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)