Make VerifyWitnessProgram use a Span stack

This allows for very cheap transformations on the range of elements that
are to be passed to ExecuteWitnessScript.

Author: sipa

src/script/interpreter.cpp

@@ -1478,9 +1478,9 @@ bool GenericTransactionSignatureChecker<T>::CheckSequence(const CScriptNum& nSeq
 template class GenericTransactionSignatureChecker<CTransaction>;
 template class GenericTransactionSignatureChecker<CMutableTransaction>;
 
-static bool ExecuteWitnessScript(std::vector<valtype>::const_iterator begin, std::vector<valtype>::const_iterator end, const CScript& scriptPubKey, unsigned int flags, SigVersion sigversion, const BaseSignatureChecker& checker, ScriptError* serror)
+static bool ExecuteWitnessScript(const Span<const valtype>& stack_span, const CScript& scriptPubKey, unsigned int flags, SigVersion sigversion, const BaseSignatureChecker& checker, ScriptError* serror)
 {
-    std::vector<valtype> stack{begin, end};
+    std::vector<valtype> stack{stack_span.begin(), stack_span.end()};
 
     // Disallow stack item size > MAX_SCRIPT_ELEMENT_SIZE in witness stack
     for (const valtype& elem : stack) {
@@ -1499,27 +1499,29 @@ static bool ExecuteWitnessScript(std::vector<valtype>::const_iterator begin, std
 static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion, const std::vector<unsigned char>& program, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
 {
     CScript scriptPubKey;
+    Span<const valtype> stack = MakeSpan(witness.stack);
 
     if (witversion == 0) {
         if (program.size() == WITNESS_V0_SCRIPTHASH_SIZE) {
             // Version 0 segregated witness program: SHA256(CScript) inside the program, CScript + inputs in witness
-            if (witness.stack.size() == 0) {
+            if (stack.size() == 0) {
                 return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY);
             }
-            scriptPubKey = CScript(witness.stack.back().begin(), witness.stack.back().end());
+            const valtype& script_bytes = SpanPopBack(stack);
+            scriptPubKey = CScript(script_bytes.begin(), script_bytes.end());
             uint256 hashScriptPubKey;
             CSHA256().Write(&scriptPubKey[0], scriptPubKey.size()).Finalize(hashScriptPubKey.begin());
             if (memcmp(hashScriptPubKey.begin(), program.data(), 32)) {
                 return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
             }
-            return ExecuteWitnessScript(witness.stack.begin(), witness.stack.end() - 1, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
+            return ExecuteWitnessScript(stack, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
         } else if (program.size() == WITNESS_V0_KEYHASH_SIZE) {
             // Special case for pay-to-pubkeyhash; signature + pubkey in witness
-            if (witness.stack.size() != 2) {
+            if (stack.size() != 2) {
                 return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH); // 2 items in witness
             }
             scriptPubKey << OP_DUP << OP_HASH160 << program << OP_EQUALVERIFY << OP_CHECKSIG;
-            return ExecuteWitnessScript(witness.stack.begin(), witness.stack.end(), scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
+            return ExecuteWitnessScript(stack, scriptPubKey, flags, SigVersion::WITNESS_V0, checker, serror);
         } else {
             return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH);
         }

src/span.h

@@ -8,6 +8,7 @@
 #include <type_traits>
 #include <cstddef>
 #include <algorithm>
+#include <assert.h>
 
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
@@ -27,6 +28,8 @@ class Span
     constexpr C* data() const noexcept { return m_data; }
     constexpr C* begin() const noexcept { return m_data; }
     constexpr C* end() const noexcept { return m_data + m_size; }
+    constexpr C& front() const noexcept { return m_data[0]; }
+    constexpr C& back() const noexcept { return m_data[m_size - 1]; }
     constexpr std::ptrdiff_t size() const noexcept { return m_size; }
     constexpr C& operator[](std::ptrdiff_t pos) const noexcept { return m_data[pos]; }
 
@@ -57,4 +60,15 @@ constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
 template<typename V>
 constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
 
+/** Pop the last element off a span, and return a reference to that element. */
+template <typename T>
+T& SpanPopBack(Span<T>& span)
+{
+    size_t size = span.size();
+    assert(size > 0);
+    T& back = span[size - 1];
+    span = Span<T>(span.data(), size - 1);
+    return back;
+}
+
 #endif