[wallet] allow transaction without change if keypool is empty

Author: Sjors

src/wallet/wallet.cpp

@@ -2629,13 +2629,14 @@ bool CWallet::CreateTransaction(interfaces::Chain::Lock& locked_chain, const std
                 //  rediscover unknown transactions that were written with keys of ours to recover
                 //  post-backup change.
 
-                // Reserve a new key pair from key pool
+                // Reserve a new key pair from key pool. If it fails, provide a dummy
+                // destination in case we don't need change.
                 CTxDestination dest;
                 if (!reservedest.GetReservedDestination(dest, true)) {
-                    strFailReason = _("Can't generate a change-address key. Please call keypoolrefill first.").translated;
-                    return false;
+                    strFailReason = _("Transaction needs a change address, but we can't generate it. Please call keypoolrefill first.").translated;
                 }
                 scriptChange = GetScriptForDestination(dest);
+                assert(!dest.empty() || scriptChange.empty());
             }
             CTxOut change_prototype_txout(0, scriptChange);
             coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
@@ -2851,6 +2852,11 @@ bool CWallet::CreateTransaction(interfaces::Chain::Lock& locked_chain, const std
                 coin_selection_params.use_bnb = false;
                 continue;
             }
+
+            // Give up if change keypool ran out and we failed to find a solution without change:
+            if (scriptChange.empty() && nChangePosInOut != -1) {
+                return false;
+            }
         }
 
         // Shuffle selected coins and fill in final vin

test/functional/rpc_fundrawtransaction.py

@@ -500,11 +500,16 @@ def test_locked_wallet(self):
         self.nodes[1].getnewaddress()
         self.nodes[1].getrawchangeaddress()
         inputs = []
-        outputs = {self.nodes[0].getnewaddress():1.1}
+        outputs = {self.nodes[0].getnewaddress():1.09999500}
         rawtx = self.nodes[1].createrawtransaction(inputs, outputs)
+        # fund a transaction that does not require a new key for the change output
+        self.nodes[1].fundrawtransaction(rawtx)
+
         # fund a transaction that requires a new key for the change output
         # creating the key must be impossible because the wallet is locked
-        assert_raises_rpc_error(-4, "Can't generate a change-address key. Please call keypoolrefill first.", self.nodes[1].fundrawtransaction, rawtx)
+        outputs = {self.nodes[0].getnewaddress():1.1}
+        rawtx = self.nodes[1].createrawtransaction(inputs, outputs)
+        assert_raises_rpc_error(-4, "Transaction needs a change address, but we can't generate it. Please call keypoolrefill first.", self.nodes[1].fundrawtransaction, rawtx)
 
         # Refill the keypool.
         self.nodes[1].walletpassphrase("test", 100)

test/functional/wallet_keypool.py

@@ -5,6 +5,7 @@
 """Test the wallet keypool and interaction with wallet encryption/locking."""
 
 import time
+from decimal import Decimal
 
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_raises_rpc_error
@@ -53,12 +54,12 @@ def run_test(self):
         assert_raises_rpc_error(-12, "Keypool ran out", nodes[0].getrawchangeaddress)
 
         # drain the external keys
-        addr.add(nodes[0].getnewaddress())
-        addr.add(nodes[0].getnewaddress())
-        addr.add(nodes[0].getnewaddress())
-        addr.add(nodes[0].getnewaddress())
-        addr.add(nodes[0].getnewaddress())
-        addr.add(nodes[0].getnewaddress())
+        addr.add(nodes[0].getnewaddress(address_type="bech32"))
+        addr.add(nodes[0].getnewaddress(address_type="bech32"))
+        addr.add(nodes[0].getnewaddress(address_type="bech32"))
+        addr.add(nodes[0].getnewaddress(address_type="bech32"))
+        addr.add(nodes[0].getnewaddress(address_type="bech32"))
+        addr.add(nodes[0].getnewaddress(address_type="bech32"))
         assert len(addr) == 6
         # the next one should fail
         assert_raises_rpc_error(-12, "Error: Keypool ran out, please call keypoolrefill first", nodes[0].getnewaddress)
@@ -82,5 +83,52 @@ def run_test(self):
         assert_equal(wi['keypoolsize_hd_internal'], 100)
         assert_equal(wi['keypoolsize'], 100)
 
+        # create a blank wallet
+        nodes[0].createwallet(wallet_name='w2', blank=True)
+        w2 = nodes[0].get_wallet_rpc('w2')
+
+        # refer to initial wallet as w1
+        w1 = nodes[0].get_wallet_rpc('')
+
+        # import private key and fund it
+        address = addr.pop()
+        privkey = w1.dumpprivkey(address)
+        res = w2.importmulti([{'scriptPubKey': {'address': address}, 'keys': [privkey], 'timestamp': 'now'}])
+        assert_equal(res[0]['success'], True)
+        w1.walletpassphrase('test', 100)
+
+        res = w1.sendtoaddress(address=address, amount=0.00010000)
+        nodes[0].generate(1)
+        destination = addr.pop()
+
+        # Using a fee rate (10 sat / byte) well above the minimum relay rate
+        # creating a 5,000 sat transaction with change should not be possible
+        assert_raises_rpc_error(-4, "Transaction needs a change address, but we can't generate it. Please call keypoolrefill first.", w2.walletcreatefundedpsbt, inputs=[], outputs=[{addr.pop(): 0.00005000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.00010})
+
+        # creating a 10,000 sat transaction without change, with a manual input, should still be possible
+        res = w2.walletcreatefundedpsbt(inputs=w2.listunspent(), outputs=[{destination: 0.00010000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.00010})
+        assert_equal("psbt" in res, True)
+
+        # creating a 10,000 sat transaction without change should still be possible
+        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00010000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.00010})
+        assert_equal("psbt" in res, True)
+        # should work without subtractFeeFromOutputs if the exact fee is subtracted from the amount
+        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00008900}], options={"feeRate": 0.00010})
+        assert_equal("psbt" in res, True)
+
+        # dust change should be removed
+        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00008800}], options={"feeRate": 0.00010})
+        assert_equal("psbt" in res, True)
+
+        # create a transaction without change at the maximum fee rate, such that the output is still spendable:
+        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00010000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.0008824})
+        assert_equal("psbt" in res, True)
+        assert_equal(res["fee"], Decimal("0.00009706"))
+
+        # creating a 10,000 sat transaction with a manual change address should be possible
+        res = w2.walletcreatefundedpsbt(inputs=[], outputs=[{destination: 0.00010000}], options={"subtractFeeFromOutputs": [0], "feeRate": 0.00010, "changeAddress": addr.pop()})
+        assert_equal("psbt" in res, True)
+
+
 if __name__ == '__main__':
     KeyPoolTest().main()