lnchannel.available_to_spend: LOCAL now respects "fee spike buffer"

electrum/lnchannel.py

@@ -1073,7 +1073,16 @@ class Channel(AbstractChannel):
             htlc_fee_msat = fee_for_htlc_output(feerate=feerate)
             htlc_trim_func = received_htlc_trim_threshold_sat if ctx_owner == receiver else offered_htlc_trim_threshold_sat
             htlc_trim_threshold_msat = htlc_trim_func(dust_limit_sat=self.config[ctx_owner].dust_limit_sat, feerate=feerate) * 1000
-            max_send_msat = sender_balance_msat - sender_reserve_msat - ctx_fees_msat[sender]
+            if sender == initiator == LOCAL:  # see https://github.com/lightningnetwork/lightning-rfc/pull/740
+                fee_spike_buffer = calc_fees_for_commitment_tx(
+                    num_htlcs=num_htlcs_in_ctx + int(not is_htlc_dust) + 1,
+                    feerate=2 * feerate,
+                    is_local_initiator=self.constraints.is_initiator,
+                    round_to_sat=False,
+                )[sender]
+                max_send_msat = sender_balance_msat - sender_reserve_msat - fee_spike_buffer
+            else:
+                max_send_msat = sender_balance_msat - sender_reserve_msat - ctx_fees_msat[sender]
             if is_htlc_dust:
                 return min(max_send_msat, htlc_trim_threshold_msat - 1)
             else:

electrum/tests/test_lnchannel.py

@@ -622,7 +622,7 @@ class TestChannel(ElectrumTestCase):
 class TestAvailableToSpend(ElectrumTestCase):
     def test_DesyncHTLCs(self):
         alice_channel, bob_channel = create_test_channels()
-        self.assertEqual(499994624000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(499986152000, alice_channel.available_to_spend(LOCAL))
         self.assertEqual(500000000000, bob_channel.available_to_spend(LOCAL))
 
         paymentPreimage = b"\x01" * 32
@@ -636,13 +636,13 @@ class TestAvailableToSpend(ElectrumTestCase):
 
         alice_idx = alice_channel.add_htlc(htlc_dict).htlc_id
         bob_idx = bob_channel.receive_htlc(htlc_dict).htlc_id
-        self.assertEqual(89993592000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(89984088000, alice_channel.available_to_spend(LOCAL))
         self.assertEqual(500000000000, bob_channel.available_to_spend(LOCAL))
 
         force_state_transition(alice_channel, bob_channel)
         bob_channel.fail_htlc(bob_idx)
         alice_channel.receive_fail_htlc(alice_idx, error_bytes=None)
-        self.assertEqual(89993592000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(89984088000, alice_channel.available_to_spend(LOCAL))
         self.assertEqual(500000000000, bob_channel.available_to_spend(LOCAL))
         # Alice now has gotten all her original balance (5 BTC) back, however,
         # adding a new HTLC at this point SHOULD fail, since if she adds the
@@ -662,7 +662,7 @@ class TestAvailableToSpend(ElectrumTestCase):
         # Now do a state transition, which will ACK the FailHTLC, making Alice
         # able to add the new HTLC.
         force_state_transition(alice_channel, bob_channel)
-        self.assertEqual(499994624000, alice_channel.available_to_spend(LOCAL))
+        self.assertEqual(499986152000, alice_channel.available_to_spend(LOCAL))
         self.assertEqual(500000000000, bob_channel.available_to_spend(LOCAL))
         alice_channel.add_htlc(htlc_dict)