skip empty change addresses for makers

works towards #418 but does not enable this behavior maker-side

joinmarket/maker.py

@@ -203,7 +203,8 @@ 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

@@ -133,17 +133,16 @@ def recv_txio(self, nick, utxo_list, cj_pub, 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().DUST_THRESHOLD:
+        # 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.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,