[REFACTOR] Initialize PrecomputedTransactionData in CheckInputScripts

Add a default constructor to `PrecomputedTransactionData`, which doesn't
initialize the struct's members. Instead they're initialized inside the
`CheckInputScripts()` function. This allows a later commit to add the
spent UTXOs to that structure.

Author: sipa

src/script/interpreter.cpp

@@ -1279,18 +1279,29 @@ uint256 GetOutputsHash(const T& txTo)
 } // namespace
 
 template <class T>
-PrecomputedTransactionData::PrecomputedTransactionData(const T& txTo)
+void PrecomputedTransactionData::Init(const T& txTo)
 {
+    assert(!m_ready);
+
     // Cache is calculated only for transactions with witness
     if (txTo.HasWitness()) {
         hashPrevouts = GetPrevoutHash(txTo);
         hashSequence = GetSequenceHash(txTo);
         hashOutputs = GetOutputsHash(txTo);
-        ready = true;
     }
+
+    m_ready = true;
+}
+
+template <class T>
+PrecomputedTransactionData::PrecomputedTransactionData(const T& txTo)
+{
+    Init(txTo);
 }
 
 // explicit instantiation
+template void PrecomputedTransactionData::Init(const CTransaction& txTo);
+template void PrecomputedTransactionData::Init(const CMutableTransaction& txTo);
 template PrecomputedTransactionData::PrecomputedTransactionData(const CTransaction& txTo);
 template PrecomputedTransactionData::PrecomputedTransactionData(const CMutableTransaction& txTo);
 
@@ -1303,7 +1314,7 @@ uint256 SignatureHash(const CScript& scriptCode, const T& txTo, unsigned int nIn
         uint256 hashPrevouts;
         uint256 hashSequence;
         uint256 hashOutputs;
-        const bool cacheready = cache && cache->ready;
+        const bool cacheready = cache && cache->m_ready;
 
         if (!(nHashType & SIGHASH_ANYONECANPAY)) {
             hashPrevouts = cacheready ? cache->hashPrevouts : GetPrevoutHash(txTo);

src/script/interpreter.h

@@ -121,7 +121,12 @@ bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned i
 struct PrecomputedTransactionData
 {
     uint256 hashPrevouts, hashSequence, hashOutputs;
-    bool ready = false;
+    bool m_ready = false;
+
+    PrecomputedTransactionData() = default;
+
+    template <class T>
+    void Init(const T& tx);
 
     template <class T>
     explicit PrecomputedTransactionData(const T& tx);

src/validation.cpp

@@ -1016,7 +1016,7 @@ bool MemPoolAccept::AcceptSingleTransaction(const CTransactionRef& ptx, ATMPArgs
     // scripts (ie, other policy checks pass). We perform the inexpensive
     // checks first and avoid hashing and signature verification unless those
     // checks pass, to mitigate CPU exhaustion denial-of-service attacks.
-    PrecomputedTransactionData txdata(*ptx);
+    PrecomputedTransactionData txdata;
 
     if (!PolicyScriptChecks(args, workspace, txdata)) return false;
 
@@ -1512,6 +1512,10 @@ bool CheckInputScripts(const CTransaction& tx, TxValidationState &state, const C
         return true;
     }
 
+    if (!txdata.m_ready) {
+        txdata.Init(tx);
+    }
+
     for (unsigned int i = 0; i < tx.vin.size(); i++) {
         const COutPoint &prevout = tx.vin[i].prevout;
         const Coin& coin = inputs.AccessCoin(prevout);
@@ -2075,15 +2079,19 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
 
     CBlockUndo blockundo;
 
+    // Precomputed transaction data pointers must not be invalidated
+    // until after `control` has run the script checks (potentially
+    // in multiple threads). Preallocate the vector size so a new allocation
+    // doesn't invalidate pointers into the vector, and keep txsdata in scope
+    // for as long as `control`.
     CCheckQueueControl<CScriptCheck> control(fScriptChecks && g_parallel_script_checks ? &scriptcheckqueue : nullptr);
+    std::vector<PrecomputedTransactionData> txsdata(block.vtx.size());
 
     std::vector<int> prevheights;
     CAmount nFees = 0;
     int nInputs = 0;
     int64_t nSigOpsCost = 0;
     blockundo.vtxundo.reserve(block.vtx.size() - 1);
-    std::vector<PrecomputedTransactionData> txdata;
-    txdata.reserve(block.vtx.size()); // Required so that pointers to individual PrecomputedTransactionData don't get invalidated
     for (unsigned int i = 0; i < block.vtx.size(); i++)
     {
         const CTransaction &tx = *(block.vtx[i]);
@@ -2130,13 +2138,12 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
             return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-blk-sigops");
         }
 
-        txdata.emplace_back(tx);
         if (!tx.IsCoinBase())
         {
             std::vector<CScriptCheck> vChecks;
             bool fCacheResults = fJustCheck; /* Don't cache results if we're actually connecting blocks (still consult the cache, though) */
             TxValidationState tx_state;
-            if (fScriptChecks && !CheckInputScripts(tx, tx_state, view, flags, fCacheResults, fCacheResults, txdata[i], g_parallel_script_checks ? &vChecks : nullptr)) {
+            if (fScriptChecks && !CheckInputScripts(tx, tx_state, view, flags, fCacheResults, fCacheResults, txsdata[i], g_parallel_script_checks ? &vChecks : nullptr)) {
                 // Any transaction validation failure in ConnectBlock is a block consensus failure
                 state.Invalid(BlockValidationResult::BLOCK_CONSENSUS,
                               tx_state.GetRejectReason(), tx_state.GetDebugMessage());