lnwallet: take remote initiator's balance into account

When we send non-dust HTLCs as the non-initiator, the remote party will
have to pay the extra commitment fee. To account for this we figure out
if they can afford paying this fee, if not we report that we only have
balance available for dust HTLCs, since these HTLCs won't increase the
commitment fee.

lnwallet/channel.go

@@ -6049,7 +6049,7 @@ func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
 	// Compute the current balances for this commitment. This will take
 	// into account HTLCs to determine the commit weight, which the
 	// initiator must pay the fee for.
-	ourBalance, _, commitWeight, filteredView, err := lc.computeView(
+	ourBalance, theirBalance, commitWeight, filteredView, err := lc.computeView(
 		view, false, false,
 	)
 	if err != nil {
@@ -6091,7 +6091,42 @@ func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
 			return 0, commitWeight
 		}
 
-		ourBalance -= htlcCommitFee
+		return ourBalance - htlcCommitFee, commitWeight
+	}
+
+	// If we're not the initiator, we must check whether the remote has
+	// enough balance to pay for the fee of our HTLC. We'll start by also
+	// subtracting our counterparty's reserve from their balance.
+	theirReserve := lnwire.NewMSatFromSatoshis(
+		lc.channelState.RemoteChanCfg.ChanReserve,
+	)
+	if theirReserve <= theirBalance {
+		theirBalance -= theirReserve
+	} else {
+		theirBalance = 0
+	}
+
+	// We'll use the dustlimit and htlcFee to find the largest HTLC value
+	// that will be considered dust on the commitment.
+	dustlimit := lnwire.NewMSatFromSatoshis(
+		lc.channelState.LocalChanCfg.DustLimit,
+	)
+
+	// For an extra HTLC fee to be paid on our commitment, the HTLC must be
+	// large enough to make a non-dust HTLC timeout transaction.
+	htlcFee := lnwire.NewMSatFromSatoshis(
+		htlcTimeoutFee(feePerKw),
+	)
+
+	// The HTLC output will be manifested on the commitment if it
+	// is non-dust after paying the HTLC fee.
+	nonDustHtlcAmt := dustlimit + htlcFee
+
+	// If they cannot pay the fee if we add another non-dust HTLC, we'll
+	// report our available balance just below the non-dust amount, to
+	// avoid attempting HTLCs larger than this size.
+	if theirBalance < htlcCommitFee && ourBalance >= nonDustHtlcAmt {
+		ourBalance = nonDustHtlcAmt - 1
 	}
 
 	return ourBalance, commitWeight