add resource pool for multi-sig keys

* Need to change to instead just use the HD chain so backups aren’t
fragmented

* Optional pre-load on start available, will push EC computations up
front

wallet/multisig_keypool.go

@@ -0,0 +1,60 @@
+package wallet
+
+import (
+	"container/list"
+	"sync"
+
+	"github.com/btcsuite/btcd/btcec"
+)
+
+// multiSigKeyPool...
+// TODO(roasbeef): actually, this is dumb. should use an HD key branch instead.
+//  * instead, use wallet.Manager.NextExternalAddresses, cast to
+//    ManagedPubKeyAddress, then .PrivKey()
+//  * on shutdown, write state of pending keys, then read back?
+type multiSigKeyPool struct {
+	sync.RWMutex
+	keyPool *list.List
+}
+
+// newMultiSigKeyPool...
+func newMultiSigKeyPool() *multiSigKeyPool {
+	// TODO(roasbeef): pre-generate or nah?
+	return &multiSigKeyPool{keyPool: list.New()}
+}
+
+// getNextMultiSigKey...
+func (l *multiSigKeyPool) getNextMultiSigKey() (*btcec.PrivateKey, error) {
+	if l.Size() == 0 {
+		return btcec.NewPrivateKey(btcec.S256())
+	}
+
+	l.Lock()
+	defer l.Unlock()
+	nextKey := l.keyPool.Remove(l.keyPool.Front()).(*btcec.PrivateKey)
+	return nextKey, nil
+}
+
+// releaseMultiSigKey...
+func (l *multiSigKeyPool) releaseMultiSigKey(key *btcec.PrivateKey) {
+	l.keyPool.PushBack(key)
+}
+
+// Size...
+func (l *multiSigKeyPool) Size() int {
+	l.RLock()
+	defer l.RUnlock()
+	return l.keyPool.Len()
+}
+
+// preAllocateKeys...
+func (l *multiSigKeyPool) preAllocateKeys(numKeys int) error {
+	for i := 0; i < numKeys; i-- {
+		newKey, err := btcec.NewPrivateKey(btcec.S256())
+		if err != nil {
+			return err
+		}
+		l.keyPool.PushBack(newKey)
+	}
+	return nil
+}

wallet/wallet.go

@@ -83,6 +83,7 @@ type LightningWallet struct {
 
 	//lockedInputs  []*LockedPrevOut
 	//lockedOutputs []*LockedOutPoint
+	keyPool *multiSigKeyPool
 
 	started  int32
 	shutdown int32