Auto merge of #4986 - str4d:zcash-script-precompute, r=str4d

zcash_script: Add API to verify scripts with precomputed tx data

This enables users of libzcash_script to more efficiently verify transactions with multiple inputs.

Closes zcash/zcash#4876.

src/script/zcash_script.cpp

@@ -71,6 +71,69 @@ struct ECCryptoClosure
 ECCryptoClosure instance_of_eccryptoclosure;
 }
 
+struct PrecomputedTransaction {
+    const CTransaction tx;
+    const PrecomputedTransactionData txdata;
+
+    PrecomputedTransaction(CTransaction txIn) : tx(txIn), txdata(txIn) {}
+};
+
+void* zcash_script_new_precomputed_tx(
+    const unsigned char* txTo,
+    unsigned int txToLen,
+    zcash_script_error* err)
+{
+    try {
+        TxInputStream stream(SER_NETWORK, PROTOCOL_VERSION, txTo, txToLen);
+        CTransaction tx;
+        stream >> tx;
+        if (GetSerializeSize(tx, SER_NETWORK, PROTOCOL_VERSION) != txToLen) {
+            set_error(err, zcash_script_ERR_TX_SIZE_MISMATCH);
+            return nullptr;
+        }
+
+        // Regardless of the verification result, the tx did not error.
+        set_error(err, zcash_script_ERR_OK);
+        auto preTx = new PrecomputedTransaction(tx);
+        return preTx;
+    } catch (const std::exception&) {
+        set_error(err, zcash_script_ERR_TX_DESERIALIZE); // Error deserializing
+        return nullptr;
+    }
+}
+
+void zcash_script_free_precomputed_tx(void* pre_preTx)
+{
+    PrecomputedTransaction* preTx = static_cast<PrecomputedTransaction*>(pre_preTx);
+    delete preTx;
+    preTx = nullptr;
+}
+
+int zcash_script_verify_precomputed(
+    const void* pre_preTx,
+    unsigned int nIn,
+    const unsigned char* scriptPubKey,
+    unsigned int scriptPubKeyLen,
+    int64_t amount,
+    unsigned int flags,
+    uint32_t consensusBranchId,
+    zcash_script_error* err)
+{
+    const PrecomputedTransaction* preTx = static_cast<const PrecomputedTransaction*>(pre_preTx);
+    if (nIn >= preTx->tx.vin.size())
+        return set_error(err, zcash_script_ERR_TX_INDEX);
+
+    // Regardless of the verification result, the tx did not error.
+    set_error(err, zcash_script_ERR_OK);
+    return VerifyScript(
+        preTx->tx.vin[nIn].scriptSig,
+        CScript(scriptPubKey, scriptPubKey + scriptPubKeyLen),
+        flags,
+        TransactionSignatureChecker(&preTx->tx, nIn, amount, preTx->txdata),
+        consensusBranchId,
+        NULL);
+}
+
 int zcash_script_verify(
     const unsigned char *scriptPubKey, unsigned int scriptPubKeyLen,
     int64_t amount,

src/script/zcash_script.h

@@ -33,7 +33,7 @@
 extern "C" {
 #endif
 
-#define ZCASH_SCRIPT_API_VER 0
+#define ZCASH_SCRIPT_API_VER 1
 
 typedef enum zcash_script_error_t
 {
@@ -51,10 +51,46 @@ enum
     zcash_script_SCRIPT_FLAGS_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9), // enable CHECKLOCKTIMEVERIFY (BIP65)
 };
 
+/// Deserializes the given transaction and precomputes values to improve
+/// script verification performance.
+///
+/// Returns a pointer to the precomputed transaction. Free this with
+/// zcash_script_free_precomputed_tx once you are done.
+///
+/// If not NULL, err will contain an error/success code for the operation.
+void* zcash_script_new_precomputed_tx(
+    const unsigned char* txTo,
+    unsigned int txToLen,
+    zcash_script_error* err);
+
+/// Frees a precomputed transaction previously created with
+/// zcash_script_new_precomputed_tx.
+void zcash_script_free_precomputed_tx(void* preTx);
+
+/// Returns 1 if the input nIn of the precomputed transaction pointed to by
+/// preTx correctly spends the scriptPubKey pointed to by scriptPubKey under
+/// the additional constraints specified by flags.
+///
+/// If not NULL, err will contain an error/success code for the operation.
+/// Note that script verification failure is indicated by err being set to
+/// zcash_script_ERR_OK and a return value of 0.
+EXPORT_SYMBOL int zcash_script_verify_precomputed(
+    const void* preTx,
+    unsigned int nIn,
+    const unsigned char* scriptPubKey,
+    unsigned int scriptPubKeyLen,
+    int64_t amount,
+    unsigned int flags,
+    uint32_t consensusBranchId,
+    zcash_script_error* err);
+
 /// Returns 1 if the input nIn of the serialized transaction pointed to by
 /// txTo correctly spends the scriptPubKey pointed to by scriptPubKey under
 /// the additional constraints specified by flags.
-/// If not NULL, err will contain an error/success code for the operation
+///
+/// If not NULL, err will contain an error/success code for the operation.
+/// Note that script verification failure is indicated by err being set to
+/// zcash_script_ERR_OK and a return value of 0.
 EXPORT_SYMBOL int zcash_script_verify(
     const unsigned char *scriptPubKey, unsigned int scriptPubKeyLen,
     int64_t amount,