(twentySigops.begin(), twentySigops.end());
 
     BOOST_CHECK(!::AreInputsStandard(CTransaction(txToNonStd2), coins));
-    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txToNonStd2), coins), 20U);
+    auto coinsTxToNonStd2{coins.AccessCoins(CTransaction(txToNonStd2))};
+    BOOST_CHECK_EQUAL(GetP2SHSigOpCount(CTransaction(txToNonStd2), std::span{coinsTxToNonStd2}), 20U);
 }
 
 BOOST_AUTO_TEST_SUITE_END()