Fixes #4262.

lib/pure/asyncdispatch.nim

@@ -500,6 +500,7 @@ when defined(windows) or defined(nimdoc):
       raise newException(ValueError,
         "No handles or timers registered in dispatcher.")
 
+    if p.handles.len != 0:
       let at = p.adjustedTimeout(timeout)
       var llTimeout =
         if at == -1: winlean.INFINITE
@@ -1283,6 +1284,8 @@ else:
 
   proc poll*(timeout = 500) =
     let p = getGlobalDispatcher()
+
+    if p.selector.len > 0:
       for info in p.selector.select(p.adjustedTimeout(timeout)):
         let data = PData(info.key.data)
         assert data.fd == info.key.fd.AsyncFD

lib/pure/selectors.nim

@@ -375,6 +375,10 @@ proc contains*(s: Selector, key: SelectorKey): bool =
   when not defined(nimdoc):
     return key.fd in s and s.fds[key.fd] == key
 
+proc len*(s: Selector): int =
+  ## Retrieves the number of registered file descriptors in this Selector.
+  return s.fds.len
+
 {.deprecated: [TEvent: Event, PSelectorKey: SelectorKey,
    TReadyInfo: ReadyInfo, PSelector: Selector].}
 

tests/async/tpolltimeouts.nim

@@ -0,0 +1,19 @@
+discard """
+  output: "true"
+"""
+# Issue https://github.com/nim-lang/Nim/issues/4262
+import asyncdispatch, times
+
+proc foo(): Future[int] {.async.} =
+  return 1
+
+proc bar(): Future[int] {.async.} =
+  return await foo()
+
+let start = epochTime()
+let barFut = bar()
+
+while not barFut.finished:
+  poll(2000)
+
+echo(epochTime() - start < 1.0)