[tor-bugs] #23993 [Core Tor/Tor]: sched: Refactor and fix interactions between channel and scheduler

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#23993: sched: Refactor and fix interactions between channel and scheduler
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     Reporter:  dgoulet       |      Owner:  dgoulet
         Type:  defect        |     Status:  assigned
     Priority:  Medium        |  Milestone:  Tor: 0.3.3.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-sched
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 This is a top level ticket for all the things we need to address in the
 channel subsystem and scheduler.

 Channels interact with the scheduler using a few entry points and
 assumptions provided by the circuit and channel subsystem. However, it
 turns out that the channel abstraction is often violated from the
 connection subsystem with `channel_t` and `channeltls_t` objects.

 We also have too many cells queues that goes from connection inbuf to
 circuit queue to channel queue to connection outbuf. But #23709 showed us
 that the channel queues aren't quite working as intended.

 Furthermore, we put scheduler state data in the `channel_t` object that
 are actually only relevant to the Vanilla scheduler but not to the other
 types (KIST and KISTLite for now).

 Along with pastly, we've discussed changes to simplify the scheduler
 interface, to make scheduler data stored in a channel tied to a scheduler
 type and make some changes to improve the channel subsystem.

 The child tickets address all of the above (and maybe more) in order to
 have a leaner channel subsystem that actually provides properties and
 correct assumptions to the scheduler.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23993>
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