Back port of IsSolvable to check scripts solvability.

This is a modified version of btc@0c8ea6380c9f402ed9777fd015b117ba13125a35

src/policy/policy.h

@@ -27,7 +27,7 @@ static const unsigned int MAX_P2SH_SIGOPS = 15;
  * with. However scripts violating these flags may still be present in valid
  * blocks and we must accept those blocks.
  */
-static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY_FLAGS |
+static constexpr unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY_FLAGS |
                                                          SCRIPT_VERIFY_DERSIG |
                                                          SCRIPT_VERIFY_STRICTENC |
                                                          SCRIPT_VERIFY_MINIMALDATA |
@@ -36,7 +36,7 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_LOW_S;
 
 /** For convenience, standard but not mandatory verify flags. */
-static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;
+static constexpr unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;
 
 // Sanity check the magic numbers when we change them
 BOOST_STATIC_ASSERT(DEFAULT_BLOCK_MAX_SIZE <= MAX_BLOCK_SIZE_CURRENT);

src/script/sign.cpp

@@ -380,3 +380,18 @@ bool DummySignatureCreator::CreateSig(std::vector<unsigned char>& vchSig, const
     vchSig[6 + 33 + 32] = SIGHASH_ALL;
     return true;
 }
+
+bool IsSolvable(const CKeyStore& store, const CScript& script)
+{
+    // This check is to make sure that the script we created can actually be solved for and signed by us
+    // if we were to have the private keys. This is just to make sure that the script is valid and that,
+    // if found in a transaction, we would still accept and relay that transaction. In particular,
+    DummySignatureCreator creator(&store);
+    SignatureData sigs;
+    if (ProduceSignature(creator, script, sigs, false)) {
+        // VerifyScript check is just defensive, and should never fail.
+        assert(VerifyScript(sigs.scriptSig, script, STANDARD_SCRIPT_VERIFY_FLAGS, creator.Checker()));
+        return true;
+    }
+    return false;
+}

src/script/sign.h

@@ -81,4 +81,10 @@ SignatureData CombineSignatures(const CScript& scriptPubKey, const BaseSignature
 SignatureData DataFromTransaction(const CMutableTransaction& tx, unsigned int nIn);
 void UpdateTransaction(CMutableTransaction& tx, unsigned int nIn, const SignatureData& data);
 
+/* Check whether we know how to sign for an output like this, assuming we
+  * have all private keys. While this function does not need private keys, the passed
+  * keystore is used to look up public keys and redeemscripts by hash.
+  * Solvability is unrelated to whether we consider this output to be ours. */
+bool IsSolvable(const CKeyStore& store, const CScript& script);
+
 #endif // BITCOIN_SCRIPT_SIGN_H