chainntnfs: allow multiple subscriber to the same input

This commit changes the `subscribedInputs` to store a map of subscribers
so multiple subscribers are allowed to receive events from the same
outpoint.

chainntnfs/mempool.go

@@ -2,6 +2,7 @@ package chainntnfs
 
 import (
 	"sync"
+	"sync/atomic"
 
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/wire"
@@ -19,7 +20,11 @@ type MempoolNotifier struct {
 
 	// subscribedInputs stores the inputs that we want to watch their
 	// spending event for.
-	subscribedInputs lnutils.SyncMap[wire.OutPoint, *MempoolSpendEvent]
+	subscribedInputs *lnutils.SyncMap[wire.OutPoint,
+		*lnutils.SyncMap[uint64, *MempoolSpendEvent]]
+
+	// sCounter is used to generate unique subscription IDs.
+	sCounter atomic.Uint64
 
 	// quit is closed when the notifier is torn down.
 	quit chan struct{}
@@ -64,9 +69,10 @@ func (m *MempoolSpendEvent) Cancel() {
 // notifier.
 func NewMempoolNotifier() *MempoolNotifier {
 	return &MempoolNotifier{
-		subscribedInputs: lnutils.SyncMap[
-			wire.OutPoint, *MempoolSpendEvent,
-		]{},
+		subscribedInputs: &lnutils.SyncMap[
+			wire.OutPoint, *lnutils.SyncMap[
+				uint64, *MempoolSpendEvent,
+			]]{},
 		quit: make(chan struct{}),
 	}
 }
@@ -77,9 +83,20 @@ func (m *MempoolNotifier) SubscribeInput(
 	outpoint wire.OutPoint) *MempoolSpendEvent {
 
 	Log.Debugf("Subscribing mempool event for input %s", outpoint)
+
+	// Get the current subscribers for this input or create a new one.
+	clients := &lnutils.SyncMap[uint64, *MempoolSpendEvent]{}
+	clients, _ = m.subscribedInputs.LoadOrStore(outpoint, clients)
+
+	// Create a new subscription.
 	sub := newMempoolSpendEvent()
 
-	m.subscribedInputs.Store(outpoint, sub)
+	// Add the subscriber with a unique id.
+	subscriptionID := m.sCounter.Add(1)
+	clients.Store(subscriptionID, sub)
+
+	// Update the subscribed inputs.
+	m.subscribedInputs.Store(outpoint, clients)
 
 	return sub
 }
@@ -148,24 +165,16 @@ func (m *MempoolNotifier) findRelevantInputs(tx *btcutil.Tx) inputsWithTx {
 // notifySpent iterates all the spentInputs and notifies the subscribers about
 // the spent details.
 func (m *MempoolNotifier) notifySpent(spentInputs inputsWithTx) {
-	// notify is a helper closure that constructs a spend detail and sends
-	// it to the subscribers.
+	// notifySingle sends a notification to a single subscriber about the
+	// spending event.
 	//
 	// NOTE: must be used inside a goroutine.
-	notify := func(detail *SpendDetail, op wire.OutPoint) {
+	notifySingle := func(id uint64, sub *MempoolSpendEvent,
+		op wire.OutPoint, detail *SpendDetail) {
+
 		defer m.wg.Done()
 
-		txid := detail.SpendingTx.TxHash()
-		Log.Debugf("Notifying the spend of %s in tx %s", op, txid)
-
-		// Load the subscriber.
-		sub, loaded := m.subscribedInputs.Load(op)
-		if !loaded {
-			// TODO(yy): this is possible if the subscriber cancels
-			// the subscription?
-			Log.Errorf("Sub not found for %s", op)
-			return
-		}
+		Log.Debugf("Notifying client %d", id)
 
 		// Send the spend details to the subscriber.
 		select {
@@ -176,8 +185,18 @@ func (m *MempoolNotifier) notifySpent(spentInputs inputsWithTx) {
 			Log.Debugf("Subscription canceled, skipped notifying "+
 				"mempool spent for input %s", op)
 
-			// Delete the subscriber.
-			m.subscribedInputs.Delete(op)
+			// Find all the subscribers for this outpoint.
+			clients, loaded := m.subscribedInputs.Load(op)
+			if !loaded {
+				Log.Errorf("Client %d not found", id)
+				return
+			}
+
+			// Delete the specific subscriber.
+			clients.Delete(id)
+
+			// Update the subscribers map.
+			m.subscribedInputs.Store(op, clients)
 
 		case <-m.quit:
 			Log.Debugf("Mempool notifier quit, skipped notifying "+
@@ -185,9 +204,37 @@ func (m *MempoolNotifier) notifySpent(spentInputs inputsWithTx) {
 		}
 	}
 
+	// notifyAll is a helper closure that constructs a spend detail and
+	// sends it to all the subscribers of that particular input.
+	//
+	// NOTE: must be used inside a goroutine.
+	notifyAll := func(detail *SpendDetail, op wire.OutPoint) {
+		defer m.wg.Done()
+
+		txid := detail.SpendingTx.TxHash()
+		Log.Debugf("Notifying the spend of %s in tx %s", op, txid)
+
+		// Load the subscriber.
+		subs, loaded := m.subscribedInputs.Load(op)
+		if !loaded {
+			Log.Errorf("Sub not found for %s", op)
+			return
+		}
+
+		// Iterate all the subscribers for this input and notify them.
+		subs.ForEach(func(id uint64, sub *MempoolSpendEvent) error {
+			m.wg.Add(1)
+			go notifySingle(id, sub, op, detail)
+
+			return nil
+		})
+	}
+
 	// Iterate the spent inputs to notify the subscribers concurrently.
 	for op, tx := range spentInputs {
+		op, tx := op, tx
+
 		m.wg.Add(1)
-		go notify(tx, op)
+		go notifyAll(tx, op)
 	}
 }