graph: fix graph cache population for channels with both policies disabled

Fix a bug where channels with both policies disabled were not added to
the graph cache during startup. When a policy update later re-enabled
one of the directions, the update would succeed in the database but fail
to update the graph cache (since the channel structure was never added),
preventing the channel from being used for routing.

graph/db/graph_cache.go

@@ -121,13 +121,9 @@ func (c *GraphCache) AddChannel(info *models.CachedEdgeInfo,
 		return
 	}
 
-	if policy1 != nil && policy1.IsDisabled() &&
-		policy2 != nil && policy2.IsDisabled() {
-
-		return
-	}
-
-	// Create the edge entry for both nodes.
+	// Create the edge entry for both nodes. We always add the channel
+	// structure to the cache, even if both policies are currently disabled,
+	// so that later policy updates can find and update the channel entry.
 	c.mtx.Lock()
 	c.updateOrAddEdge(info.NodeKey1Bytes, &DirectedChannel{
 		ChannelID: info.ChannelID,
@@ -143,6 +139,19 @@ func (c *GraphCache) AddChannel(info *models.CachedEdgeInfo,
 	})
 	c.mtx.Unlock()
 
+	// Skip adding policies if both are disabled, as the channel is
+	// currently unusable for routing. However, we still add the channel
+	// structure above so that policy updates can later enable it.
+	if policy1 != nil && policy1.IsDisabled() &&
+		policy2 != nil && policy2.IsDisabled() {
+
+		log.Debugf("Skipping policies for channel %v: both "+
+			"policies are disabled (channel structure still "+
+			"cached for future updates)", info.ChannelID)
+
+		return
+	}
+
 	// The policy's node is always the to_node. So if policy 1 has to_node
 	// of node 2 then we have the policy 1 as seen from node 1.
 	if policy1 != nil {