[tor-bugs] #10478 [Tor]: Use prepend_policy or similar in preference to mark_circuit_unusable_for_new_conns

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#10478: Use prepend_policy or similar in preference to
mark_circuit_unusable_for_new_conns
-------------------------------------+------------------------------------
 Reporter:  nickm                    |          Owner:
     Type:  defect                   |         Status:  new
 Priority:  major                    |      Milestone:  Tor: 0.2.5.x-final
Component:  Tor                      |        Version:
 Keywords:  024-backport tor-client  |  Actual Points:
Parent ID:                           |         Points:
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 For some error cases, probably including END_STREAM_REASON_TIMEOUT,
 END_STREAM_REASON_RESOLVEFAILED, END_STREAM_REASON_CONNECTREFUSED, our
 current practice of tracking failures with client_dns_incr_failures and
 marking the whole circuit as unusable for connections is probably no good.

 After all, a TIMEOUT can just mean that the particular address was
 filtered, or that it legitimately timed out, or something like that.

 Instead, we should use prepend_policy or some similar mechanism to mark
 that circuit, not as unsuitable for all new streams, but as simply
 unsuitable for the particular address.  (I say "something like
 prepend_policy" and not "prepend_policy" because we should really drop the
 original hostname and not just the resolved address.)   We might want to
 keep a counter of the number of successful and failed streams per circuit.

 This is going to be important if we make more failure types retriable
 (#7870).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/10478>
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