lightningd: add large-channels / wumbo option.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-added: `large-channels` option to negotiate opening larger channels.

doc/lightningd-config.5

@@ -281,6 +281,14 @@ extremely busy node for you to even notice\.
 
 .SH Lightning channel and HTLC options
 
+ \fBlarge-channels\fR
+Removes capacity limits for channel creation\.  Version 1\.0 of the specification
+limited channel sizes to 16777216 satoshi\.  With this option (which your
+node will advertize to peers), your node will accept larger incoming channels
+and if the peer supports it, will open larger channels\.  Note: this option
+is spelled \fBlarge-channels\fR but it's pronounced \fBwumbo\fR\.
+
+
  \fBwatchtime-blocks\fR=\fIBLOCKS\fR
 How long we need to spot an outdated close attempt: on opening a channel
 we tell our peer that this is how long they’ll have to wait if they

doc/lightningd-config.5.md

@@ -230,6 +230,13 @@ extremely busy node for you to even notice.
 
 ### Lightning channel and HTLC options
 
+ **large-channels**
+Removes capacity limits for channel creation.  Version 1.0 of the specification
+limited channel sizes to 16777215 satoshi.  With this option (which your
+node will advertize to peers), your node will accept larger incoming channels
+and if the peer supports it, will open larger channels.  Note: this option
+is spelled **large-channels** but it's pronounced **wumbo**.
+
  **watchtime-blocks**=*BLOCKS*
 How long we need to spot an outdated close attempt: on opening a channel
 we tell our peer that this is how long they’ll have to wait if they

lightningd/options.c

@@ -742,6 +742,14 @@ static char *opt_start_daemon(struct lightningd *ld)
 	errx(1, "Died with signal %u", WTERMSIG(exitcode));
 }
 
+static char *opt_set_wumbo(struct lightningd *ld)
+{
+	feature_set_or(ld->feature_set,
+		       take(feature_set_for_feature(NULL,
+						    OPTIONAL_FEATURE(OPT_LARGE_CHANNELS))));
+	return NULL;
+}
+
 static void register_opts(struct lightningd *ld)
 {
 	/* This happens before plugins started */
@@ -774,6 +782,11 @@ static void register_opts(struct lightningd *ld)
 			       &ld->wallet_dsn,
 			       "Location of the wallet database.");
 
+	/* This affects our features, so set early. */
+	opt_register_early_noarg("--large-channels|--wumbo",
+				 opt_set_wumbo, ld,
+				 "Allow channels larger than 0.16777215 BTC");
+
 	opt_register_noarg("--help|-h", opt_lightningd_usage, ld,
 				 "Print this message.");
 	opt_register_arg("--rgb", opt_set_rgb, NULL, ld,
@@ -1188,6 +1201,10 @@ static void add_config(struct lightningd *ld,
 					 ? "false" : "true");
 		} else if (opt->cb == (void *)opt_set_hsm_password) {
 			json_add_bool(response, "encrypted-hsm", ld->encrypted_hsm);
+		} else if (opt->cb == (void *)opt_set_wumbo) {
+			json_add_bool(response, "wumbo",
+				      feature_offered(ld->feature_set->bits[INIT_FEATURE],
+						      OPT_LARGE_CHANNELS));
 		} else {
 			/* Insert more decodes here! */
 			assert(!"A noarg option was added but was not handled");

tests/test_connection.py

@@ -3,7 +3,7 @@ from decimal import Decimal
 from fixtures import *  # noqa: F401,F403
 from fixtures import TEST_NETWORK
 from flaky import flaky  # noqa: F401
-from pyln.client import RpcError
+from pyln.client import RpcError, Millisatoshi
 from utils import (
     DEVELOPER, only_one, wait_for, sync_blockheight, VALGRIND, TIMEOUT,
     SLOW_MACHINE, expected_features
@@ -2220,3 +2220,47 @@ def test_pay_disconnect_stress(node_factory, executor):
                 pass
 
         fut.result()
+
+
+def test_wumbo_channels(node_factory, bitcoind):
+    f = bytes.fromhex(expected_features())
+
+    # OPT_LARGE_CHANNELS = 18 (19 for us). 0x080000
+    f = (f[:-3] + bytes([f[-3] | 0x08]) + f[-2:]).hex()
+
+    l1, l2, l3 = node_factory.get_nodes(3,
+                                        opts=[{'large-channels': None},
+                                              {'large-channels': None},
+                                              {}])
+    conn = l1.rpc.connect(l2.info['id'], 'localhost', port=l2.port)
+    assert conn['features'] == f
+    assert only_one(l1.rpc.listpeers(l2.info['id'])['peers'])['features'] == f
+
+    # Now, can we open a giant channel?
+    l1.fundwallet(1 << 26)
+    l1.rpc.fundchannel(l2.info['id'], 1 << 24)
+
+    # Get that mined, and announced.
+    bitcoind.generate_block(6, wait_for_mempool=1)
+
+    # Connect l3, get gossip.
+    l3.rpc.connect(l1.info['id'], 'localhost', port=l1.port)
+    wait_for(lambda: len(l3.rpc.listnodes(l1.info['id'])['nodes']) == 1)
+    wait_for(lambda: 'features' in only_one(l3.rpc.listnodes(l1.info['id'])['nodes']))
+
+    # Make sure channel capacity is what we expected.
+    assert ([c['amount_msat'] for c in l3.rpc.listchannels()['channels']]
+            == [Millisatoshi(str(1 << 24) + "sat")] * 2)
+
+    # Make sure we can't open a wumbo channel if we don't agree.
+    with pytest.raises(RpcError, match='Amount exceeded'):
+        l1.rpc.fundchannel(l3.info['id'], 1 << 24)
+
+    # But we can open and announce a normal one.
+    l1.rpc.fundchannel(l3.info['id'], 'all')
+    bitcoind.generate_block(6, wait_for_mempool=1)
+    wait_for(lambda: l1.channel_state(l3) == 'CHANNELD_NORMAL')
+
+    # Make sure l2 sees correct size.
+    wait_for(lambda: [c['amount_msat'] for c in l2.rpc.listchannels(l1.get_channel_scid(l3))['channels']]
+             == [Millisatoshi(str((1 << 24) - 1) + "sat")] * 2)