Add MissingDataBehavior and make TransactionSignatureChecker handle it

This allows specifying how *TransactionSignatureChecker will behave when
presented with missing transaction data such as amounts spent, BIP341 data,
or spent outputs.

As all call sites still (implicitly) use MissingDataBehavior::ASSERT_FAIL,
this commit introduces no change in behavior.

src/script/interpreter.cpp

@@ -1488,8 +1488,20 @@ static const CHashWriter HASHER_TAPLEAF = TaggedHash("TapLeaf");
 static const CHashWriter HASHER_TAPBRANCH = TaggedHash("TapBranch");
 static const CHashWriter HASHER_TAPTWEAK = TaggedHash("TapTweak");
 
+static bool HandleMissingData(MissingDataBehavior mdb)
+{
+    switch (mdb) {
+    case MissingDataBehavior::ASSERT_FAIL:
+        assert(!"Missing data");
+        break;
+    case MissingDataBehavior::FAIL:
+        return false;
+    }
+    assert(!"Unknown MissingDataBehavior value");
+}
+
 template<typename T>
-bool SignatureHashSchnorr(uint256& hash_out, const ScriptExecutionData& execdata, const T& tx_to, uint32_t in_pos, uint8_t hash_type, SigVersion sigversion, const PrecomputedTransactionData& cache)
+bool SignatureHashSchnorr(uint256& hash_out, const ScriptExecutionData& execdata, const T& tx_to, uint32_t in_pos, uint8_t hash_type, SigVersion sigversion, const PrecomputedTransactionData& cache, MissingDataBehavior mdb)
 {
     uint8_t ext_flag, key_version;
     switch (sigversion) {
@@ -1509,7 +1521,9 @@ bool SignatureHashSchnorr(uint256& hash_out, const ScriptExecutionData& execdata
         assert(false);
     }
     assert(in_pos < tx_to.vin.size());
-    assert(cache.m_bip341_taproot_ready && cache.m_spent_outputs_ready);
+    if (!(cache.m_bip341_taproot_ready && cache.m_spent_outputs_ready)) {
+        return HandleMissingData(mdb);
+    }
 
     CHashWriter ss = HASHER_TAPSIGHASH;
 
@@ -1696,7 +1710,7 @@ bool GenericTransactionSignatureChecker<T>::CheckSchnorrSignature(Span<const uns
     }
     uint256 sighash;
     assert(this->txdata);
-    if (!SignatureHashSchnorr(sighash, execdata, *txTo, nIn, hashtype, sigversion, *this->txdata)) {
+    if (!SignatureHashSchnorr(sighash, execdata, *txTo, nIn, hashtype, sigversion, *this->txdata, m_mdb)) {
         return set_error(serror, SCRIPT_ERR_SCHNORR_SIG_HASHTYPE);
     }
     if (!VerifySchnorrSignature(sig, pubkey, sighash)) return set_error(serror, SCRIPT_ERR_SCHNORR_SIG);

src/script/interpreter.h

@@ -247,11 +247,21 @@ public:
     virtual ~BaseSignatureChecker() {}
 };
 
+/** Enum to specify what *TransactionSignatureChecker's behavior should be
+ *  when dealing with missing transaction data.
+ */
+enum class MissingDataBehavior
+{
+    ASSERT_FAIL,  //!< Abort execution through assertion failure (for consensus code)
+    FAIL,         //!< Just act as if the signature was invalid
+};
+
 template <class T>
 class GenericTransactionSignatureChecker : public BaseSignatureChecker
 {
 private:
     const T* txTo;
+    const MissingDataBehavior m_mdb;
     unsigned int nIn;
     const CAmount amount;
     const PrecomputedTransactionData* txdata;
@@ -261,8 +271,8 @@ protected:
     virtual bool VerifySchnorrSignature(Span<const unsigned char> sig, const XOnlyPubKey& pubkey, const uint256& sighash) const;
 
 public:
-    GenericTransactionSignatureChecker(const T* txToIn, unsigned int nInIn, const CAmount& amountIn) : txTo(txToIn), nIn(nInIn), amount(amountIn), txdata(nullptr) {}
+    GenericTransactionSignatureChecker(const T* txToIn, unsigned int nInIn, const CAmount& amountIn, MissingDataBehavior mdb = MissingDataBehavior::ASSERT_FAIL) : txTo(txToIn), m_mdb(mdb), nIn(nInIn), amount(amountIn), txdata(nullptr) {}
-    GenericTransactionSignatureChecker(const T* txToIn, unsigned int nInIn, const CAmount& amountIn, const PrecomputedTransactionData& txdataIn) : txTo(txToIn), nIn(nInIn), amount(amountIn), txdata(&txdataIn) {}
+    GenericTransactionSignatureChecker(const T* txToIn, unsigned int nInIn, const CAmount& amountIn, const PrecomputedTransactionData& txdataIn, MissingDataBehavior mdb = MissingDataBehavior::ASSERT_FAIL) : txTo(txToIn), m_mdb(mdb), nIn(nInIn), amount(amountIn), txdata(&txdataIn) {}
     bool CheckECDSASignature(const std::vector<unsigned char>& scriptSig, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion) const override;
     bool CheckSchnorrSignature(Span<const unsigned char> sig, Span<const unsigned char> pubkey, SigVersion sigversion, const ScriptExecutionData& execdata, ScriptError* serror = nullptr) const override;
     bool CheckLockTime(const CScriptNum& nLockTime) const override;