lightningd: add a feerate offset when updating feerates as opener

Adding a fee offset as the channel opener reduces the likelihood of a
disconnect by the peer do to slight variation in feerate calculation
between nodes.

Changelog-Fixed: Some peer disconnects due to update_fee disagreements are avoided.

lightningd/channel_control.c

@@ -56,6 +56,13 @@ void channel_update_feerates(struct lightningd *ld, const struct channel *channe
 	else
 		min_feerate = feerate_min(ld, NULL);
 	max_feerate = feerate_max(ld, NULL);
+	/* The channel opener should use a slightly higher than minimal feerate
+	 * in order to avoid excessive feerate disagreements */
+	if (channel->opener == LOCAL) {
+		feerate += ld->config.feerate_offset;
+		if (feerate > max_feerate)
+			feerate = max_feerate;
+	}
 
 	if (channel->ignore_fee_limits || ld->config.ignore_fee_limits) {
 		min_feerate = 1;

lightningd/lightningd.h

@@ -93,6 +93,9 @@ struct config {
 
 	/* Percent of CONSERVATIVE/2 feerate we'll use for commitment txs. */
 	u64 commit_fee_percent;
+
+	/* Commit feerate offset above min_feerate to use as a channel opener */
+	u32 feerate_offset;
 };
 
 typedef STRMAP(const char *) alt_subdaemon_map;

lightningd/options.c

@@ -948,6 +948,7 @@ static const struct config testnet_config = {
 
 	.max_fee_multiplier = 10,
 	.commit_fee_percent = 100,
+	.feerate_offset = 5,
 };
 
 /* aka. "Dude, where's my coins?" */
@@ -1022,6 +1023,7 @@ static const struct config mainnet_config = {
 
 	.max_fee_multiplier = 10,
 	.commit_fee_percent = 100,
+	.feerate_offset = 5,
 };
 
 static void check_config(struct lightningd *ld)

tests/test_closing.py

@@ -1251,7 +1251,7 @@ def test_penalty_htlc_tx_fulfill(node_factory, bitcoind, chainparams, anchors):
 
     # reconnect with l1, which will fulfill the payment
     l2.rpc.connect(l1.info['id'], 'localhost', l1.port)
-    l2.daemon.wait_for_log('got commitsig .*: feerate 11000, blockheight: 0, 0 added, 1 fulfilled, 0 failed, 0 changed')
+    l2.daemon.wait_for_log('got commitsig .*: feerate 11005, blockheight: 0, 0 added, 1 fulfilled, 0 failed, 0 changed')
 
     # l2 moves on for closed l3
     bitcoind.generate_block(1)
@@ -1463,7 +1463,7 @@ def test_penalty_htlc_tx_timeout(node_factory, bitcoind, chainparams, anchors):
 
     # reconnect with l1, which will fulfill the payment
     l2.rpc.connect(l1.info['id'], 'localhost', l1.port)
-    l2.daemon.wait_for_log('got commitsig .*: feerate {}, blockheight: 0, 0 added, 1 fulfilled, 0 failed, 0 changed'.format(3750 if anchors else 11000))
+    l2.daemon.wait_for_log('got commitsig .*: feerate {}, blockheight: 0, 0 added, 1 fulfilled, 0 failed, 0 changed'.format(3755 if anchors else 11005))
 
     # l2 moves on for closed l3
     bitcoind.generate_block(1, wait_for_mempool=1)
@@ -2650,12 +2650,12 @@ def test_onchain_different_fees(node_factory, bitcoind, executor):
 
     # Both sides should have correct feerate
     assert l1.db_query('SELECT min_possible_feerate, max_possible_feerate FROM channels;') == [{
-        'min_possible_feerate': 5000,
-        'max_possible_feerate': 11000
+        'min_possible_feerate': 5005,
+        'max_possible_feerate': 11005
     }]
     assert l2.db_query('SELECT min_possible_feerate, max_possible_feerate FROM channels;') == [{
-        'min_possible_feerate': 5000,
-        'max_possible_feerate': 11000
+        'min_possible_feerate': 5005,
+        'max_possible_feerate': 11005
     }]
 
     bitcoind.generate_block(5)

tests/test_connection.py

@@ -2412,7 +2412,7 @@ def test_update_fee(node_factory, bitcoind):
     # Make payments.
     l1.pay(l2, 200000000)
     # First payment causes fee update.
-    l2.daemon.wait_for_log('peer updated fee to 11000')
+    l2.daemon.wait_for_log('peer updated fee to 11005')
     l2.pay(l1, 100000000)
 
