Abstract out IsSolvable from Witnessifier

src/policy/policy.h

@@ -49,28 +49,28 @@ static const unsigned int DUST_RELAY_TX_FEE = 3000;
  * 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 |
-                                                         SCRIPT_VERIFY_DERSIG |
-                                                         SCRIPT_VERIFY_STRICTENC |
-                                                         SCRIPT_VERIFY_MINIMALDATA |
-                                                         SCRIPT_VERIFY_NULLDUMMY |
-                                                         SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
-                                                         SCRIPT_VERIFY_CLEANSTACK |
-                                                         SCRIPT_VERIFY_MINIMALIF |
-                                                         SCRIPT_VERIFY_NULLFAIL |
-                                                         SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
-                                                         SCRIPT_VERIFY_CHECKSEQUENCEVERIFY |
-                                                         SCRIPT_VERIFY_LOW_S |
-                                                         SCRIPT_VERIFY_WITNESS |
-                                                         SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM |
-                                                         SCRIPT_VERIFY_WITNESS_PUBKEYTYPE;
+static constexpr unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY_FLAGS |
+                                                             SCRIPT_VERIFY_DERSIG |
+                                                             SCRIPT_VERIFY_STRICTENC |
+                                                             SCRIPT_VERIFY_MINIMALDATA |
+                                                             SCRIPT_VERIFY_NULLDUMMY |
+                                                             SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
+                                                             SCRIPT_VERIFY_CLEANSTACK |
+                                                             SCRIPT_VERIFY_MINIMALIF |
+                                                             SCRIPT_VERIFY_NULLFAIL |
+                                                             SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
+                                                             SCRIPT_VERIFY_CHECKSEQUENCEVERIFY |
+                                                             SCRIPT_VERIFY_LOW_S |
+                                                             SCRIPT_VERIFY_WITNESS |
+                                                             SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM |
+                                                             SCRIPT_VERIFY_WITNESS_PUBKEYTYPE;
 
 /** 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;
 
 /** Used as the flags parameter to sequence and nLocktime checks in non-consensus code. */
-static const unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = LOCKTIME_VERIFY_SEQUENCE |
-                                                           LOCKTIME_MEDIAN_TIME_PAST;
+static constexpr unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = LOCKTIME_VERIFY_SEQUENCE |
+                                                               LOCKTIME_MEDIAN_TIME_PAST;
 
 CAmount GetDustThreshold(const CTxOut& txout, const CFeeRate& dustRelayFee);
 

src/script/sign.cpp

@@ -422,3 +422,22 @@ 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,
+    // it will reject witness outputs that require signing with an uncompressed public key.
+    DummySignatureCreator creator(&store);
+    SignatureData sigs;
+    // Make sure that STANDARD_SCRIPT_VERIFY_FLAGS includes SCRIPT_VERIFY_WITNESS_PUBKEYTYPE, the most
+    // important property this function is designed to test for.
+    static_assert(STANDARD_SCRIPT_VERIFY_FLAGS & SCRIPT_VERIFY_WITNESS_PUBKEYTYPE, "IsSolvable requires standard script flags to include WITNESS_PUBKEYTYPE");
+    if (ProduceSignature(creator, script, sigs)) {
+        // VerifyScript check is just defensive, and should never fail.
+        assert(VerifyScript(sigs.scriptSig, script, &sigs.scriptWitness, 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

src/wallet/rpcwallet.cpp

@@ -1204,12 +1204,7 @@ public:
         if (pwallet) {
             CScript basescript = GetScriptForDestination(keyID);
             CScript witscript = GetScriptForWitness(basescript);
-            SignatureData sigs;
-            // 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.
-            if (!ProduceSignature(DummySignatureCreator(pwallet), witscript, sigs) ||
-                !VerifyScript(sigs.scriptSig, witscript, &sigs.scriptWitness, MANDATORY_SCRIPT_VERIFY_FLAGS | SCRIPT_VERIFY_WITNESS_PUBKEYTYPE, DummySignatureCreator(pwallet).Checker())) {
+            if (!IsSolvable(*pwallet, witscript)) {
                 return false;
             }
             return ExtractDestination(witscript, result);
@@ -1228,12 +1223,7 @@ public:
                 return true;
             }
             CScript witscript = GetScriptForWitness(subscript);
-            SignatureData sigs;
-            // 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.
-            if (!ProduceSignature(DummySignatureCreator(pwallet), witscript, sigs) ||
-                !VerifyScript(sigs.scriptSig, witscript, &sigs.scriptWitness, MANDATORY_SCRIPT_VERIFY_FLAGS | SCRIPT_VERIFY_WITNESS_PUBKEYTYPE, DummySignatureCreator(pwallet).Checker())) {
+            if (!IsSolvable(*pwallet, witscript)) {
                 return false;
             }
             return ExtractDestination(witscript, result);