refactor: Remove SignetTxs::m_valid and use optional instead

m_valid implies the block solution has been checked, which is not the
case. It only means the txs could be parsed. C++17 comes with
std::optional, so just use that instead.

src/signet.cpp

@@ -65,7 +65,7 @@ static uint256 ComputeModifiedMerkleRoot(const CMutableTransaction& cb, const CB
     return ComputeMerkleRoot(std::move(leaves));
 }
 
-SignetTxs SignetTxs::Create(const CBlock& block, const CScript& challenge)
+Optional<SignetTxs> SignetTxs::Create(const CBlock& block, const CScript& challenge)
 {
     CMutableTransaction tx_to_spend;
     tx_to_spend.nVersion = 0;
@@ -83,12 +83,12 @@ SignetTxs SignetTxs::Create(const CBlock& block, const CScript& challenge)
     // responses from block coinbase tx
 
     // find and delete signet signature
-    if (block.vtx.empty()) return invalid(); // no coinbase tx in block; invalid
+    if (block.vtx.empty()) return nullopt; // no coinbase tx in block; invalid
     CMutableTransaction modified_cb(*block.vtx.at(0));
 
     const int cidx = GetWitnessCommitmentIndex(block);
     if (cidx == NO_WITNESS_COMMITMENT) {
-        return invalid(); // require a witness commitment
+        return nullopt; // require a witness commitment
     }
 
     CScript& witness_commitment = modified_cb.vout.at(cidx).scriptPubKey;
@@ -101,9 +101,9 @@ SignetTxs SignetTxs::Create(const CBlock& block, const CScript& challenge)
             VectorReader v(SER_NETWORK, INIT_PROTO_VERSION, signet_solution, 0);
             v >> tx_spending.vin[0].scriptSig;
             v >> tx_spending.vin[0].scriptWitness.stack;
-            if (!v.empty()) return invalid(); // extraneous data encountered
+            if (!v.empty()) return nullopt; // extraneous data encountered
         } catch (const std::exception&) {
-            return invalid(); // parsing error
+            return nullopt; // parsing error
         }
     }
     uint256 signet_merkle = ComputeModifiedMerkleRoot(modified_cb, block);
@@ -117,7 +117,7 @@ SignetTxs SignetTxs::Create(const CBlock& block, const CScript& challenge)
     tx_to_spend.vin[0].scriptSig << block_data;
     tx_spending.vin[0].prevout = COutPoint(tx_to_spend.GetHash(), 0);
 
-    return {tx_to_spend, tx_spending};
+    return SignetTxs{tx_to_spend, tx_spending};
 }
 
 // Signet block solution checker
@@ -129,19 +129,19 @@ bool CheckSignetBlockSolution(const CBlock& block, const Consensus::Params& cons
     }
 
     const CScript challenge(consensusParams.signet_challenge.begin(), consensusParams.signet_challenge.end());
-    const SignetTxs signet_txs(block, challenge);
+    const Optional<SignetTxs> signet_txs = SignetTxs::Create(block, challenge);
 
-    if (!signet_txs.m_valid) {
+    if (!signet_txs) {
         LogPrint(BCLog::VALIDATION, "CheckSignetBlockSolution: Errors in block (block solution parse failure)\n");
         return false;
     }
 
-    const CScript& scriptSig = signet_txs.m_to_sign.vin[0].scriptSig;
-    const CScriptWitness& witness = signet_txs.m_to_sign.vin[0].scriptWitness;
+    const CScript& scriptSig = signet_txs->m_to_sign.vin[0].scriptSig;
+    const CScriptWitness& witness = signet_txs->m_to_sign.vin[0].scriptWitness;
 
-    TransactionSignatureChecker sigcheck(&signet_txs.m_to_sign, /*nIn=*/ 0, /*amount=*/ signet_txs.m_to_spend.vout[0].nValue);
+    TransactionSignatureChecker sigcheck(&signet_txs->m_to_sign, /*nIn=*/ 0, /*amount=*/ signet_txs->m_to_spend.vout[0].nValue);
 
-    if (!VerifyScript(scriptSig, signet_txs.m_to_spend.vout[0].scriptPubKey, &witness, BLOCK_SCRIPT_VERIFY_FLAGS, sigcheck)) {
+    if (!VerifyScript(scriptSig, signet_txs->m_to_spend.vout[0].scriptPubKey, &witness, BLOCK_SCRIPT_VERIFY_FLAGS, sigcheck)) {
         LogPrint(BCLog::VALIDATION, "CheckSignetBlockSolution: Errors in block (block solution invalid)\n");
         return false;
     }

src/signet.h

@@ -9,6 +9,8 @@
 #include <primitives/block.h>
 #include <primitives/transaction.h>
 
+#include <optional.h>
+
 /**
  * Extract signature and check whether a block has a valid solution
  */
@@ -22,21 +24,14 @@ bool CheckSignetBlockSolution(const CBlock& block, const Consensus::Params& cons
  * 2. It skips the nonce.
  */
 class SignetTxs {
-private:
-    struct invalid {};
-    SignetTxs(invalid i) : m_to_spend(), m_to_sign(), m_valid(false) { }
-
     template<class T1, class T2>
-    SignetTxs(const T1& to_spend, const T2& to_sign) : m_to_spend{to_spend}, m_to_sign{to_sign}, m_valid(true) { }
-
-    static SignetTxs Create(const CBlock& block, const CScript& challenge);
+    SignetTxs(const T1& to_spend, const T2& to_sign) : m_to_spend{to_spend}, m_to_sign{to_sign} { }
 
 public:
-    SignetTxs(const CBlock& block, const CScript& challenge) : SignetTxs(Create(block, challenge)) { }
+    static Optional<SignetTxs> Create(const CBlock& block, const CScript& challenge);
 
     const CTransaction m_to_spend;
     const CTransaction m_to_sign;
-    const bool m_valid;
 };
 
 #endif // BITCOIN_SIGNET_H

src/test/fuzz/signet.cpp

@@ -29,6 +29,6 @@ void test_one_input(const std::vector<uint8_t>& buffer)
     }
     (void)CheckSignetBlockSolution(*block, Params().GetConsensus());
     if (GetWitnessCommitmentIndex(*block) != NO_WITNESS_COMMITMENT) {
-        (void)SignetTxs(*block, ConsumeScript(fuzzed_data_provider));
+        (void)SignetTxs::Create(*block, ConsumeScript(fuzzed_data_provider));
     }
 }