GS: Publish to fanout when mesh unhealthy (#638)

* Send to fanout when mesh unhealthy

* don't use fanout when floodPublish

libp2p/protocols/pubsub/gossipsub.nim

@@ -482,13 +482,23 @@ method publish*(g: GossipSub,
 
   if topic in g.topics: # if we're subscribed use the mesh
     peers.incl(g.mesh.getOrDefault(topic))
-  else: # not subscribed, send to fanout peers
-    # try optimistically
-    peers.incl(g.fanout.getOrDefault(topic))
-    if peers.len == 0:
-      # ok we had nothing.. let's try replenish inline
+
+  if peers.len < g.parameters.dLow and g.parameters.floodPublish == false:
+    # not subscribed or bad mesh, send to fanout peers
+    # disable for floodPublish, since we already sent to every good peer
+    #
+    var fanoutPeers = g.fanout.getOrDefault(topic).toSeq()
+    if fanoutPeers.len == 0:
       g.replenishFanout(topic)
-      peers.incl(g.fanout.getOrDefault(topic))
+      fanoutPeers = g.fanout.getOrDefault(topic).toSeq()
+
+    fanoutPeers.shuffle()
+    if fanoutPeers.len + peers.len > g.parameters.d:
+      fanoutPeers.setLen(g.parameters.d - peers.len)
+    
+    for fanPeer in fanoutPeers:
+      peers.incl(fanPeer)
+      if peers.len > g.parameters.d: break
 
     # even if we couldn't publish,
     # we still attempted to publish

libp2p/protocols/pubsub/gossipsub/behavior.nim

@@ -489,9 +489,11 @@ proc replenishFanout*(g: GossipSub, topic: string) {.raises: [Defect].} =
   logScope: topic
   trace "about to replenish fanout"
 
+  let currentMesh = g.mesh.getOrDefault(topic)
   if g.fanout.peers(topic) < g.parameters.dLow:
     trace "replenishing fanout", peers = g.fanout.peers(topic)
     for peer in g.gossipsub.getOrDefault(topic):
+      if peer in currentMesh: continue
       if g.fanout.addPeer(topic, peer):
         if g.fanout.peers(topic) == g.parameters.d:
           break

tests/pubsub/testgossipsub.nim

@@ -565,6 +565,71 @@ suite "GossipSub":
     await allFuturesThrowing(nodesFut.concat())
     check observed == 2
 
+  asyncTest "e2e - GossipSub send over fanout A -> B for subscribed topic":
+    var passed = newFuture[void]()
+    proc handler(topic: string, data: seq[byte]) {.async, gcsafe.} =
+      check topic == "foobar"
+      passed.complete()
+
+    let
+      nodes = generateNodes(
+        2,
+        gossip = true)
+
+      # start switches
+      nodesFut = await allFinished(
+        nodes[0].switch.start(),
+        nodes[1].switch.start(),
+      )
+
+    # start pubsub
+    await allFuturesThrowing(
+      allFinished(
+        nodes[0].start(),
+        nodes[1].start(),
+    ))
+
+    await subscribeNodes(nodes)
+
+    nodes[1].subscribe("foobar", handler)
+    nodes[0].subscribe("foobar", handler)
+    await waitSub(nodes[0], nodes[1], "foobar")
+    await waitSub(nodes[1], nodes[0], "foobar")
+
+    nodes[0].unsubscribe("foobar", handler)
+
+    let gsNode = GossipSub(nodes[1])
+    check await checkExpiring(gsNode.mesh.getOrDefault("foobar").len == 0)
+
+    nodes[0].subscribe("foobar", handler)
+
+    check GossipSub(nodes[0]).mesh.getOrDefault("foobar").len == 0
+
+    tryPublish await nodes[0].publish("foobar", "Hello!".toBytes()), 1
+
+    check:
+      GossipSub(nodes[0]).fanout.getOrDefault("foobar").len > 0
+      GossipSub(nodes[0]).mesh.getOrDefault("foobar").len == 0
+
+    await passed.wait(2.seconds)
+
+    trace "test done, stopping..."
+
+    await nodes[0].stop()
+    await nodes[1].stop()
+
+    await allFuturesThrowing(
+      nodes[0].switch.stop(),
+      nodes[1].switch.stop()
+    )
+
+    await allFuturesThrowing(
+      nodes[0].stop(),
+      nodes[1].stop()
+    )
+
+    await allFuturesThrowing(nodesFut.concat())
+
   asyncTest "e2e - GossipSub send over mesh A -> B":
     var passed: Future[bool] = newFuture[bool]()
     proc handler(topic: string, data: seq[byte]) {.async, gcsafe.} =