Move checkCloseError to nativesockets

lib/pure/nativesockets.nim

@@ -620,3 +620,28 @@ proc selectWrite*(writefds: var seq[SocketHandle],
 when defined(Windows):
   var wsa: WSAData
   if wsaStartup(0x0101'i16, addr wsa) != 0: raiseOSError(osLastError())
+
+proc checkCloseError*(ret: cint) =
+  ## Asserts that the return value of close() or closeSocket() syscall
+  ## does not indicate a programming error (such as invalid descriptor).
+  ## This must only be used when an error has already occurred and
+  ## you are performing a cleanup.
+  ## Otherwise, error handling must be performed as usual.
+  ##
+  ## This procedure must be called right after performing the syscall. Example:
+  ##
+  ## .. code-block:: nim
+  ##
+  ##  let ret = someSysCall()
+  ##  if ret != 0:
+  ##    let errcode = osLastError()
+  ##    checkCloseError sock.closeSocket()
+  ##    raise newException(OSError, osErrorMsg(errcode))
+
+  if ret != 0:
+    let errcode = osLastError()
+    when useWinVersion:
+      doAssert(errcode.int32 notin {WSANOTINITIALISED, WSAENOTSOCK,
+                                    WSAEINPROGRESS, WSAEINTR, WSAEWOULDBLOCK})
+    else:
+      doAssert(errcode.int32 notin {EBADF})