Validate pubsub subscriptions (#627)

* Check topic before subscribing
* Block subscribe to invalid topics

libp2p/protocols/pubsub/pubsub.nim

@@ -452,6 +452,11 @@ proc subscribe*(p: PubSub,
   ##               on every received message
   ##
 
+  # Check that this is an allowed topic
+  if p.subscriptionValidator != nil and p.subscriptionValidator(topic) == false:
+    warn "Trying to subscribe to a topic not passing validation!", topic
+    return
+
   p.topics.withValue(topic, handlers) do:
     # Already subscribed, just adding another handler
     handlers[].add(handler)

tests/pubsub/testgossipsub2.nim

@@ -147,6 +147,9 @@ suite "GossipSub":
         nodes[1].start(),
     ))
 
+    # We must subscribe before setting the validator
+    nodes[0].subscribe("foobar", handler)
+
     var gossip = GossipSub(nodes[0])
     let invalidDetected = newFuture[void]()
     gossip.subscriptionValidator =
@@ -162,7 +165,6 @@ suite "GossipSub":
 
     await subscribeNodes(nodes)
 
-    nodes[0].subscribe("foobar", handler)
     nodes[1].subscribe("foobar", handler)
 
     await invalidDetected.wait(10.seconds)