channeldb: fix race condition in link node pruning

diff --git a/channeldb/db.go b/channeldb/db.go
index 00b29f6..6064975 100644
--- a/channeldb/db.go
+++ b/channeldb/db.go
@@ -1363,11 +1363,7 @@ func (c *ChannelStateDB) FetchClosedChannelForID(cid lnwire.ChannelID) (
 // the pending funds in a channel that has been forcibly closed have been
 // swept.
 func (c *ChannelStateDB) MarkChanFullyClosed(chanPoint *wire.OutPoint) error {
-	var (
-		openChannels  []*OpenChannel
-		pruneLinkNode *btcec.PublicKey
-	)
-	err := kvdb.Update(c.backend, func(tx kvdb.RwTx) error {
+	return kvdb.Update(c.backend, func(tx kvdb.RwTx) error {
 		var b bytes.Buffer
 		if err := graphdb.WriteOutpoint(&b, chanPoint); err != nil {
 			return err
@@ -1413,44 +1409,72 @@ func (c *ChannelStateDB) MarkChanFullyClosed(chanPoint *wire.OutPoint) error {
 		// other open channels with this peer. If we don't we'll
 		// garbage collect it to ensure we don't establish persistent
 		// connections to peers without open channels.
-		pruneLinkNode = chanSummary.RemotePub
-		openChannels, err = c.fetchOpenChannels(
-			tx, pruneLinkNode,
-		)
+		remotePub := chanSummary.RemotePub
+		openChannels, err := c.fetchOpenChannels(tx, remotePub)
 		if err != nil {
 			return fmt.Errorf("unable to fetch open channels for "+
 				"peer %x: %v",
-				pruneLinkNode.SerializeCompressed(), err)
+				remotePub.SerializeCompressed(), err)
 		}
 
-		return nil
-	}, func() {
-		openChannels = nil
-		pruneLinkNode = nil
-	})
-	if err != nil {
-		return err
-	}
+		if len(openChannels) > 0 {
+			return nil
+		}
+
+		// If there are no open channels with this peer, prune the
+		// link node. We do this within the same transaction to avoid
+		// a race condition where a new channel could be opened
+		// between this check and the deletion.
+		log.Infof("Pruning link node %x with zero open "+
+			"channels from database",
+			remotePub.SerializeCompressed())
 
-	// Decide whether we want to remove the link node, based upon the number
-	// of still open channels.
-	return c.pruneLinkNode(openChannels, pruneLinkNode)
+		err = deleteLinkNode(tx, remotePub)
+		if err != nil {
+			return fmt.Errorf("unable to delete link "+
+				"node: %w", err)
+		}
+
+		return nil
+	}, func() {})
 }
 
 // pruneLinkNode determines whether we should garbage collect a link node from
-// the database due to no longer having any open channels with it. If there are
-// any left, then this acts as a no-op.
-func (c *ChannelStateDB) pruneLinkNode(openChannels []*OpenChannel,
-	remotePub *btcec.PublicKey) error {
+// the database due to no longer having any open channels with it.
+//
+// NOTE: This function should be called after an initial check shows no open
+// channels exist. It will double-check within a write transaction to avoid a
+// race condition where a channel could be opened between the initial check
+// and the deletion.
+func (c *ChannelStateDB) pruneLinkNode(remotePub *btcec.PublicKey) error {
+	return kvdb.Update(c.backend, func(tx kvdb.RwTx) error {
+		// Double-check for open channels to avoid deleting a link node
+		// if a channel was opened since the caller's initial check.
+		//
+		// NOTE: This avoids a race condition where a channel could be
+		// opened between the initial check and the deletion.
+		openChannels, err := c.fetchOpenChannels(tx, remotePub)
+		if err != nil {
+			return err
+		}
 
-	if len(openChannels) > 0 {
-		return nil
-	}
+		// If channels exist now, don't prune.
+		if len(openChannels) > 0 {
+			return nil
+		}
 
-	log.Infof("Pruning link node %x with zero open channels from database",
-		remotePub.SerializeCompressed())
+		// No open channels, safe to prune the link node.
+		log.Infof("Pruning link node %x with zero open channels "+
+			"from database",
+			remotePub.SerializeCompressed())
 
-	return c.linkNodeDB.DeleteLinkNode(remotePub)
+		err = deleteLinkNode(tx, remotePub)
+		if err != nil {
+			return fmt.Errorf("unable to prune link node: %w", err)
+		}
+
+		return nil
+	}, func() {})
 }
 
 // PruneLinkNodes attempts to prune all link nodes found within the database
@@ -1479,7 +1503,11 @@ func (c *ChannelStateDB) PruneLinkNodes() error {
 			return err
 		}
 
-		err = c.pruneLinkNode(openChannels, linkNode.IdentityPub)
+		if len(openChannels) > 0 {
+			continue
+		}
+
+		err = c.pruneLinkNode(linkNode.IdentityPub)
 		if err != nil {
 			return err
 		}