lnd: add new preimageBeacon struct

In this commit, we add the preimageBeacon  which is an implementation
of both the contractcourt.WitnessBeacon and lnwallet.PreimageCache
interfaces. This will be used when closing channels to check to see if
the know the preimage, and also to communicate to any active contract
resolvers once we discover a preimage either on-chain or off-chain.

witness_beacon.go

@@ -0,0 +1,145 @@
+package main
+
+import (
+	"sync"
+
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/contractcourt"
+	"github.com/lightningnetwork/lnd/lnwallet"
+	"github.com/roasbeef/btcd/chaincfg/chainhash"
+)
+
+// preimageSubcriber reprints an active subscription to be notified once the
+// daemon discovers new preimages, either on chain or off-chain.
+type preimageSubcriber struct {
+	updateChan chan []byte
+
+	quit chan struct{}
+}
+
+// preimageBeacon is an implementation of the contractcourt.WitnessBeacon
+// interface, and the lnwallet.PreimageCache interface. This implementation is
+// concerned with a single witness type: sha256 hahsh preimages.
+type preimageBeacon struct {
+	sync.RWMutex
+
+	invoices *invoiceRegistry
+
+	wCache *channeldb.WitnessCache
+
+	clientCounter uint64
+	subscribers   map[uint64]*preimageSubcriber
+}
+
+// NewPreimageBeacon creates a new preimageBeacon instance given an invoice
+// registry and witness cache.
+func NewPreimageBeacon(invoices *invoiceRegistry,
+	wCache *channeldb.WitnessCache) *preimageBeacon {
+
+	return &preimageBeacon{
+		invoices:    invoices,
+		wCache:      wCache,
+		subscribers: make(map[uint64]*preimageSubcriber),
+	}
+}
+
+// SubcribeUpdates returns a channel that will be sent upon *each* time a new
+// preimage is discovered.
+func (p *preimageBeacon) SubcribeUpdates() *contractcourt.WitnessSubcription {
+	p.Lock()
+	defer p.Unlock()
+
+	clientID := p.clientCounter
+	client := &preimageSubcriber{
+		updateChan: make(chan []byte, 10),
+		quit:       make(chan struct{}),
+	}
+
+	p.subscribers[p.clientCounter] = client
+
+	p.clientCounter++
+
+	srvrLog.Debugf("Creating new witness beacon subscriber, id=%v",
+		p.clientCounter)
+
+	return &contractcourt.WitnessSubcription{
+		WitnessUpdates: client.updateChan,
+		CancelSubcription: func() {
+			p.Lock()
+			defer p.Unlock()
+
+			delete(p.subscribers, clientID)
+
+			close(client.quit)
+		},
+	}
+}
+
+// LookupPreImage attempts to lookup a preimage in the global cache.  True is
+// returned for the second argument if the preimage is found.
+func (p *preimageBeacon) LookupPreimage(payHash []byte) ([]byte, bool) {
+	p.RLock()
+	defer p.RUnlock()
+
+	// First, we'll check the invoice registry to see if we already know of
+	// the preimage as it's on that we created ourselves.
+	var invoiceKey chainhash.Hash
+	copy(invoiceKey[:], payHash)
+	invoice, err := p.invoices.LookupInvoice(invoiceKey)
+	switch {
+	case err == channeldb.ErrInvoiceNotFound:
+		// If we get this error, then it simply means that this invoice
+		// wasn't found, so we don't treat it as a critical error.
+	case err != nil:
+		return nil, false
+	}
+
+	// If we've found the invoice, then we can return the preimage
+	// directly.
+	if err != channeldb.ErrInvoiceNotFound {
+		return invoice.Terms.PaymentPreimage[:], true
+	}
+
+	// Otherwise, we'll perform a final check using the witness cache.
+	preimage, err := p.wCache.LookupWitness(
+		channeldb.Sha256HashWitness, payHash,
+	)
+	if err != nil {
+		ltndLog.Errorf("unable to lookup witness: %v", err)
+		return nil, false
+	}
+
+	return preimage, true
+}
+
+// AddPreImage adds a newly discovered preimage to the global cache, and also
+// signals any subscribers of the newly discovered witness.
+func (p *preimageBeacon) AddPreimage(pre []byte) error {
+	p.Lock()
+	defer p.Unlock()
+
+	srvrLog.Infof("Adding preimage=%x to witness cache", pre[:])
+
+	// First, we'll add the witness to the decaying witness cache.
+	err := p.wCache.AddWitness(channeldb.Sha256HashWitness, pre)
+	if err != nil {
+		return err
+	}
+
+	// With the preimage added to our state, we'll now send a new
+	// notification to all subscribers.
+	for _, client := range p.subscribers {
+		go func(c *preimageSubcriber) {
+			select {
+			case c.updateChan <- pre:
+			case <-c.quit:
+				return
+			}
+		}(client)
+	}
+
+	return nil
+}
+
+var _ contractcourt.WitnessBeacon = (*preimageBeacon)(nil)
+var _ lnwallet.PreimageCache = (*preimageBeacon)(nil)