chainntnfs+chainreg: add interface MempoolWatcher

This commit adds the interface `MempoolWatcher` and uses it in the chain
registry.

chainntnfs/bitcoindnotify/bitcoind.go

@@ -80,6 +80,10 @@ type BitcoindNotifier struct {
 // time.
 var _ chainntnfs.ChainNotifier = (*BitcoindNotifier)(nil)
 
+// Ensure BitcoindNotifier implements the MempoolWatcher interface at compile
+// time.
+var _ chainntnfs.MempoolWatcher = (*BitcoindNotifier)(nil)
+
 // New returns a new BitcoindNotifier instance. This function assumes the
 // bitcoind node detailed in the passed configuration is already running, and
 // willing to accept RPC requests and new zmq clients.

chainntnfs/btcdnotify/btcd.go

@@ -96,6 +96,9 @@ type BtcdNotifier struct {
 // Ensure BtcdNotifier implements the ChainNotifier interface at compile time.
 var _ chainntnfs.ChainNotifier = (*BtcdNotifier)(nil)
 
+// Ensure BtcdNotifier implements the MempoolWatcher interface at compile time.
+var _ chainntnfs.MempoolWatcher = (*BtcdNotifier)(nil)
+
 // New returns a new BtcdNotifier instance. This function assumes the btcd node
 // detailed in the passed configuration is already running, and willing to
 // accept new websockets clients.

chainntnfs/interface.go

@@ -808,3 +808,16 @@ type ConfirmHintCache interface {
 	// the cache.
 	PurgeConfirmHint(confRequests ...ConfRequest) error
 }
+
+// MempoolWatcher defines an interface that allows the caller to query
+// information in the mempool.
+type MempoolWatcher interface {
+	// SubscribeMempoolSpent allows the caller to register a subscription
+	// to watch for a spend of an outpoint in the mempool.The event will be
+	// dispatched once the outpoint is spent in the mempool.
+	SubscribeMempoolSpent(op wire.OutPoint) (*MempoolSpendEvent, error)
+
+	// CancelMempoolSpendEvent allows the caller to cancel a subscription to
+	// watch for a spend of an outpoint in the mempool.
+	CancelMempoolSpendEvent(sub *MempoolSpendEvent)
+}