policy: GetScriptForTransactionInput to figure out P2SH, witness, taproot

Github-Pull: bitcoin#28408
Rebased-From: 62b0ebd

Author: luke-jr

src/policy/policy.cpp

@@ -372,3 +372,50 @@ int64_t GetVirtualTransactionInputSize(const CTxIn& txin, int64_t nSigOpCost, un
 {
     return GetVirtualTransactionSize(GetTransactionInputWeight(txin), nSigOpCost, bytes_per_sigop);
 }
+
+std::pair<CScript, unsigned int> GetScriptForTransactionInput(CScript prevScript, const CTxIn& txin)
+{
+    bool p2sh = false;
+    if (prevScript.IsPayToScriptHash()) {
+        std::vector <std::vector<unsigned char> > stack;
+        if (!EvalScript(stack, txin.scriptSig, SCRIPT_VERIFY_NONE, BaseSignatureChecker(), SigVersion::BASE)) {
+            return std::make_pair(CScript(), 0);
+        }
+        if (stack.empty()) {
+            return std::make_pair(CScript(), 0);
+        }
+        prevScript = CScript(stack.back().begin(), stack.back().end());
+        p2sh = true;
+    }
+
+    int witnessversion = 0;
+    std::vector<unsigned char> witnessprogram;
+
+    if (!prevScript.IsWitnessProgram(witnessversion, witnessprogram)) {
+        // For P2SH, scriptSig is always push-only, so the actual script is only the last stack item
+        // For non-P2SH, prevScript is likely the real script, but not part of this transaction, and scriptSig could very well be executable, so return the latter instead
+        return std::make_pair(p2sh ? prevScript : txin.scriptSig, WITNESS_SCALE_FACTOR);
+    }
+
+    Span stack{txin.scriptWitness.stack};
+
+    if (witnessversion == 0 && witnessprogram.size() == WITNESS_V0_SCRIPTHASH_SIZE) {
+        if (stack.empty()) return std::make_pair(CScript(), 0);  // invalid
+        auto& script_data = stack.back();
+        prevScript = CScript(script_data.begin(), script_data.end());
+        return std::make_pair(prevScript, 1);
+    }
+
+    if (witnessversion == 1 && witnessprogram.size() == WITNESS_V1_TAPROOT_SIZE && !p2sh) {
+        if (stack.size() >= 2 && !stack.back().empty() && stack.back()[0] == ANNEX_TAG) {
+            SpanPopBack(stack);
+        }
+        if (stack.size() >= 2) {
+            SpanPopBack(stack);  // Ignore control block
+            prevScript = CScript(stack.back().begin(), stack.back().end());
+            return std::make_pair(prevScript, 1);
+        }
+    }
+
+    return std::make_pair(CScript(), 0);
+}

src/policy/policy.h

@@ -15,6 +15,7 @@
 #include <cstdint>
 #include <string>
 #include <unordered_set>
+#include <utility>
 
 class CCoinsViewCache;
 class CFeeRate;
@@ -185,4 +186,6 @@ static inline int64_t GetVirtualTransactionInputSize(const CTxIn& tx)
     return GetVirtualTransactionInputSize(tx, 0, 0);
 }
 
+std::pair<CScript, unsigned int> GetScriptForTransactionInput(CScript prevScript, const CTxIn&);
+
 #endif // BITCOIN_POLICY_POLICY_H