ln update_fee: enforce that feerate is over default min relay fee

(this was always already the case when we are the funder, but we were
not checking it when remote is responsible for update_fee)

electrum/lnchannel.py

@@ -61,6 +61,7 @@ from .address_synchronizer import TX_HEIGHT_LOCAL
 from .lnutil import CHANNEL_OPENING_TIMEOUT
 from .lnutil import ChannelBackupStorage, ImportedChannelBackupStorage, OnchainChannelBackupStorage
 from .lnutil import format_short_channel_id
+from .simple_config import FEERATE_PER_KW_MIN_RELAY_LIGHTNING
 
 if TYPE_CHECKING:
     from .lnworker import LNWallet
@@ -1346,6 +1347,8 @@ class Channel(AbstractChannel):
         # feerate uses sat/kw
         if self.constraints.is_initiator != from_us:
             raise Exception(f"Cannot update_fee: wrong initiator. us: {from_us}")
+        if feerate < FEERATE_PER_KW_MIN_RELAY_LIGHTNING:
+            raise Exception(f"Cannot update_fee: feerate lower than min relay fee. {feerate} sat/kw. us: {from_us}")
         sender = LOCAL if from_us else REMOTE
         ctx_owner = -sender
         ctn = self.get_next_ctn(ctx_owner)

electrum/lnutil.py

@@ -169,6 +169,9 @@ class ChannelConfig(StoredObject):
             raise Exception(
                 "both to_local and to_remote amounts for the initial commitment "
                 "transaction are less than or equal to channel_reserve_satoshis")
+        from .simple_config import FEERATE_PER_KW_MIN_RELAY_LIGHTNING
+        if initial_feerate_per_kw < FEERATE_PER_KW_MIN_RELAY_LIGHTNING:
+            raise Exception(f"feerate lower than min relay fee. {initial_feerate_per_kw} sat/kw.")
 
 
 @attr.s

electrum/lnworker.py

@@ -2096,12 +2096,13 @@ class LNWallet(LNWorker):
 
     def current_feerate_per_kw(self):
         from .simple_config import FEE_LN_ETA_TARGET, FEERATE_FALLBACK_STATIC_FEE, FEERATE_REGTEST_HARDCODED
+        from .simple_config import FEERATE_PER_KW_MIN_RELAY_LIGHTNING
         if constants.net is constants.BitcoinRegtest:
             return FEERATE_REGTEST_HARDCODED // 4
         feerate_per_kvbyte = self.network.config.eta_target_to_fee(FEE_LN_ETA_TARGET)
         if feerate_per_kvbyte is None:
             feerate_per_kvbyte = FEERATE_FALLBACK_STATIC_FEE
-        return max(253, feerate_per_kvbyte // 4)
+        return max(FEERATE_PER_KW_MIN_RELAY_LIGHTNING, feerate_per_kvbyte // 4)
 
     def create_channel_backup(self, channel_id):
         chan = self._channels[channel_id]

electrum/simple_config.py

@@ -34,6 +34,12 @@ FEERATE_STATIC_VALUES = [1000, 2000, 5000, 10000, 20000, 30000,
                          50000, 70000, 100000, 150000, 200000, 300000]
 FEERATE_REGTEST_HARDCODED = 180000  # for eclair compat
 
+# The min feerate_per_kw that can be used in lightning so that
+# the resulting onchain tx pays the min relay fee.
+# This would be FEERATE_DEFAULT_RELAY / 4 if not for rounding errors,
+# see https://github.com/ElementsProject/lightning/commit/2e687b9b352c9092b5e8bd4a688916ac50b44af0
+FEERATE_PER_KW_MIN_RELAY_LIGHTNING = 253
+
 FEE_RATIO_HIGH_WARNING = 0.05  # warn user if fee/amount for on-chain tx is higher than this
 
 

electrum/tests/test_lnchannel.py

@@ -532,7 +532,7 @@ class TestChannel(ElectrumTestCase):
         self.assertEqual(bob_channel.total_msat(SENT), 5 * one_bitcoin_in_msat, "bob satoshis sent incorrect")
 
 
-    def alice_to_bob_fee_update(self, fee=111):
+    def alice_to_bob_fee_update(self, fee=1111):
         aoldctx = self.alice_channel.get_next_commitment(REMOTE).outputs()
         self.alice_channel.update_fee(fee, True)
         anewctx = self.alice_channel.get_next_commitment(REMOTE).outputs()