Error on missing amount in signrawtransaction*

Signatures using segregated witness commit to the amount being spent,
so that value must be passed into signrawtransactionwithkey and
signrawtransactionwithwallet. This ensures an error is issued if that
doesn't happen, rather than just assuming the value is 0 and producing
a signature that is almost certainly invalid.

Author: ajtowns

src/rpc/rawtransaction.cpp

@@ -811,7 +811,7 @@ UniValue SignTransaction(CMutableTransaction& mtx, const UniValue& prevTxsUnival
                 }
                 Coin newcoin;
                 newcoin.out.scriptPubKey = scriptPubKey;
-                newcoin.out.nValue = 0;
+                newcoin.out.nValue = MAX_MONEY;
                 if (prevOut.exists("amount")) {
                     newcoin.out.nValue = AmountFromValue(find_value(prevOut, "amount"));
                 }
@@ -884,6 +884,11 @@ UniValue SignTransaction(CMutableTransaction& mtx, const UniValue& prevTxsUnival
 
         UpdateInput(txin, sigdata);
 
+        // amount must be specified for valid segwit signature
+        if (amount == MAX_MONEY && !txin.scriptWitness.IsNull()) {
+            throw JSONRPCError(RPC_TYPE_ERROR, strprintf("Missing amount for %s", coin.out.ToString()));
+        }
+
         ScriptError serror = SCRIPT_ERR_OK;
         if (!VerifyScript(txin.scriptSig, prevPubKey, &txin.scriptWitness, STANDARD_SCRIPT_VERIFY_FLAGS, TransactionSignatureChecker(&txConst, i, amount), &serror)) {
             if (serror == SCRIPT_ERR_INVALID_STACK_OPERATION) {