diff --git a/tests/untestable/tasynchttpserver_extra.nim b/tests/untestable/tasynchttpserver_extra.nim
new file mode 100644
index 0000000000000..96faaf578da04
--- /dev/null
+++ b/tests/untestable/tasynchttpserver_extra.nim
@@ -0,0 +1,71 @@
+#[
+D20210108T105903
+adapted from the runnableExamples in asynchttpserver.nim
+
+## works on linux
+
+## TODO
+* support osx + other platforms; the missing part is `/proc/{pid}/fd`
+maybe something based on lsof could work
+* turn this into a proper test that doesn't require user interaction
+
+* D20210107T122027:here BUG: why -1 in `result = int(fdLim.rlim_cur) - 1`  in asyncdispatch.maxDescriptors ?
+]#
+
+when true:
+  # This example will create an HTTP server on port 8080. The server will
+  # respond to all requests with a `200 OK` response code and "Hello World"
+  # as the response body.
+  import asynchttpserver
+  import asyncdispatch
+  import os, osproc, strformat, strutils
+
+  proc getCmdOutput(cmd: string): string =
+    var (outp, status) = execCmdEx(cmd)
+    doAssert status == 0, cmd
+    doAssert status == 0
+    outp.stripLineEnd
+    result = outp
+
+  proc getNumOpenFiles(): int =
+    let pid = getCurrentProcessId()
+    # xxx remove the pipe so that we only spawn 1 process, not 2, leading to 3 extra fds, not 6
+    let s = getCmdOutput fmt"ls -1 /proc/{pid}/fd | wc -l"
+    result = s.parseInt
+
+  proc getNumOpenFilesMax(): int =
+    # this could potentially change over time
+    when false:
+      result = getCmdOutput(fmt"ulimit -Sn").parseInt
+    result = maxDescriptors() + 1 # + 1 because of D20210107T122027
+
+  proc shouldAcceptRequest2(numFdPerRequest = 1): bool =
+    let num = getNumOpenFiles()
+    let numMax = getNumOpenFilesMax()
+    let numExtra = 6 # because of `getNumOpenFiles`
+    result = num + numFdPerRequest + numExtra <= numMax
+    echo ("shouldAcceptRequest2", num, numMax, result)
+
+  var count = 0
+  proc main {.async.} =
+    const port = 8080
+    var server = newAsyncHttpServer()
+    proc cb(req: Request) {.async.} =
+      let count2 = atomicInc(count) # count.inc would be buggy
+      echo (count2, req.reqMethod, req.url, req.headers)
+      let headers = {"Date": "Tue, 29 Apr 2014 23:40:08 GMT",
+          "Content-type": "text/plain; charset=utf-8"}
+      await sleepAsync(5_000)
+      await req.respond(Http200, &"Hello World: {count2}\n", headers.newHttpHeaders())
+
+    echo fmt"test this with: curl localhost:{port}/"
+    server.listen Port(port)
+    while true:
+      if shouldAcceptRequest2(1):
+        await server.acceptRequest(cb)
+      else:
+        # too many files opened, likely caused by too many concurrent connections (`maxFDs` exceeded)
+        poll()
+
+  asyncCheck main()
+  runForever()