[tor-bugs] #25374 [Core Tor/Tor]: Create a better-designed system for handling computation outside the event loop

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#25374: Create a better-designed system for handling computation outside the event
loop
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     Reporter:  nickm         |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Low           |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:  Sponsor8      |
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 Right now, we do a couple of things in `run_main_loop_once` that happen
 outside the event loop (because we want to re-scan for events event loop
 before they happen):
   * Making events on active_linked_connection_lst active.
   * Running connection_ap_attach_pending.

 But we can do this much better.  With Libevent 2.1, instead of making the
 loop exit for this, we can should do all of these things in a separate
 event callback, and call `event_base_loopcontinue()` at the end of that
 event's callback so that the event_base will get rescanned before we
 return. With earlier versions of Libevent, we can do something similar
 with event_base_loopbreak().

 Doing this won't lower the number of wakeups we do, but it should simplify
 our overall event loop logic, and make other event loop simplifications
 easier.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25374>
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