lnwallet/channel: take remote balance into availableBalance calculation

Since the remote balance affect how much we are allowed to send (if they
are the initiator) because of the added HTLC fee, we do the available
balance caluclation also from their perspective and report our minimum
balance from the two commit views as our available balance.

lnwallet/channel.go

@@ -5974,9 +5974,19 @@ func (lc *LightningChannel) availableBalance() (lnwire.MilliSatoshi, int64) {
 	// add updates concurrently, causing our balance to go down if we're
 	// the initiator, but this is a problem on the protocol level.
 	ourLocalCommitBalance, commitWeight := lc.availableCommitmentBalance(
-		htlcView,
+		htlcView, false,
 	)
 
+	// Do the same caluclation from the remote commitment point of view.
+	ourRemoteCommitBalance, _ := lc.availableCommitmentBalance(
+		htlcView, true,
+	)
+
+	// Return which ever balance is lowest.
+	if ourRemoteCommitBalance < ourLocalCommitBalance {
+		return ourRemoteCommitBalance, commitWeight
+	}
+
 	return ourLocalCommitBalance, commitWeight
 }
 
@@ -5985,21 +5995,26 @@ func (lc *LightningChannel) availableBalance() (lnwire.MilliSatoshi, int64) {
 // account for sending HTLCs of different sizes, it will take into account
 // whether the HTLCs will be manifested on the commitment, increasing the
 // commitment fee we must pay as an initiator, eating into our balance.
-func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
-	lnwire.MilliSatoshi, int64) {
+func (lc *LightningChannel) availableCommitmentBalance(view *htlcView,
+	remoteChain bool) (lnwire.MilliSatoshi, int64) {
 
 	// 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 := lc.computeView(
-		view, false, false,
+		view, remoteChain, false,
 	)
 	feePerKw := filteredView.feePerKw
 
 	// Set the dustlimit based on who's commitment we are looking at.
 	dustlimit := lnwire.NewMSatFromSatoshis(
 		lc.channelState.LocalChanCfg.DustLimit,
 	)
+	if remoteChain {
+		dustlimit = lnwire.NewMSatFromSatoshis(
+			lc.channelState.RemoteChanCfg.DustLimit,
+		)
+	}
 
 	// Given the commitment weight, find the commitment fee in case of no
 	// added HTLC ouput.
@@ -6019,6 +6034,13 @@ func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
 		htlcTimeoutFee(feePerKw),
 	)
 
+	// On the remote commitment, the HTLC success transaction will be used.
+	if remoteChain {
+		htlcFee = lnwire.NewMSatFromSatoshis(
+			htlcSuccessFee(feePerKw),
+		)
+	}
+
 	// The HTLC output will be manifested on the commitment if it
 	// is non-dust after paying the HTLC fee.
 	nonDustHtlcAmt := dustlimit + htlcFee