[tor-bugs] #11553 [Tor]: Be more verbose when warning about not finding a circuit ID

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#11553: Be more verbose when warning about not finding a circuit ID
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     Reporter:  ra       |      Owner:
         Type:           |     Status:  needs_review
  enhancement            |  Milestone:  Tor: 0.2.5.x-final
     Priority:  major    |    Version:  Tor: 0.2.5.3-alpha
    Component:  Tor      |   Keywords:  tor-relay 024-backport 023-backport
   Resolution:           |  Parent ID:
Actual Points:           |
       Points:           |
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Comment (by andrea):

 Replying to [comment:15 cypherpunks]:
 > What chance to fill test_circ_id with zeroes by crypto_rand? Non zero.
 > Non zero chance to get zero circ_id and to violate specification.

 Good near-catch.  Not actually going to cause it to try to use a zero circ
 ID since get_unique_circ_id_by_chan() returns zero to indicate failure,
 but that amounts to a lucky coincidence.  Hitting the 2^-15^ or 2^-31^
 probability to zero circ_id would cause it to exit the loop without
 actually making MAX_CIRCID_ATTEMPTS tries, so it increases the probability
 to fail to find a circuit ID.  For the narrow circuit ID case, this is the
 dominant source of failures to find a circuit ID when N/max_range is less
 than about 0.85, if my analysis is correct.  This code should check for
 the zero case in the loop condition, I think.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/11553#comment:16>
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