Only pass things committed to by tx's witness hash to CScriptCheck

This clarifies a bit more the ways in which the new script execution
cache could break consensus in the future if additional data from
the CCoins object were to be used as a part of script execution.

After this change, any such consensus breaks should be very visible
to reviewers, hopefully ensuring no such changes can be made.

src/test/script_P2SH_tests.cpp

@@ -112,7 +112,8 @@ BOOST_AUTO_TEST_CASE(sign)
         {
             CScript sigSave = txTo[i].vin[0].scriptSig;
             txTo[i].vin[0].scriptSig = txTo[j].vin[0].scriptSig;
-            bool sigOK = CScriptCheck(CCoins(txFrom, 0), txTo[i], 0, SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_STRICTENC, false, &txdata)();
+            const CTxOut& output = txFrom.vout[txTo[i].vin[0].prevout.n];
+            bool sigOK = CScriptCheck(output.scriptPubKey, output.nValue, txTo[i], 0, SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_STRICTENC, false, &txdata)();
             if (i == j)
                 BOOST_CHECK_MESSAGE(sigOK, strprintf("VerifySignature %d %d", i, j));
             else

src/test/transaction_tests.cpp

@@ -481,7 +481,8 @@ BOOST_AUTO_TEST_CASE(test_big_witness_transaction) {
 
     for(uint32_t i = 0; i < mtx.vin.size(); i++) {
         std::vector<CScriptCheck> vChecks;
-        CScriptCheck check(coins, tx, i, SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS, false, &txdata);
+        const CTxOut& output = coins.vout[tx.vin[i].prevout.n];
+        CScriptCheck check(output.scriptPubKey, output.nValue, tx, i, SCRIPT_VERIFY_P2SH | SCRIPT_VERIFY_WITNESS, false, &txdata);
         vChecks.push_back(CScriptCheck());
         check.swap(vChecks.back());
         control.Add(vChecks);

src/validation.cpp

@@ -1119,8 +1119,16 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, const CCoinsVi
                 const CCoins* coins = inputs.AccessCoins(prevout.hash);
                 assert(coins);
 
+                // We very carefully only pass in things to CScriptCheck which
+                // are clearly committed to by tx' witness hash. This provides
+                // a sanity check that our caching is not introducing consensus
+                // failures through additional data in, eg, the coins being
+                // spent being checked as a part of CScriptCheck.
+                const CScript& scriptPubKey = coins->vout[prevout.n].scriptPubKey;
+                const CAmount amount = coins->vout[prevout.n].nValue;
+
                 // Verify signature
-                CScriptCheck check(*coins, tx, i, flags, cacheStore, &txdata);
+                CScriptCheck check(scriptPubKey, amount, tx, i, flags, cacheStore, &txdata);
                 if (pvChecks) {
                     pvChecks->push_back(CScriptCheck());
                     check.swap(pvChecks->back());
@@ -1132,7 +1140,7 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, const CCoinsVi
                         // arguments; if so, don't trigger DoS protection to
                         // avoid splitting the network between upgraded and
                         // non-upgraded nodes.
-                        CScriptCheck check2(*coins, tx, i,
+                        CScriptCheck check2(scriptPubKey, amount, tx, i,
                                 flags & ~STANDARD_NOT_MANDATORY_VERIFY_FLAGS, cacheStore, &txdata);
                         if (check2())
                             return state.Invalid(false, REJECT_NONSTANDARD, strprintf("non-mandatory-script-verify-flag (%s)", ScriptErrorString(check.GetScriptError())));

src/validation.h

@@ -404,8 +404,8 @@ private:
 
 public:
     CScriptCheck(): amount(0), ptxTo(0), nIn(0), nFlags(0), cacheStore(false), error(SCRIPT_ERR_UNKNOWN_ERROR) {}
-    CScriptCheck(const CCoins& txFromIn, const CTransaction& txToIn, unsigned int nInIn, unsigned int nFlagsIn, bool cacheIn, PrecomputedTransactionData* txdataIn) :
+    CScriptCheck(const CScript& scriptPubKeyIn, const CAmount amountIn, const CTransaction& txToIn, unsigned int nInIn, unsigned int nFlagsIn, bool cacheIn, PrecomputedTransactionData* txdataIn) :
-        scriptPubKey(txFromIn.vout[txToIn.vin[nInIn].prevout.n].scriptPubKey), amount(txFromIn.vout[txToIn.vin[nInIn].prevout.n].nValue),
+        scriptPubKey(scriptPubKeyIn), amount(amountIn),
         ptxTo(&txToIn), nIn(nInIn), nFlags(nFlagsIn), cacheStore(cacheIn), error(SCRIPT_ERR_UNKNOWN_ERROR), txdata(txdataIn) { }
 
     bool operator()();