[tor-bugs] #23687 [Core Tor/Tor]: kist: Channel in waiting_to_write goes missing until a destroy cell

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#23687: kist: Channel in waiting_to_write goes missing until a destroy cell
------------------------------+--------------------------------
     Reporter:  dgoulet       |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  High          |  Milestone:  Tor: 0.3.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-sched, kist
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Here is the gist with some logs I added to tor:

 {{{
 Sep 28 20:03:09.472 [info] kist_scheduler_run(): Case 4: channel 471. Out
 queue len: 0, in queue len: 0, cmux queue len: 15. Is there more to flush:
 yes. Channel state 'open' (sched: 3)
 Sep 28 20:03:09.472 [info] channel_write_cell_queue_entry(): Channel 471
 cell sent. cmux queue len: 15
 Sep 28 20:03:09.483 [info] kist_scheduler_run(): Case 3: channel 471. Out
 queue len: 0, in queue len: 0, cmux queue len: 14. Is there more to flush:
 yes. Channel state 'open' (sched: 2)
 Sep 28 20:03:09.484 [info] channel_write_to_kernel(): Writing 0 bytes to
 kernel for chan 471
 [...]
 Sep 28 20:04:13.738 [info] circuitmux_append_destroy_cell(): Destroy cell
 on channel 471
 Sep 28 20:04:13.738 [info] channel_write_cell_queue_entry(): Channel 471
 cell sent. cmux queue len: 0
 }}}

 Notice the 60+ seconds gap from the channel going to
 `SCHED_CHAN_WAITING_TO_WRITE` (sched: 2) and then nothing until a destroy
 cell happens. Then suddenly at that point, no more cells in the cmux queue
 nor any calls to the flush function.

 So questions are:

 1. Why the channel in waiting_to_write never got scheduled again?

 2. Where are those cells from the cmux queue? Is something else in tor
 moving those to the outbuf?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23687>
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