attempt to make asynchttpserver better; fixes #15925; [backport:1.0]

lib/pure/asyncdispatch.nim

@@ -1934,3 +1934,25 @@ proc waitFor*[T](fut: Future[T]): T =
     poll()
 
   fut.read
+
+proc activeDescriptors*(): int {.inline.} =
+  ## Returns the current number of active file descriptors for the current
+  ## event loop. This is a cheap operation that does not involve a system call.
+  when defined(windows):
+    result = getGlobalDispatcher().handles.len
+  else:
+    result = getGlobalDispatcher().selector.count
+
+when defined(posix):
+  import posix
+
+proc maxDescriptors*(): int {.raises: OSError.} =
+  ## Returns the maximum number of active file descriptors for the current
+  ## process. This involves a system call.
+  when defined(windows):
+    result = 16_700_000
+  else:
+    var fdLim: RLimit
+    if getrlimit(RLIMIT_NOFILE, fdLim) < 0:
+      raiseOSError(osLastError())
+    result = int(fdLim.rlim_cur) - 1