lnwallet: add new RegisterFundingIntent method

In this commit, we add a new method `RegisterFundingIntent` that allows
a caller to "inject" a pre-populated chanfunding.Intent into a funding
workflow. As an example, if we've already agreed upon the "shape" of the
funding output _outside_ the protocol, then we can use this to pass down
the details of the output, then leverage the normal wire protocol to
carry out the remainder of the funding flow.

lnwallet/wallet.go

@@ -277,6 +277,11 @@ type LightningWallet struct {
 	// the currently locked outpoints.
 	lockedOutPoints map[wire.OutPoint]struct{}
 
+	// fundingIntents houses all the "interception" registered by a caller
+	// using the RegisterFundingIntent method.
+	intentMtx      sync.RWMutex
+	fundingIntents map[[32]byte]chanfunding.Intent
+
 	quit chan struct{}
 
 	wg sync.WaitGroup
@@ -297,6 +302,7 @@ func NewLightningWallet(Cfg Config) (*LightningWallet, error) {
 		nextFundingID:    0,
 		fundingLimbo:     make(map[uint64]*ChannelReservation),
 		lockedOutPoints:  make(map[wire.OutPoint]struct{}),
+		fundingIntents:   make(map[[32]byte]chanfunding.Intent),
 		quit:             make(chan struct{}),
 	}, nil
 }
@@ -435,6 +441,21 @@ func (l *LightningWallet) InitChannelReservation(
 	return <-req.resp, <-req.err
 }
 
+// RegisterFundingIntent allows a caller to signal to the wallet that if a
+// pending channel ID of expectedID is found, then it can skip constructing a
+// new chanfunding.Assembler, and instead use the specified chanfunding.Intent.
+// As an example, this lets some of the parameters for funding transaction to
+// be negotiated outside the regular funding protocol.
+func (l *LightningWallet) RegisterFundingIntent(expectedID [32]byte,
+	shimIntent chanfunding.Intent) error {
+
+	l.intentMtx.Lock()
+	l.fundingIntents[expectedID] = shimIntent
+	l.intentMtx.Unlock()
+
+	return nil
+}
+
 // handleFundingReserveRequest processes a message intending to create, and
 // validate a funding reservation request.
 func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg) {
@@ -478,11 +499,19 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 		err           error
 	)
 
-	// If we're on the receiving end of a single funder channel then we
-	// don't need to perform any coin selection, and the remote contributes
-	// all funds. Otherwise, attempt to obtain enough coins to meet the
-	// required funding amount.
-	if req.LocalFundingAmt != 0 {
+	// If we've just received an inbound funding request that we have a
+	// registered shim intent to, then we'll obtain the backing intent now.
+	// In this case, we're doing a special funding workflow that allows
+	// more advanced constructions such as channel factories to be
+	// instantiated.
+	l.intentMtx.Lock()
+	fundingIntent, ok := l.fundingIntents[req.PendingChanID]
+	l.intentMtx.Unlock()
+
+	// Otherwise, this is a normal funding flow, so we'll use the chan
+	// funder in the attached request to provision the inputs/outputs
+	// that'll ultimately be used to construct the funding transaction.
+	if !ok {
 		// Coin selection is done on the basis of sat/kw, so we'll use
 		// the fee rate passed in to perform coin selection.
 		var err error
@@ -700,6 +729,16 @@ func (l *LightningWallet) handleFundingCancelRequest(req *fundingReserveCancelMs
 
 	delete(l.fundingLimbo, req.pendingFundingID)
 
+	pid := pendingReservation.pendingChanID
+
+	l.intentMtx.Lock()
+	if intent, ok := l.fundingIntents[pid]; ok {
+		intent.Cancel()
+
+		delete(l.fundingIntents, pendingReservation.pendingChanID)
+	}
+	l.intentMtx.Unlock()
+
 	req.err <- nil
 }
 
@@ -1149,6 +1188,10 @@ func (l *LightningWallet) handleFundingCounterPartySigs(msg *addCounterPartySigs
 	delete(l.fundingLimbo, res.reservationID)
 	l.limboMtx.Unlock()
 
+	l.intentMtx.Lock()
+	delete(l.fundingIntents, res.pendingChanID)
+	l.intentMtx.Unlock()
+
 	// As we're about to broadcast the funding transaction, we'll take note
 	// of the current height for record keeping purposes.
 	//
@@ -1347,6 +1390,10 @@ func (l *LightningWallet) handleSingleFunderSigs(req *addSingleFunderSigsMsg) {
 	l.limboMtx.Lock()
 	delete(l.fundingLimbo, req.pendingFundingID)
 	l.limboMtx.Unlock()
+
+	l.intentMtx.Lock()
+	delete(l.fundingIntents, pendingReservation.pendingChanID)
+	l.intentMtx.Unlock()
 }
 
 // WithCoinSelectLock will execute the passed function closure in a