wallet: Show fee in results for signrawtransaction* for segwit inputs

doc/release-notes-pr12911.md

@@ -0,0 +1,7 @@
+RPC changes
+------------
+
+### Low-level changes
+
+- `signrawtransactionwithkey` and `signrawtransactionwithwallet` will now include a `fee` entry in the results,
+  for cases where the fee is known.

src/rpc/rawtransaction.cpp

@@ -741,6 +741,8 @@ static UniValue signrawtransactionwithkey(const JSONRPCRequest& request)
                     {
                         {RPCResult::Type::STR_HEX, "hex", "The hex-encoded raw transaction with signature(s)"},
                         {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
+                        {RPCResult::Type::NUM, "fee", "The fee (input amounts minus output amounts), if known"},
+                        {RPCResult::Type::NUM, "feerate", "The fee rate (in " + CURRENCY_UNIT + "/kB), if fee is known"},
                         {RPCResult::Type::ARR, "errors", "Script verification errors (if there are any)",
                         {
                             {RPCResult::Type::OBJ, "", "",

src/rpc/rawtransaction_util.cpp

@@ -281,20 +281,26 @@ void SignTransaction(CMutableTransaction& mtx, const SigningProvider* keystore,
     // transaction to avoid rehashing.
     const CTransaction txConst(mtx);
     // Sign what we can:
+    CAmount inout_amount = 0;
+    bool known_inputs = true;
     for (unsigned int i = 0; i < mtx.vin.size(); i++) {
         CTxIn& txin = mtx.vin[i];
         auto coin = coins.find(txin.prevout);
         if (coin == coins.end() || coin->second.IsSpent()) {
+            known_inputs = false;
             TxInErrorToJSON(txin, vErrors, "Input not found or already spent");
             continue;
         }
         const CScript& prevPubKey = coin->second.out.scriptPubKey;
         const CAmount& amount = coin->second.out.nValue;
+        inout_amount += amount;
+        known_inputs &= amount > 0;
 
         SignatureData sigdata = DataFromTransaction(mtx, i, coin->second.out);
         // Only sign SIGHASH_SINGLE if there's a corresponding output:
         if (!fHashSingle || (i < mtx.vout.size())) {
             ProduceSignature(*keystore, MutableTransactionSignatureCreator(&mtx, i, amount, nHashType), prevPubKey, sigdata);
+            known_inputs &= sigdata.witness;
         }
 
         UpdateInput(txin, sigdata);
@@ -319,8 +325,20 @@ void SignTransaction(CMutableTransaction& mtx, const SigningProvider* keystore,
     }
     bool fComplete = vErrors.empty();
 
-    result.pushKV("hex", EncodeHexTx(CTransaction(mtx)));
+    CTransaction tx(mtx);
+    result.pushKV("hex", EncodeHexTx(tx));
     result.pushKV("complete", fComplete);
+    if (known_inputs) {
+        for (const CTxOut& txout : mtx.vout) {
+            inout_amount -= txout.nValue;
+        }
+        result.pushKV("fee", ValueFromAmount(inout_amount));
+        result.pushKV("feerate",
+            ValueFromAmount(
+                CFeeRate(inout_amount, GetVirtualTransactionSize(tx)).GetFeePerK()
+            )
+        );
+    }
     if (!vErrors.empty()) {
         if (result.exists("errors")) {
             vErrors.push_backV(result["errors"].getValues());

src/wallet/rpcwallet.cpp

@@ -3288,6 +3288,8 @@ UniValue signrawtransactionwithwallet(const JSONRPCRequest& request)
                     {
                         {RPCResult::Type::STR_HEX, "hex", "The hex-encoded raw transaction with signature(s)"},
                         {RPCResult::Type::BOOL, "complete", "If the transaction has a complete set of signatures"},
+                        {RPCResult::Type::NUM, "fee", "The fee (input amounts minus output amounts), if known"},
+                        {RPCResult::Type::NUM, "feerate", "The fee rate (in " + CURRENCY_UNIT + "/kB), if fee is known"},
                         {RPCResult::Type::ARR, "errors", "Script verification errors (if there are any)",
                         {
                             {RPCResult::Type::OBJ, "", "",

test/functional/feature_segwit.py

@@ -19,6 +19,7 @@
 from test_framework.script import CScript, OP_HASH160, OP_CHECKSIG, OP_0, hash160, OP_EQUAL, OP_DUP, OP_EQUALVERIFY, OP_1, OP_2, OP_CHECKMULTISIG, OP_TRUE, OP_DROP
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
+    assert_approx,
     assert_equal,
     assert_raises_rpc_error,
     connect_nodes,
@@ -278,6 +279,34 @@ def run_test(self):
         # Mine a block to clear the gbt cache again.
         self.nodes[0].generate(1)
 
+        self.log.info("Signing with all-segwit inputs reveals fee rate")
+        # Test that signing a tx with all-segwit inputs returns the fee
+        # in the results
+        addr = self.nodes[0].getnewaddress(address_type='p2sh-segwit')
+        txid = self.nodes[0].sendtoaddress(addr, 1)
+        tx = self.nodes[0].getrawtransaction(txid, True)
+        n = -1
+        value = -1
+        for o in tx["vout"]:
+            if o["scriptPubKey"]["addresses"][0] == addr:
+                n = o["n"]
+                value = Decimal(o["value"])
+                break
+        assert n > -1 # failure means we could not find the address in the outputs despite sending to it
+        assert_approx(value, 1.0, 0.01) # failure means we got an unexpected amount of coins, despite trying to send 1
+        fee = Decimal("0.00010000")
+        value_out = value - fee
+        self.nodes[0].generatetoaddress(1, self.nodes[0].getnewaddress())
+        raw = self.nodes[0].createrawtransaction([{"txid" : txid, "vout" : n}], [{self.nodes[0].getnewaddress() : value_out}])
+        signed = self.nodes[0].signrawtransactionwithwallet(raw)
+        assert_equal(signed["complete"], True)
+        txsize = self.nodes[0].decoderawtransaction(signed['hex'])['vsize']
+        exp_feerate = 1000 * fee / Decimal(txsize)
+        assert_approx(signed["feerate"], exp_feerate, Decimal("0.00000010"))
+        # discrepancy = 100000000 * (exp_feerate - signed["feerate"])
+        # assert -10 < discrepancy < 10
+        assert Decimal(signed["fee"]) == fee
+
         self.log.info("Verify behaviour of importaddress and listunspent")
 
         # Some public keys to be used later