[tor-bugs] #32265 [Metrics/Exit Scanner]: MS: Format an exit list from a previous exit list and exitmap output

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#32265: MS: Format an exit list from a previous exit list and exitmap output
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 Reporter:  irl                   |          Owner:  irl
     Type:  task                  |         Status:  needs_revision
 Priority:  Medium                |      Milestone:
Component:  Metrics/Exit Scanner  |        Version:
 Severity:  Normal                |     Resolution:
 Keywords:                        |  Actual Points:
Parent ID:  #29654                |         Points:
 Reviewer:  karsten               |        Sponsor:
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Changes (by karsten):

 * status:  needs_review => needs_revision


Comment:

 Glad to see that the rewrite is progressing so quickly!

 Couple remarks/questions:
  - Why 48 hours and not 24 hours? Doesn't the current exit scanner keep
 scan results for 24 hours? I might be wrong, though. Let's use whatever
 the current scanner does.
  - Rather than downloading exit lists from CollecTor, wouldn't it be
 sufficient to just read the latest exit list previously written by this
 scanner? And if there's none, just assume that no previous scans have
 happened. In theory, this should be all we need to learn.
  - It seems that `LastStatus` is only taken from exit lists downloaded
 from CollecTor but never set by new measurements. We should make a plan
 what to do with this field. Take it out? Populate it with consensus valid-
 after times?
  - Does exitmap with the plugin use previous scans as input to decide
 which relays to scan? I believe that it uses some logic to avoid scanning
 relays too frequently. This has two effects: it doesn't generate more load
 on the network and on single relays than necessary, and it ensures that
 new relays are scanned sooner. As a result, the new scanner could be run
 once or twice per hour, rather than every 2 or 3 hours (at 45 minutes
 runtime).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32265#comment:5>
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