link+lnwallet: move bandwidth channel reserve validation into channel

Since we want to handle the edge case where paying the HTLC fee would
take the initiator below the reserve, we move the subtraction of the
reserve into availableBalance where this calculation will be performed.

lnwallet/channel.go

@@ -6057,6 +6057,17 @@ func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
 		return 0, 0
 	}
 
+	// We can never spend from the channel reserve, so we'll subtract it
+	// from our available balance.
+	ourReserve := lnwire.NewMSatFromSatoshis(
+		lc.channelState.LocalChanCfg.ChanReserve,
+	)
+	if ourReserve <= ourBalance {
+		ourBalance -= ourReserve
+	} else {
+		ourBalance = 0
+	}
+
 	// Given the commitment weight, find the commitment fee in case of no
 	// added HTLC output.
 	feePerKw := filteredView.feePerKw
@@ -6067,6 +6078,9 @@ func (lc *LightningChannel) availableCommitmentBalance(view *htlcView) (
 	// If we are the channel initiator, we must to subtract the commitment
 	// fee from our available balance.
 	if lc.channelState.IsInitiator {
+		if ourBalance < baseCommitFee {
+			return 0, commitWeight
+		}
 		ourBalance -= baseCommitFee
 	}
 
@@ -6278,13 +6292,14 @@ func (lc *LightningChannel) MaxFeeRate(maxAllocation float64) chainfee.SatPerKWe
 
 	// The maximum fee depends of the available balance that can be
 	// committed towards fees.
-	balance, weight := lc.availableBalance()
+	commit := lc.channelState.LocalCommitment
 	feeBalance := float64(
-		balance.ToSatoshis() + lc.channelState.LocalCommitment.CommitFee,
+		commit.LocalBalance.ToSatoshis() + commit.CommitFee,
 	)
 	maxFee := feeBalance * maxAllocation
 
 	// Ensure the fee rate doesn't dip below the fee floor.
+	_, weight := lc.availableBalance()
 	maxFeeRate := maxFee / (float64(weight) / 1000)
 	return chainfee.SatPerKWeight(
 		math.Max(maxFeeRate, float64(chainfee.FeePerKwFloor)),

lnwallet/channel_test.go

@@ -4599,6 +4599,10 @@ func TestChanAvailableBandwidth(t *testing.T) {
 	}
 	defer cleanUp()
 
+	aliceReserve := lnwire.NewMSatFromSatoshis(
+		aliceChannel.channelState.LocalChanCfg.ChanReserve,
+	)
+
 	assertBandwidthEstimateCorrect := func(aliceInitiate bool) {
 		// With the HTLC's added, we'll now query the AvailableBalance
 		// method for the current available channel bandwidth from
@@ -4625,11 +4629,14 @@ func TestChanAvailableBandwidth(t *testing.T) {
 		// Now, we'll obtain the current available bandwidth in Alice's
 		// latest commitment and compare that to the prior estimate.
 		aliceBalance := aliceChannel.channelState.LocalCommitment.LocalBalance
-		if aliceBalance != aliceAvailableBalance {
+
+		// The balance we have available for new HTLCs should be the
+		// current local commitment balance, minus the channel reserve.
+		expBalance := aliceBalance - aliceReserve
+		if expBalance != aliceAvailableBalance {
 			_, _, line, _ := runtime.Caller(1)
 			t.Fatalf("line: %v, incorrect balance: expected %v, "+
-				"got %v", line, aliceBalance,
-				aliceAvailableBalance)
+				"got %v", line, expBalance, aliceAvailableBalance)
 		}
 	}