[tor-bugs] #28090 [Core Tor/Tor]: Keepalive padding cell bypasses the cell scheduler

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#28090: Keepalive padding cell bypasses the cell scheduler
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     Reporter:  dgoulet   |      Owner:  (none)
         Type:  defect    |     Status:  new
     Priority:  Medium    |  Milestone:  Tor: 0.3.6.x-final
    Component:  Core      |    Version:
  Tor/Tor                 |
     Severity:  Normal    |   Keywords:  tor-relay, tor-scheduler, tor-kist
Actual Points:            |  Parent ID:
       Points:            |   Reviewer:
      Sponsor:            |
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 When sending a keepalive cell, we look if the connection outbuf is empty
 but with the KIST scheduler, that buffer is optimized to be emptied very
 quickly.

 That cell should be able to ask the scheduler if it can queue cells in the
 outbuf. In an _ideal_ world, that cell would be handled by the scheduler
 but since it is not a circuit level "cell", we can not quickly fix that.

 The fix for padding cell is to ask the scheduler, for a specific channel,
 if it can queue in the outbuf. The KIST scheduler would do two things:
 check for pending cells and if the kernel would allow it (tcp buffer
 size).

 This made the #28089 issue more pathological.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28090>
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