opening: Add dev-allowdustreserve option to opt into dust reserves

Technically this is a non-conformance with the spec, hence the `dev`
flag to opt-in, however I'm being told that it is also implemented in
other implementations. I'll follow this up with a proposal to the spec
to remove the checks we now bypass.

Makefile

@@ -234,7 +234,6 @@ DEFAULT_TARGETS :=
 CPPFLAGS += -DBINTOPKGLIBEXECDIR="\"$(shell sh tools/rel.sh $(bindir) $(pkglibexecdir))\""
 CFLAGS = $(CPPFLAGS) $(CWARNFLAGS) $(CDEBUGFLAGS) $(COPTFLAGS) -I $(CCANDIR) $(EXTERNAL_INCLUDE_FLAGS) -I . -I/usr/local/include $(SQLITE3_CFLAGS) $(POSTGRES_INCLUDE) $(FEATURES) $(COVFLAGS) $(DEV_CFLAGS) -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS $(PIE_CFLAGS) $(COMPAT_CFLAGS) -DBUILD_ELEMENTS=1
 
-CFLAGS += -DZERORESERVE=1
 # If CFLAGS is already set in the environment of make (to whatever value, it
 # does not matter) then it would export it to subprocesses with the above value
 # we set, including CWARNFLAGS which by default contains -Wall -Werror. This

openingd/openingd.c

@@ -146,27 +146,28 @@ static void set_reserve_absolute(struct state * state, const struct amount_sat d
 {
 	status_debug("Setting their reserve to %s",
 		     type_to_string(tmpctx, struct amount_sat, &reserve_sat));
-#ifdef ZERORESERVE
-	state->localconf.channel_reserve = reserve_sat;
-#else
-	/* BOLT #2:
-	 *
-	 * The sending node:
-	 *...
-	 * - MUST set `channel_reserve_satoshis` greater than or equal to
-         *   `dust_limit_satoshis` from the `open_channel` message.
-	 */
-	if (amount_sat_greater(dust_limit, reserve_sat)) {
-		status_debug(
-		    "Their reserve is too small, bumping to dust_limit: %s < %s",
-		    type_to_string(tmpctx, struct amount_sat, &reserve_sat),
-		    type_to_string(tmpctx, struct amount_sat, &dust_limit));
-		state->localconf.channel_reserve
-			= dust_limit;
-	} else {
+	if (state->allowdustreserve) {
 		state->localconf.channel_reserve = reserve_sat;
+	} else {
+		/* BOLT #2:
+		 *
+		 * The sending node:
+		 *...
+		 * - MUST set `channel_reserve_satoshis` greater than or equal
+		 *to `dust_limit_satoshis` from the `open_channel` message.
+		 */
+		if (amount_sat_greater(dust_limit, reserve_sat)) {
+			status_debug("Their reserve is too small, bumping to "
+				     "dust_limit: %s < %s",
+				     type_to_string(tmpctx, struct amount_sat,
+						    &reserve_sat),
+				     type_to_string(tmpctx, struct amount_sat,
+						    &dust_limit));
+			state->localconf.channel_reserve = dust_limit;
+		} else {
+			state->localconf.channel_reserve = reserve_sat;
+		}
 	}
-#endif
 }
 
 /* We always set channel_reserve_satoshis to 1%, rounded down. */
@@ -464,8 +465,8 @@ static u8 *funder_channel_start(struct state *state, u8 channel_flags)
 				type_to_string(msg, struct channel_id,
 					       &state->channel_id));
 
-#ifndef ZERORESERVE
-	if (amount_sat_greater(state->remoteconf.dust_limit,
+	if (!state->allowdustreserve &&
+	    amount_sat_greater(state->remoteconf.dust_limit,
 			       state->localconf.channel_reserve)) {
 		negotiation_failed(state,
 				   "dust limit %s"
@@ -476,7 +477,6 @@ static u8 *funder_channel_start(struct state *state, u8 channel_flags)
 						  &state->localconf.channel_reserve));
 		return NULL;
 	}
-#endif
 
 	if (!check_config_bounds(tmpctx, state->funding_sats,
 				 state->feerate_per_kw,
@@ -972,7 +972,8 @@ static u8 *fundee_channel(struct state *state, const u8 *open_channel_msg)
 	 * - MUST set `dust_limit_satoshis` less than or equal to
          *   `channel_reserve_satoshis` from the `open_channel` message.
 	 */
-	if (amount_sat_greater(state->remoteconf.dust_limit,
+	if (!state->allowdustreserve &&
+	    amount_sat_greater(state->remoteconf.dust_limit,
 			       state->localconf.channel_reserve)) {
 		negotiation_failed(state,
 				   "Our channel reserve %s"

tests/test_opening.py

@@ -1401,17 +1401,20 @@ def test_zeroreserve(node_factory, bitcoind):
      - l2 enforces default reserve
      - l3 enforces sub-dust reserves
     """
-    ZEROCONF = True
     plugin_path = Path(__file__).parent / "plugins" / "zeroreserve.py"
     opts = [
         {
             'plugin': str(plugin_path),
             'reserve': '0sat',
+            'dev-allowdustreserve': True,
+        },
+        {
+            'dev-allowdustreserve': True,
         },
-        {},
         {
             'plugin': str(plugin_path),
-            'reserve': '123sat'
+            'reserve': '123sat',
+            'dev-allowdustreserve': True,
         }
     ]
     l1, l2, l3 = node_factory.get_nodes(3, opts=opts)
@@ -1441,16 +1444,16 @@ def test_zeroreserve(node_factory, bitcoind):
     l1c3 = l1.rpc.listpeers(l3.info['id'])['peers'][0]['channels'][0]
 
     # l1 imposed a 0sat reserve on l2, while l2 imposed the default 1% reserve on l1
-    assert l1c1['their_channel_reserve_satoshis'] == l2c1['our_channel_reserve_satoshis'] == (0 if ZEROCONF else 546)
+    assert l1c1['their_channel_reserve_satoshis'] == l2c1['our_channel_reserve_satoshis'] == 0
     assert l1c1['our_channel_reserve_satoshis'] == l2c1['their_channel_reserve_satoshis'] == 10000
 
     # l2 imposed the default 1% on l3, while l3 imposed a custom 123sat fee on l2
     assert l2c2['their_channel_reserve_satoshis'] == l3c2['our_channel_reserve_satoshis'] == 10000
-    assert l2c2['our_channel_reserve_satoshis'] == l3c2['their_channel_reserve_satoshis'] == (123 if ZEROCONF else 546)
+    assert l2c2['our_channel_reserve_satoshis'] == l3c2['their_channel_reserve_satoshis'] == 123
 
     # l3 imposed a custom 321sat fee on l1, while l1 imposed a custom 0sat fee on l3
-    assert l3c3['their_channel_reserve_satoshis'] == l1c3['our_channel_reserve_satoshis'] == (321 if ZEROCONF else 546)
-    assert l3c3['our_channel_reserve_satoshis'] == l1c3['their_channel_reserve_satoshis'] == (0 if ZEROCONF else 546)
+    assert l3c3['their_channel_reserve_satoshis'] == l1c3['our_channel_reserve_satoshis'] == 321
+    assert l3c3['our_channel_reserve_satoshis'] == l1c3['their_channel_reserve_satoshis'] == 0
 
     # Now do some drain tests on c1, as that should be drainable
     # completely by l2 being the fundee