From 3a445746256b88f0aa718d19694d6bc4dceb6a3a Mon Sep 17 00:00:00 2001
From: "Johan T. Halseth" <johanth@gmail.com>
Date: Tue, 4 Jun 2019 09:58:04 +0200
Subject: [PATCH] chainntnfs/neutrinonotify: async call GetUtxo from
 RegisterSpendNtfn

Since GetUtxo is a potentially long running call, we would see
RegisterSpendNtfn block in some cases, in particular on starting the
chain watcher, causing lnd to hang on startup.

This commit makes the call to GetUtxo in a go routine, letting
RegisterSpendNtfn return immediately when the notification events are
created.
---
 chainntnfs/neutrinonotify/neutrino.go | 105 ++++++++++++++------------
 1 file changed, 58 insertions(+), 47 deletions(-)

diff --git a/chainntnfs/neutrinonotify/neutrino.go b/chainntnfs/neutrinonotify/neutrino.go
index e23c49d0..bc5c27d7 100644
--- a/chainntnfs/neutrinonotify/neutrino.go
+++ b/chainntnfs/neutrinonotify/neutrino.go
@@ -740,58 +740,69 @@ func (n *NeutrinoNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint,
 	}
 
 	// With the filter updated, we'll dispatch our historical rescan to
-	// ensure we detect the spend if it happened in the past. We'll ensure
-	// that neutrino is caught up to the starting height before we attempt
-	// to fetch the UTXO from the chain. If we're behind, then we may miss a
-	// notification dispatch.
-	for {
-		n.bestBlockMtx.RLock()
-		currentHeight := uint32(n.bestBlock.Height)
-		n.bestBlockMtx.RUnlock()
+	// ensure we detect the spend if it happened in the past.
+	n.wg.Add(1)
+	go func() {
+		defer n.wg.Done()
 
-		if currentHeight >= historicalDispatch.StartHeight {
-			break
+		// We'll ensure that neutrino is caught up to the starting
+		// height before we attempt to fetch the UTXO from the chain.
+		// If we're behind, then we may miss a notification dispatch.
+		for {
+			n.bestBlockMtx.RLock()
+			currentHeight := uint32(n.bestBlock.Height)
+			n.bestBlockMtx.RUnlock()
+
+			if currentHeight >= historicalDispatch.StartHeight {
+				break
+			}
+
+			select {
+			case <-time.After(time.Millisecond * 200):
+			case <-n.quit:
+				return
+			}
 		}
 
-		time.Sleep(time.Millisecond * 200)
-	}
-
-	spendReport, err := n.p2pNode.GetUtxo(
-		neutrino.WatchInputs(inputToWatch),
-		neutrino.StartBlock(&waddrmgr.BlockStamp{
-			Height: int32(historicalDispatch.StartHeight),
-		}),
-		neutrino.EndBlock(&waddrmgr.BlockStamp{
-			Height: int32(historicalDispatch.EndHeight),
-		}),
-		neutrino.QuitChan(n.quit),
-	)
-	if err != nil && !strings.Contains(err.Error(), "not found") {
-		return nil, err
-	}
-
-	// If a spend report was returned, and the transaction is present, then
-	// this means that the output is already spent.
-	var spendDetails *chainntnfs.SpendDetail
-	if spendReport != nil && spendReport.SpendingTx != nil {
-		spendingTxHash := spendReport.SpendingTx.TxHash()
-		spendDetails = &chainntnfs.SpendDetail{
-			SpentOutPoint:     &spendRequest.OutPoint,
-			SpenderTxHash:     &spendingTxHash,
-			SpendingTx:        spendReport.SpendingTx,
-			SpenderInputIndex: spendReport.SpendingInputIndex,
-			SpendingHeight:    int32(spendReport.SpendingTxHeight),
+		spendReport, err := n.p2pNode.GetUtxo(
+			neutrino.WatchInputs(inputToWatch),
+			neutrino.StartBlock(&waddrmgr.BlockStamp{
+				Height: int32(historicalDispatch.StartHeight),
+			}),
+			neutrino.EndBlock(&waddrmgr.BlockStamp{
+				Height: int32(historicalDispatch.EndHeight),
+			}),
+			neutrino.QuitChan(n.quit),
+		)
+		if err != nil && !strings.Contains(err.Error(), "not found") {
+			chainntnfs.Log.Errorf("Failed getting UTXO: %v", err)
+			return
 		}
-	}
 
-	// Finally, no matter whether the rescan found a spend in the past or
-	// not, we'll mark our historical rescan as complete to ensure the
-	// outpoint's spend hint gets updated upon connected/disconnected
-	// blocks.
-	err = n.txNotifier.UpdateSpendDetails(spendRequest, spendDetails)
-	if err != nil {
-		return nil, err
-	}
+		// If a spend report was returned, and the transaction is present, then
+		// this means that the output is already spent.
+		var spendDetails *chainntnfs.SpendDetail
+		if spendReport != nil && spendReport.SpendingTx != nil {
+			spendingTxHash := spendReport.SpendingTx.TxHash()
+			spendDetails = &chainntnfs.SpendDetail{
+				SpentOutPoint:     &spendRequest.OutPoint,
+				SpenderTxHash:     &spendingTxHash,
+				SpendingTx:        spendReport.SpendingTx,
+				SpenderInputIndex: spendReport.SpendingInputIndex,
+				SpendingHeight:    int32(spendReport.SpendingTxHeight),
+			}
+		}
+
+		// Finally, no matter whether the rescan found a spend in the past or
+		// not, we'll mark our historical rescan as complete to ensure the
+		// outpoint's spend hint gets updated upon connected/disconnected
+		// blocks.
+		err = n.txNotifier.UpdateSpendDetails(spendRequest, spendDetails)
+		if err != nil {
+			chainntnfs.Log.Errorf("Failed to update spend details: %v", err)
+			return
+		}
+	}()
 
 	return ntfn.Event, nil
 }