skip empty change addresses for makers

joinmarket/maker.py

@@ -234,7 +234,8 @@ def verify_unsigned_tx(self, txd):
                 if outs['value'] != expected_change_value:
                     return False, 'wrong change, i expect ' + str(
                             expected_change_value)
-        if times_seen_cj_addr != 1 or times_seen_change_addr != 1:
+        if (times_seen_cj_addr != 1 or # allow "sweeping" empty change
+            (expected_change_value != 0 and times_seen_change_addr != 1)):
             fmt = ('cj or change addr not in tx '
                    'outputs once, #cjaddr={}, #chaddr={}').format
             return False, (fmt(times_seen_cj_addr, times_seen_change_addr))

joinmarket/taker.py

@@ -179,18 +179,16 @@ def recv_txio(self, nick, utxo_list, auth_pub, cj_addr, change_addr):
             change_amount = (total_input - self.cj_amount -
                 self.active_orders[nick]['txfee'] + real_cjfee)
 
-            # certain malicious and/or incompetent liquidity providers send
-            # inputs totalling less than the coinjoin amount! this leads to
-            # a change output of zero satoshis, so the invalid transaction
-            # fails harmlessly; let's fail earlier, with a clear message.
-            if change_amount < jm_single().BITCOIN_DUST_THRESHOLD:
-                fmt = ('ERROR counterparty requires sub-dust change. No '
-                       'action required. nick={}'
+            # change must either be above the DUST_THRESHOLD, or exactly zero
+            if change_amount != 0 and change_amount < jm_single().DUST_THRESHOLD:
+                fmt = ('ERROR counterparty requires sub-dust change. nick={}'
                        'totalin={:d} cjamount={:d} change={:d}').format
-                log.warn(fmt(nick, total_input, self.cj_amount, change_amount))
+                log.debug(fmt(nick, total_input, self.cj_amount, change_amount))
                 return              # timeout marks this maker as nonresponsive
 
-            self.outputs.append({'address': change_addr, 'value': change_amount})
+            if change_amount != 0:  # FIXME add support for this from maker-side
+                self.outputs.append({'address': change_addr, 'value': change_amount})
+
             fmt = ('fee breakdown for {} totalin={:d} '
                    'cjamount={:d} txfee={:d} realcjfee={:d}').format
             log.debug(fmt(nick, total_input, self.cj_amount,