Don't modify iterables while iterating them (#219)

* don't modify iterables while iterating

* assert handlers to properly close connections

libp2p/muxers/mplex/mplex.nim

@@ -118,6 +118,7 @@ method handle*(m: Mplex) {.async, gcsafe.} =
                   # doAssert(stream.closed, "connection not closed by handler!")
                 except CatchableError as exc:
                   trace "exception in stream handler", exc = exc.msg
+                  doAssert(stream.closed, "stream not closed by protocol handler")
                 finally:
                   m.conns.keepItIf(it != stream)
                   m.handlerFuts.keepItIf(it != fut)

libp2p/protocols/pubsub/gossipsub.nim

@@ -215,7 +215,7 @@ proc heartbeat(g: GossipSub) {.async.} =
       await g.heartbeatLock.acquire()
       trace "running heartbeat"
 
-      for t in g.topics.keys:
+      for t in toSeq(g.topics.keys):
         await g.rebalanceMesh(t)
 
       await g.dropFanoutPeers()