     # Now shutdown cleanly.
@@ -2454,22 +2454,22 @@ def test_fee_limits(node_factory, bitcoind):
     l1.set_feerates((15, 15, 15, 15), False)
     l1.start()
 
-    l1.daemon.wait_for_log('Received WARNING .*: update_fee 253 outside range 1875-75000')
+    l1.daemon.wait_for_log('Received WARNING .*: update_fee 258 outside range 1875-75000')
     # They hang up on *us*
     l1.daemon.wait_for_log('Peer transient failure in CHANNELD_NORMAL: channeld: Owning subdaemon channeld died')
 
     # Disconnects, but does not error.  Make sure it's noted in their status though.
     # FIXME: does not happen for l1!
     # assert 'update_fee 253 outside range 1875-75000' in only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['status'][0]
-    assert 'update_fee 253 outside range 1875-75000' in only_one(l2.rpc.listpeerchannels(l1.info['id'])['channels'])['status'][0]
+    assert 'update_fee 258 outside range 1875-75000' in only_one(l2.rpc.listpeerchannels(l1.info['id'])['channels'])['status'][0]
 
-    assert only_one(l2.rpc.listpeerchannels()['channels'])['feerate']['perkw'] != 253
+    assert only_one(l2.rpc.listpeerchannels()['channels'])['feerate']['perkw'] != 258
     # Make l2 accept those fees, and it should recover.
     assert only_one(l2.rpc.setchannel(l1.get_channel_scid(l2), ignorefeelimits=True)['channels'])['ignore_fee_limits'] is True
     assert only_one(l2.rpc.listpeerchannels()['channels'])['ignore_fee_limits'] is True
 
     # Now we stay happy (and connected!)
-    wait_for(lambda: only_one(l2.rpc.listpeerchannels()['channels'])['feerate']['perkw'] == 253)
+    wait_for(lambda: only_one(l2.rpc.listpeerchannels()['channels'])['feerate']['perkw'] == 258)
     assert only_one(l2.rpc.listpeerchannels()['channels'])['peer_connected'] is True
 
     l1.rpc.close(l2.info['id'])
@@ -2583,19 +2583,19 @@ def test_update_fee_reconnect(node_factory, bitcoind):
     # Make l1 send out feechange; triggers disconnect/reconnect.
     # (Note: < 10% change, so no smoothing here!)
     l1.set_feerates((14000, 14000, 14000, 3750))
-    l1.daemon.wait_for_log('Setting REMOTE feerate to 14000')
-    l2.daemon.wait_for_log('Setting LOCAL feerate to 14000')
+    l1.daemon.wait_for_log('Setting REMOTE feerate to 14005')
+    l2.daemon.wait_for_log('Setting LOCAL feerate to 14005')
     l1.daemon.wait_for_log(r'dev_disconnect: \+WIRE_COMMITMENT_SIGNED')
 
     # Wait for reconnect....
-    l1.daemon.wait_for_log('Feerate:.*LOCAL now 14000')
+    l1.daemon.wait_for_log('Feerate:.*LOCAL now 14005')
 
     l1.pay(l2, 200000000)
     l2.pay(l1, 100000000)
 
     # They should both have gotten commits with correct feerate.
-    assert l1.daemon.is_in_log('got commitsig [0-9]*: feerate 14000')
-    assert l2.daemon.is_in_log('got commitsig [0-9]*: feerate 14000')
+    assert l1.daemon.is_in_log('got commitsig [0-9]*: feerate 14005')
+    assert l2.daemon.is_in_log('got commitsig [0-9]*: feerate 14005')
 
     # Now shutdown cleanly.
     l1.rpc.close(chan)
@@ -3347,7 +3347,7 @@ def test_feerate_spam(node_factory, chainparams):
     l1.pay(l2, 10**9 - slack)
 
     # It will send this once (may have happened before line_graph's wait)
-    wait_for(lambda: l1.daemon.is_in_log('Setting REMOTE feerate to 11000'))
+    wait_for(lambda: l1.daemon.is_in_log('Setting REMOTE feerate to 11005'))
     wait_for(lambda: l1.daemon.is_in_log('peer_out WIRE_UPDATE_FEE'))
 
     # Now change feerates to something l1 can't afford.