proof that refcounting can handle Nim's async (#12533)

tests/async/tasyncawait_cyclebreaker.nim

@@ -0,0 +1,73 @@
+discard """
+  output: "50000"
+  cmd: "nim c -d:nimTypeNames -d:nimCycleBreaker $file"
+"""
+import asyncdispatch, asyncnet, nativesockets, net, strutils, os
+
+var msgCount = 0
+
+const
+  swarmSize = 500
+  messagesToSend = 100
+
+var clientCount = 0
+
+proc sendMessages(client: AsyncFD) {.async.} =
+  for i in 0 ..< messagesToSend:
+    await send(client, "Message " & $i & "\c\L")
+
+proc launchSwarm(port: Port) {.async.} =
+  for i in 0 ..< swarmSize:
+    var sock = createAsyncNativeSocket()
+
+    await connect(sock, "localhost", port)
+    await sendMessages(sock)
+    closeSocket(sock)
+
+proc readMessages(client: AsyncFD) {.async.} =
+  # wrapping the AsyncFd into a AsyncSocket object
+  var sockObj = newAsyncSocket(client)
+  var (ipaddr, port) = sockObj.getPeerAddr()
+  doAssert ipaddr == "127.0.0.1"
+  (ipaddr, port) = sockObj.getLocalAddr()
+  doAssert ipaddr == "127.0.0.1"
+  while true:
+    var line = await recvLine(sockObj)
+    if line == "":
+      closeSocket(client)
+      clientCount.inc
+      break
+    else:
+      if line.startswith("Message "):
+        msgCount.inc
+      else:
+        doAssert false
+
+proc createServer(port: Port) {.async.} =
+  var server = createAsyncNativeSocket()
+  block:
+    var name: Sockaddr_in
+    name.sin_family = typeof(name.sin_family)(toInt(AF_INET))
+    name.sin_port = htons(uint16(port))
+    name.sin_addr.s_addr = htonl(INADDR_ANY)
+    if bindAddr(server.SocketHandle, cast[ptr SockAddr](addr(name)),
+                sizeof(name).Socklen) < 0'i32:
+      raiseOSError(osLastError())
+
+  discard server.SocketHandle.listen()
+  while true:
+    asyncCheck readMessages(await accept(server))
+
+asyncCheck createServer(Port(10335))
+asyncCheck launchSwarm(Port(10335))
+while true:
+  poll()
+  GC_collectZct()
+
+  let (allocs, deallocs) = getMemCounters()
+  doAssert allocs < deallocs + 1000
+
+  if clientCount == swarmSize: break
+
+assert msgCount == swarmSize * messagesToSend
+echo msgCount