Not calling setGlobalDispatcher newDispatcher() costs 500ms

[RedBeard0531]

import asyncdispatch
import asyncfile

var asyncStdout = 1.AsyncFD.newAsyncFile()
proc doStuff: Future[void] {.async.} =
  await asyncStdout.write "hello world\n"

#setGlobalDispatcher newDispatcher() # magical Go Fast button!
waitFor doStuff()

time: 502ms
time with dispatcher: 2ms

[kvinwang]

The iterator generated by async is:

  iterator doStuffIter245003(): FutureBase {.closure.} =
    var future245004 = asyncStdout.write "hello world\x0A"
    yield future245004
    future245004.read
    complete(retFuture245002)

But the future245004 completed immediately after asyncStdout.write.
Then the next yield will cause an extra polling, which will get nothing but 500ms timeout.
I guess the right code should looks like this:

  iterator doStuffIter245003(): FutureBase {.closure.} =
    var future245004 = asyncStdout.write "hello world\x0A"
    if not future245004.finished:
      yield future245004
    future245004.read
    complete(retFuture245002)

[yglukhov]

Will be fixed by #6614

[dom96]

This appears to be fixed now.

Reproduces in 0.17.2 but not 0.18.0 nor devel.