lnwallet/channel: break up availableBalance

lnwallet/channel.go

@@ -5997,11 +5997,14 @@ func (lc *LightningChannel) CompleteCooperativeClose(localSig, remoteSig []byte,
 	return closeTx, ourBalance, nil
 }
 
-// AvailableBalance returns the current available balance within the channel.
-// By available balance, we mean that if at this very instance s new commitment
-// were to be created which evals all the log entries, what would our available
-// balance me. This method is useful when deciding if a given channel can
-// accept an HTLC in the multi-hop forwarding scenario.
+// AvailableBalance returns the current balance available for sending within
+// the channel. By available balance, we mean that if at this very instance a
+// new commitment were to be created which evals all the log entries, what
+// would our available balance for adding an additional HTLC be. It takes into
+// account the fee that must be paid for adding this HTLC (if we're the
+// initiator), and that we cannot spend from the channel reserve. This method
+// is useful when deciding if a given channel can accept an HTLC in the
+// multi-hop forwarding scenario.
 func (lc *LightningChannel) AvailableBalance() lnwire.MilliSatoshi {
 	lc.RLock()
 	defer lc.RUnlock()
@@ -6021,19 +6024,50 @@ func (lc *LightningChannel) availableBalance() (lnwire.MilliSatoshi, int64) {
 	htlcView := lc.fetchHTLCView(remoteACKedIndex,
 		lc.localUpdateLog.logIndex)
 
-	// Then compute our current balance for that view.
-	ourBalance, _, commitWeight, filteredView, err :=
-		lc.computeView(htlcView, false, false)
+	// Calculate our available balance from our local commitment.
+	//
+	// NOTE: This is not always accurate, since the remote node can always
+	// 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,
+	)
+
+	return ourLocalCommitBalance, commitWeight
+}
+
+// availableCommitmentBalance attempts to calculate the balance we have
+// available for HTLCs on the local/remote commitment given the htlcView. To
+// account for sending HTLCs of different sizes, it will report the balance
+// available for sending non-dust HTLCs, which will be manifested on the
+// commitment, increasing the commitment fee we must pay as an initiator,
+// eating into our balance. It will make sure we won't violate the channel
+// reserve constraints for this amount.
+func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
+	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, err := lc.computeView(
+		view, false, false,
+	)
 	if err != nil {
 		lc.log.Errorf("Unable to fetch available balance: %v", err)
 		return 0, 0
 	}
 
-	// If we are the channel initiator, we must remember to subtract the
-	// commitment fee from our available balance.
-	commitFee := filteredView.feePerKw.FeeForWeight(commitWeight)
+	// Given the commitment weight, find the commitment fee in case of no
+	// added HTLC output.
+	feePerKw := filteredView.feePerKw
+	baseCommitFee := lnwire.NewMSatFromSatoshis(
+		feePerKw.FeeForWeight(commitWeight),
+	)
+
+	// If we are the channel initiator, we must to subtract the commitment
+	// fee from our available balance.
 	if lc.channelState.IsInitiator {
-		ourBalance -= lnwire.NewMSatFromSatoshis(commitFee)
+		ourBalance -= baseCommitFee
 	}
 
 	return ourBalance, commitWeight