Fix RangeError defect being happened using android toolchain. (#538)

* Fix RangeError defect being happened using android toolchain.

* Set proper type for `Tnfds`.

* Update comment.

Author: cheatfate

chronos/ioselects/ioselectors_poll.nim

@@ -220,7 +220,10 @@ proc selectInto2*[T](s: Selector[T], timeout: int,
   verifySelectParams(timeout, -1, int(high(cint)))
 
   let
-    maxEventsCount = min(len(s.pollfds), len(readyKeys))
+    maxEventsCount = culong(min(len(s.pollfds), len(readyKeys)))
+      # Without `culong` conversion, this code could fail with RangeError
+      # defect on explicit Tnfds(integer) conversion (probably related to
+      # combination of nim+clang (android toolchain)).
     eventsCount =
       if maxEventsCount > 0:
         let res = handleEintr(poll(addr(s.pollfds[0]), Tnfds(maxEventsCount),

chronos/osdefs.nim

@@ -965,7 +965,7 @@ elif defined(macos) or defined(macosx):
       events*: cshort
       revents*: cshort
 
-    Tnfds* {.importc: "nfds_t", header: "<poll.h>".} = cuint
+    Tnfds* {.importc: "nfds_t", header: "<poll.h>".} = culong
 
   const
     POLLIN* = 0x0001