chainntnfs/btcd: start concurrent queues prior to connection

We would establish the connection to the chain backend and start getting
block notifications before we had started the concurrent queues, which
would lead to the OnBlockConnected call being blocked, and a deadlock
(since GetBestBlock would never return).

Instead we make sure to start the queues before establishing the
connection, consuming the notifications right away.

chainntnfs/btcdnotify/btcd.go

@@ -139,17 +139,24 @@ func (b *BtcdNotifier) Start() error {
 		return nil
 	}
 
+	// Start our concurrent queues before starting the chain connection, to
+	// ensure onBlockConnected and onRedeemingTx callbacks won't be
+	// blocked.
+	b.chainUpdates.Start()
+	b.txUpdates.Start()
+
 	// Connect to btcd, and register for notifications on connected, and
 	// disconnected blocks.
 	if err := b.chainConn.Connect(20); err != nil {
-		return err
-	}
-	if err := b.chainConn.NotifyBlocks(); err != nil {
+		b.txUpdates.Stop()
+		b.chainUpdates.Stop()
 		return err
 	}
 
 	currentHash, currentHeight, err := b.chainConn.GetBestBlock()
 	if err != nil {
+		b.txUpdates.Stop()
+		b.chainUpdates.Stop()
 		return err
 	}
 
@@ -163,8 +170,11 @@ func (b *BtcdNotifier) Start() error {
 		Hash:   currentHash,
 	}
 
-	b.chainUpdates.Start()
-	b.txUpdates.Start()
+	if err := b.chainConn.NotifyBlocks(); err != nil {
+		b.txUpdates.Stop()
+		b.chainUpdates.Stop()
+		return err
+	}
 
 	b.wg.Add(1)
 	go b.notificationDispatcher()
@@ -208,10 +218,14 @@ func (b *BtcdNotifier) Stop() error {
 func (b *BtcdNotifier) onBlockConnected(hash *chainhash.Hash, height int32, t time.Time) {
 	// Append this new chain update to the end of the queue of new chain
 	// updates.
-	b.chainUpdates.ChanIn() <- &chainUpdate{
+	select {
+	case b.chainUpdates.ChanIn() <- &chainUpdate{
 		blockHash:   hash,
 		blockHeight: height,
 		connect:     true,
+	}:
+	case <-b.quit:
+		return
 	}
 }
 
@@ -236,10 +250,14 @@ type filteredBlock struct {
 func (b *BtcdNotifier) onBlockDisconnected(hash *chainhash.Hash, height int32, t time.Time) {
 	// Append this new chain update to the end of the queue of new chain
 	// updates.
-	b.chainUpdates.ChanIn() <- &chainUpdate{
+	select {
+	case b.chainUpdates.ChanIn() <- &chainUpdate{
 		blockHash:   hash,
 		blockHeight: height,
 		connect:     false,
+	}:
+	case <-b.quit:
+		return
 	}
 }
 
@@ -247,7 +265,11 @@ func (b *BtcdNotifier) onBlockDisconnected(hash *chainhash.Hash, height int32, t
 func (b *BtcdNotifier) onRedeemingTx(tx *btcutil.Tx, details *btcjson.BlockDetails) {
 	// Append this new transaction update to the end of the queue of new
 	// chain updates.
-	b.txUpdates.ChanIn() <- &txUpdate{tx, details}
+	select {
+	case b.txUpdates.ChanIn() <- &txUpdate{tx, details}:
+	case <-b.quit:
+		return
+	}
 }
 
 // notificationDispatcher is the primary goroutine which handles client