lnchannel.available_to_spend: consider both receiver's and sender's ctx

electrum/lnchannel.py

@@ -1054,19 +1054,14 @@ class Channel(AbstractChannel):
         assert type(subject) is HTLCOwner
         sender = subject
         receiver = subject.inverted()
-        ctx_owner = receiver
+        initiator = LOCAL if self.constraints.is_initiator else REMOTE  # the initiator/funder pays on-chain fees
-        # TODO but what about the other ctx? BOLT-02 only talks about checking the receiver's ctx,
+
-        #      however the channel reserve is only meaningful if we also check the sender's ctx!
+        def consider_ctx(*, ctx_owner: HTLCOwner) -> int:
-        #      in particular, note that dust limits can be different between the parties!
-        #      but due to the racy nature of this, we cannot be sure exactly what the sender's
-        #      next ctx will look like (e.g. what feerate it will use). hmmm :/
             ctn = self.get_next_ctn(ctx_owner)
             sender_balance_msat = self.balance_minus_outgoing_htlcs(whose=sender, ctx_owner=ctx_owner, ctn=ctn)
             receiver_balance_msat = self.balance_minus_outgoing_htlcs(whose=receiver, ctx_owner=ctx_owner, ctn=ctn)
             sender_reserve_msat = self.config[receiver].reserve_sat * 1000
             receiver_reserve_msat = self.config[sender].reserve_sat * 1000
-        initiator = LOCAL if self.constraints.is_initiator else REMOTE
-        # the initiator/funder pays on-chain fees
             num_htlcs_in_ctx = len(self.included_htlcs(ctx_owner, SENT, ctn=ctn) + self.included_htlcs(ctx_owner, RECEIVED, ctn=ctn))
             feerate = self.get_feerate(ctx_owner, ctn=ctn)
             ctx_fees_msat = calc_fees_for_commitment_tx(
@@ -1075,19 +1070,9 @@ class Channel(AbstractChannel):
                 is_local_initiator=self.constraints.is_initiator,
                 round_to_sat=False,
             )
-        # note: if this supposed new HTLC is large enough to create an output, the initiator needs to pay for that too
-        # note: if sender != initiator, both the sender and the receiver need to "afford" the payment
             htlc_fee_msat = fee_for_htlc_output(feerate=feerate)
-        # TODO stuck channels. extra funder reserve? "fee spike buffer" (maybe only if "strict")
+            htlc_trim_func = received_htlc_trim_threshold_sat if ctx_owner == receiver else offered_htlc_trim_threshold_sat
-        #      see https://github.com/lightningnetwork/lightning-rfc/issues/728
+            htlc_trim_threshold_msat = htlc_trim_func(dust_limit_sat=self.config[ctx_owner].dust_limit_sat, feerate=feerate) * 1000
-        # note: in terms of on-chain outputs, as we are considering the htlc_receiver's ctx, this is a "received" HTLC
-        htlc_trim_threshold_msat = received_htlc_trim_threshold_sat(dust_limit_sat=self.config[receiver].dust_limit_sat, feerate=feerate) * 1000
-        if strict:
-            # also consider the other ctx, where the trim threshold is different
-            # note: the 'feerate' we use is not technically correct but we have no way
-            #       of knowing the actual future feerate ahead of time (this is a protocol bug)
-            htlc_trim_threshold_msat = min(htlc_trim_threshold_msat,
-                                           offered_htlc_trim_threshold_sat(dust_limit_sat=self.config[sender].dust_limit_sat, feerate=feerate) * 1000)
             max_send_msat = sender_balance_msat - sender_reserve_msat - ctx_fees_msat[sender]
             if max_send_msat < htlc_trim_threshold_msat:
                 # there will be no corresponding HTLC output
@@ -1102,6 +1087,12 @@ class Channel(AbstractChannel):
                     max_send_msat = htlc_trim_threshold_msat - 1
                 return max_send_msat
 
+        max_send_msat = min(consider_ctx(ctx_owner=receiver),
+                            consider_ctx(ctx_owner=sender))
+        max_send_msat = max(max_send_msat, 0)
+        return max_send_msat
+
+
     def included_htlcs(self, subject: HTLCOwner, direction: Direction, ctn: int = None) -> Sequence[UpdateAddHtlc]:
         """Returns list of non-dust HTLCs for subject's commitment tx at ctn,
         filtered by direction (of HTLCs).