[tor-bugs] #12862 [Tor]: Refactor code to modify circ->n_chan only by circuit_set_circid_chan_helper

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#12862: Refactor code to modify circ->n_chan only by circuit_set_circid_chan_helper
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     Reporter:           |      Owner:
  cypherpunks            |     Status:  new
         Type:           |  Milestone:  Tor: 0.2.6.x-final
  enhancement            |    Version:
     Priority:  minor    |   Keywords:  tor-relay 026-triaged-1
    Component:  Tor      |  026-deferrable
   Resolution:           |  Parent ID:
Actual Points:           |
       Points:           |
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Comment (by andrea):

 There's no bug here, and this ticket is just griping that old_chan (and
 old_id too) are copies of *chan_ptr and *circid_ptr in the first half of
 circuit_set_circid_chan_helper(), AFAICT.

 Refactoring wouldn't eliminate the old_chan/old_id local variables, since
 we also need them after we change the channel/circuit id to decide whether
 we need to circuitmux_attach_circuit().  It'd only change the references
 to old_chan/old_id to *chan_ptr/*circid_ptr in the if-clause tests prior
 to the channel/circuit id change.

 I'm not sure changing this actually counts as an improvement.  The
 existing names make it manifestly clear that it's comparing the existing
 channel or circuit id to the value we're setting to, and thus that the
 purpose of the test is to detect if we have a real change or one/both of
 them is remaining constant.  It'd also introduce an inconsistency in usage
 between the ones before the assignment and the circuitmux_attach_circuit()
 test after, where these are at present nicely symmetrical.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/12862#comment:4>
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