lnchannel: apply stricter max_htlc_value_in_flight rules for receiving

Otherwise we create invoices that eclair cannot route to us

electrum/lnchannel.py

@@ -1150,7 +1150,15 @@ class Channel(AbstractChannel):
         # check "max_htlc_value_in_flight_msat"
         ctn = self.get_next_ctn(htlc_receiver)
         current_htlc_sum = htlcsum(self.hm.htlcs_by_direction(htlc_receiver, direction=RECEIVED, ctn=ctn).values())
-        return self.config[htlc_receiver].max_htlc_value_in_flight_msat - current_htlc_sum
+        max_inflight = self.config[htlc_receiver].max_htlc_value_in_flight_msat
+        if htlc_receiver == LOCAL:
+            # in order to send, eclair applies both local and remote max values
+            # https://github.com/ACINQ/eclair/blob/9b0c00a2a28d3ba6c7f3d01fbd2d8704ebbdc75d/eclair-core/src/main/scala/fr/acinq/eclair/channel/Commitments.scala#L503
+            max_inflight = min(
+                self.config[LOCAL].max_htlc_value_in_flight_msat,
+                self.config[REMOTE].max_htlc_value_in_flight_msat
+            )
+        return max_inflight - current_htlc_sum
 
     def can_pay(self, amount_msat: int, *, check_frozen=False) -> bool:
         """Returns whether we can add an HTLC of given value."""

tests/test_lnchannel.py

@@ -738,24 +738,31 @@ class TestAvailableToSpend(ElectrumTestCase):
             local_max_inflight=1000000000,
             remote_max_inflight=2000000000)
 
+        # alice can send 20 but bob can only receive 10, because of stricter receiving rules
         self.assertEqual(2000000000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(1000000000, bob_channel.available_to_spend(REMOTE))
+
+        # bob can send 10, alice can receive 10
         self.assertEqual(1000000000, bob_channel.available_to_spend(LOCAL))
+        self.assertEqual(1000000000, alice_channel.available_to_spend(REMOTE))
 
         paymentPreimage1 = b"\x01" * 32
         htlc = UpdateAddHtlc(
             payment_hash=bitcoin.sha256(paymentPreimage1),
-            amount_msat=1500000000,
+            amount_msat=1000000000,
             cltv_abs=5,
             timestamp=0,
         )
-
+        # put 10mBTC inflight a->b
-        # put 15mBTC inflight a->b
         alice_idx1 = alice_channel.add_htlc(htlc).htlc_id
         bob_idx1 = bob_channel.receive_htlc(htlc).htlc_id
         force_state_transition(alice_channel, bob_channel)
 
-        self.assertEqual(500000000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(1000000000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(0, bob_channel.available_to_spend(REMOTE))
+
         self.assertEqual(1000000000, bob_channel.available_to_spend(LOCAL))
+        self.assertEqual(1000000000, alice_channel.available_to_spend(REMOTE))
 
         paymentPreimage2 = b"\x02" * 32
         htlc2 = UpdateAddHtlc(
@@ -764,7 +771,6 @@ class TestAvailableToSpend(ElectrumTestCase):
             cltv_abs=5,
             timestamp=0,
         )
-
         # try to add another 15mBTC HTLC while 15mBTC already inflight
         with self.assertRaises(lnutil.PaymentFailure):
             alice_idx2 = alice_channel.add_htlc(htlc2).htlc_id
@@ -775,7 +781,10 @@ class TestAvailableToSpend(ElectrumTestCase):
         force_state_transition(alice_channel, bob_channel)
 
         self.assertEqual(2000000000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(1000000000, alice_channel.available_to_spend(REMOTE))
+
         self.assertEqual(1000000000, bob_channel.available_to_spend(LOCAL))
+        self.assertEqual(1000000000, alice_channel.available_to_spend(REMOTE))
 
 
 class TestAvailableToSpendAnchors(TestAvailableToSpend):