setBlocking(false) is called on AsyncFD in newAsyncSocket proc.

lib/pure/asyncnet.nim

@@ -140,9 +140,18 @@ proc newAsyncSocket*(fd: AsyncFD, domain: Domain = AF_INET,
     sockType: SockType = SOCK_STREAM,
     protocol: Protocol = IPPROTO_TCP, buffered = true): AsyncSocket =
   ## Creates a new ``AsyncSocket`` based on the supplied params.
+  ##
+  ## The supplied ``fd``'s non-blocking state will be enabled implicitly.
+  ##
+  ## **Note**: This procedure will **NOT** register ``fd`` with the global
+  ## async dispatcher. You need to do this manually. If you have used
+  ## ``newAsyncNativeSocket`` to create ``fd`` then it's already registered.
+  ## The reason for this is that the ``AsyncFD`` type is a special type for an
+  ## FD that signifies that its been registered.
   assert fd != osInvalidSocket.AsyncFD
   new(result)
   result.fd = fd.SocketHandle
+  fd.SocketHandle.setBlocking(false)
   result.isBuffered = buffered
   result.domain = domain
   result.sockType = sockType