lnwallet/chancloser: remove the commit fee clamp, introduce max fee

In this commit, we stop clamping the ideal fee rate to the commitment
fee of the channel. This catches us up to this PR of the spec:
lightning/bolts#847.

We also do away with the old 3x ideal fee "max fee", and replace that
with an explicit max fee. This new max fee will either be the default
multiplier of the ideal fee, or a new user specified max fee value.

lnwallet/chancloser/chancloser.go

@@ -35,6 +35,12 @@ var (
 	// shutdown script previously set for that party.
 	ErrUpfrontShutdownScriptMismatch = fmt.Errorf("shutdown script does not " +
 		"match upfront shutdown script")
+
+	// ErrProposalExeceedsMaxFee is returned when as the initiator, the
+	// latest fee proposal sent by the responder exceed our max fee.
+	// responder.
+	ErrProposalExeceedsMaxFee = fmt.Errorf("latest fee proposal exceeds " +
+		"max fee")
 )
 
 // closeState represents all the possible states the channel closer state
@@ -73,6 +79,14 @@ const (
 	closeFinished
 )
 
+const (
+	// defaultMaxFeeMultiplier is a multiplier we'll apply to the ideal fee
+	// of the initiator, to decide when the negotiated fee is too high. By
+	// default, we want to bail out if we attempt to negotiate a fee that's
+	// 3x higher than our max fee.
+	defaultMaxFeeMultiplier = 3
+)
+
 // ChanCloseCfg holds all the items that a ChanCloser requires to carry out its
 // duties.
 type ChanCloseCfg struct {
@@ -89,6 +103,9 @@ type ChanCloseCfg struct {
 	// Disconnect will disconnect from the remote peer in this close.
 	Disconnect func() error
 
+	// MaxFee, is non-zero represents the highest fee that the initiator is
+	// willing to pay to close the channel.
+	MaxFee chainfee.SatPerKWeight
 	// Quit is a channel that should be sent upon in the occasion the state
 	// machine should cease all progress and shutdown.
 	Quit chan struct{}
@@ -122,6 +139,11 @@ type ChanCloser struct {
 	// offer when starting negotiation. This will be used as a baseline.
 	idealFeeSat btcutil.Amount
 
+	// maxFee is the highest fee the initiator is willing to pay to close
+	// out the channel. This is either a use specified value, or a default
+	// multiplier based of the initial starting ideal fee.
+	maxFee btcutil.Amount
+
 	// lastFeeProposal is the last fee that we proposed to the remote party.
 	// We'll use this as a pivot point to ratchet our next offer up, down, or
 	// simply accept the remote party's prior offer.
@@ -168,17 +190,12 @@ func NewChanCloser(cfg ChanCloseCfg, deliveryScript []byte,
 	// TODO(roasbeef): should factor in minimal commit
 	idealFeeSat := cfg.Channel.CalcFee(idealFeePerKw)
 
-	// If this fee is greater than the fee currently present within the
-	// commitment transaction, then we'll clamp it down to be within the proper
-	// range.
-	//
-	// TODO(roasbeef): clamp fee func?
-	channelCommitFee := cfg.Channel.StateSnapshot().CommitFee
-	if idealFeeSat > channelCommitFee {
-		chancloserLog.Infof("Ideal starting fee of %v is greater than commit "+
-			"fee of %v, clamping", int64(idealFeeSat), int64(channelCommitFee))
-
-		idealFeeSat = channelCommitFee
+	// When we're the initiator, we'll want to also factor in the highest
+	// fee we want to pay. This'll either be 3x the ideal fee, or the
+	// specified explicit max fee.
+	maxFee := idealFeeSat * defaultMaxFeeMultiplier
+	if cfg.MaxFee > 0 {
+		maxFee = cfg.Channel.CalcFee(cfg.MaxFee)
 	}
 
 	chancloserLog.Infof("Ideal fee for closure of ChannelPoint(%v) is: %v sat",
@@ -193,6 +210,7 @@ func NewChanCloser(cfg ChanCloseCfg, deliveryScript []byte,
 		cfg:                 cfg,
 		negotiationHeight:   negotiationHeight,
 		idealFeeSat:         idealFeeSat,
+		maxFee:              maxFee,
 		localDeliveryScript: deliveryScript,
 		priorFeeOffers:      make(map[btcutil.Amount]*lnwire.ClosingSigned),
 		locallyInitiated:    locallyInitiated,
@@ -483,9 +501,10 @@ func (c *ChanCloser) ProcessCloseMsg(msg lnwire.Message) ([]lnwire.Message,
 			feeProposal := calcCompromiseFee(c.chanPoint, c.idealFeeSat,
 				c.lastFeeProposal, remoteProposedFee,
 			)
-			if feeProposal > c.idealFeeSat*3 {
-				return nil, false, fmt.Errorf("couldn't find" +
-					" compromise fee")
+			if c.cfg.Channel.IsInitiator() && feeProposal > c.maxFee {
+				return nil, false, fmt.Errorf("%w: %v > %v",
+					ErrProposalExeceedsMaxFee, feeProposal,
+					c.maxFee)
 			}
 
 			// With our new fee proposal calculated, we'll craft a new close