Abstract out IsSegWitOutput from utxoupdatepsbt

This is not a pure refactor; additional functionality is added in
IsSegWitOutput which lets it recurse into P2SH when a
SigningProvider is provided that knows about the inner script.

Author: sipa

src/rpc/rawtransaction.cpp

@@ -1540,9 +1540,7 @@ UniValue utxoupdatepsbt(const JSONRPCRequest& request)
 
         const Coin& coin = view.AccessCoin(psbtx.tx->vin[i].prevout);
 
-        std::vector<std::vector<unsigned char>> solutions_data;
-        txnouttype which_type = Solver(coin.out.scriptPubKey, solutions_data);
-        if (which_type == TX_WITNESS_V0_SCRIPTHASH || which_type == TX_WITNESS_V0_KEYHASH || which_type == TX_WITNESS_UNKNOWN) {
+        if (IsSegWitOutput(DUMMY_SIGNING_PROVIDER, coin.out.scriptPubKey)) {
             input.witness_utxo = coin.out;
         }
     }

src/script/sign.cpp

@@ -505,3 +505,19 @@ FlatSigningProvider Merge(const FlatSigningProvider& a, const FlatSigningProvide
     ret.origins.insert(b.origins.begin(), b.origins.end());
     return ret;
 }
+
+bool IsSegWitOutput(const SigningProvider& provider, const CScript& script)
+{
+    std::vector<valtype> solutions;
+    auto whichtype = Solver(script, solutions);
+    if (whichtype == TX_WITNESS_V0_SCRIPTHASH || whichtype == TX_WITNESS_V0_KEYHASH || whichtype == TX_WITNESS_UNKNOWN) return true;
+    if (whichtype == TX_SCRIPTHASH) {
+        auto h160 = uint160(solutions[0]);
+        CScript subscript;
+        if (provider.GetCScript(h160, subscript)) {
+            whichtype = Solver(subscript, solutions);
+            if (whichtype == TX_WITNESS_V0_SCRIPTHASH || whichtype == TX_WITNESS_V0_KEYHASH || whichtype == TX_WITNESS_UNKNOWN) return true;
+        }
+    }
+    return false;
+}

src/script/sign.h

@@ -232,4 +232,7 @@ void UpdateInput(CTxIn& input, const SignatureData& data);
  * Solvability is unrelated to whether we consider this output to be ours. */
 bool IsSolvable(const SigningProvider& provider, const CScript& script);
 
+/** Check whether a scriptPubKey is known to be segwit. */
+bool IsSegWitOutput(const SigningProvider& provider, const CScript& script);
+
 #endif // BITCOIN_SCRIPT_SIGN_H