[tor-bugs] #21910 [Core Tor/Tor]: Refactor connection_edge_process_relay_cell()

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#21910: Refactor connection_edge_process_relay_cell()
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     Reporter:  dgoulet       |      Owner:
         Type:  enhancement   |     Status:  new
     Priority:  Low           |  Milestone:  Tor: unspecified
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  refactor, easy
Actual Points:                |  Parent ID:
       Points:  1             |   Reviewer:
      Sponsor:                |
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 Ticket #16706 is one of the possible many issues we had and will have with
 this function.

 It is quite big with many many return callsite and it is confusing on how
 it behaves. For instance, if `-reason` is returned, the caller should
 teardown the circuit and log warn but yet this functions already does many
 `LOG_PROTOCOL_WARN` in that case.

 One thing we could do is maybe return a different error code (or set an
 error code) depending on what's happening (should close circ, cell
 dropped, error). For instance, currently, returning 0 can either mean that
 a cell was dropped or successfully relayed.

 Auditing every callsite of this function would be important to understand
 how it is actually used so we can properly improve it and make it less
 error prone with dubious logging (or improved logging).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21910>
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