channeldb/graph: only set laggard zombie pubkey

channeldb/graph.go

@@ -1997,12 +1997,54 @@ func delChannelEdge(edges, edgeIndex, chanIndex, zombieIndex,
 		return nil
 	}
 
+	nodeKey1, nodeKey2 := makeZombiePubkeys(&edgeInfo, edge1, edge2)
+
 	return markEdgeZombie(
-		zombieIndex, byteOrder.Uint64(chanID), edgeInfo.NodeKey1Bytes,
-		edgeInfo.NodeKey2Bytes,
+		zombieIndex, byteOrder.Uint64(chanID), nodeKey1, nodeKey2,
 	)
 }
 
+// makeZombiePubkeys derives the node pubkeys to store in the zombie index for a
+// particular pair of channel policies. The return values are one of:
+//  1. (pubkey1, pubkey2)
+//  2. (pubkey1, blank)
+//  3. (blank, pubkey2)
+//
+// A blank pubkey means that corresponding node will be unable to resurrect a
+// channel on its own. For example, node1 may continue to publish recent
+// updates, but node2 has fallen way behind. After marking an edge as a zombie,
+// we don't want another fresh update from node1 to resurrect, as the edge can
+// only become live once node2 finally sends something recent.
+//
+// In the case where we have neither update, we allow either party to resurrect
+// the channel. If the channel were to be marked zombie again, it would be
+// marked with the correct lagging channel since we received an update from only
+// one side.
+func makeZombiePubkeys(info *ChannelEdgeInfo,
+	e1, e2 *ChannelEdgePolicy) ([33]byte, [33]byte) {
+
+	switch {
+
+	// If we don't have either edge policy, we'll return both pubkeys so
+	// that the channel can be resurrected by either party.
+	case e1 == nil && e2 == nil:
+		return info.NodeKey1Bytes, info.NodeKey2Bytes
+
+	// If we're missing edge1, or if both edges are present but edge1 is
+	// older, we'll return edge1's pubkey and a blank pubkey for edge2. This
+	// means that only an update from edge1 will be able to resurrect the
+	// channel.
+	case e1 == nil || (e2 != nil && e1.LastUpdate.Before(e2.LastUpdate)):
+		return info.NodeKey1Bytes, [33]byte{}
+
+	// Otherwise, we're missing edge2 or edge2 is the older side, so we
+	// return a blank pubkey for edge1. In this case, only an update from
+	// edge2 can resurect the channel.
+	default:
+		return [33]byte{}, info.NodeKey2Bytes
+	}
+}
+
 // UpdateEdgePolicy updates the edge routing policy for a single directed edge
 // within the database for the referenced channel. The `flags` attribute within
 // the ChannelEdgePolicy determines which of the directed edges are being