lnrouter: simplify max fee sanity checks

electrum/lnrouter.py

@@ -74,16 +74,11 @@ class RouteEdge(NamedTuple):
     def is_sane_to_use(self, amount_msat: int) -> bool:
         # TODO revise ad-hoc heuristics
         # cltv cannot be more than 2 weeks
-        if self.cltv_expiry_delta > 14 * 144: return False
+        if self.cltv_expiry_delta > 14 * 144:
+            return False
         total_fee = self.fee_for_edge(amount_msat)
-        # fees below 50 sat are fine
-        if total_fee > 50_000:
-            # fee cannot be higher than amt
-            if total_fee > amount_msat: return False
-            # fee cannot be higher than 5000 sat
-            if total_fee > 5_000_000: return False
-            # unless amt is tiny, fee cannot be more than 10%
-            if amount_msat > 1_000_000 and total_fee > amount_msat/10: return False
+        if not is_fee_sane(total_fee, payment_amount_msat=amount_msat):
+            return False
         return True
 
 
@@ -105,18 +100,23 @@ def is_route_sane_to_use(route: LNPaymentRoute, invoice_amount_msat: int, min_fi
     total_fee = amt - invoice_amount_msat
     # TODO revise ad-hoc heuristics
     # cltv cannot be more than 2 months
-    if cltv > 60 * 144: return False
-    # fees below 50 sat are fine
-    if total_fee > 50_000:
-        # fee cannot be higher than amt
-        if total_fee > invoice_amount_msat: return False
-        # fee cannot be higher than 5000 sat
-        if total_fee > 5_000_000: return False
-        # unless amt is tiny, fee cannot be more than 10%
-        if invoice_amount_msat > 1_000_000 and total_fee > invoice_amount_msat/10: return False
+    if cltv > 60 * 144:
+        return False
+    if not is_fee_sane(total_fee, payment_amount_msat=invoice_amount_msat):
+        return False
     return True
 
 
+def is_fee_sane(fee_msat: int, *, payment_amount_msat: int) -> bool:
+    # fees <= 2 sat are fine
+    if fee_msat <= 2_000:
+        return True
+    # fees <= 1 % of payment are fine
+    if 100 * fee_msat <= payment_amount_msat:
+        return True
+    return False
+
+
 class LNPathFinder(Logger):
 
     def __init__(self, channel_db: ChannelDB):