discovery: lock the channelMtx before making the funding script

As we move the funding transaction validation logic out of the builder
and into the gossiper, we want to ensure that the behaviour stays
consistent with what we have today. So we should aquire this lock before
performing any expensive checks such as building the funding tx or
valdating it.

Author: ellemouton

discovery/gossiper.go

@@ -2634,6 +2634,14 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 		tapscriptRoot = nMsg.optionalMsgFields.tapscriptRoot
 	}
 
+	// Before we start validation or add the edge to the database, we obtain
+	// the mutex for this channel ID. We do this to ensure no other
+	// goroutine has read the database and is now making decisions based on
+	// this DB state, before it writes to the DB. It also ensures that we
+	// don't perform the expensive validation check on the same channel
+	// announcement at the same time.
+	d.channelMtx.Lock(scid.ToUint64())
+
 	// We only make use of the funding script later on during funding
 	// transaction validation if AssumeChannelValid is not true.
 	if !(d.cfg.AssumeChannelValid || d.cfg.IsAlias(scid)) {
@@ -2642,6 +2650,8 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 			tapscriptRoot,
 		)
 		if err != nil {
+			defer d.channelMtx.Unlock(scid.ToUint64())
+
 			log.Errorf("Unable to make funding script %v", err)
 			nMsg.err <- err
 
@@ -2656,12 +2666,6 @@ func (d *AuthenticatedGossiper) handleChanAnnouncement(nMsg *networkMsg,
 	// We will add the edge to the channel router. If the nodes present in
 	// this channel are not present in the database, a partial node will be
 	// added to represent each node while we wait for a node announcement.
-	//
-	// Before we add the edge to the database, we obtain the mutex for this
-	// channel ID. We do this to ensure no other goroutine has read the
-	// database and is now making decisions based on this DB state, before
-	// it writes to the DB.
-	d.channelMtx.Lock(scid.ToUint64())
 	err = d.cfg.Graph.AddEdge(edge, ops...)
 	if err != nil {
 		log.Debugf("Graph rejected edge for short_chan_id(%v): %v",