[tor-bugs] #31937 [Core Tor/Tor]: decide and document that stuff in the CacheDirectory can be deleted while Tor is running

[Tor Bug Tracker & Wiki]

#31937: decide and document that stuff in the CacheDirectory can be deleted while
Tor is running
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     Reporter:  arma          |      Owner:  (none)
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 As of #24272 and #22703 we have a separate CacheDirectory that Tor can use
 to put transient things like cached directory info.

 _hc asked just now if it's ok to put the CacheDirectory, for Tor on
 Android, in a place where it might get blown away periodically.

 On #24272 I see an example value being "/var/cache/".

 And on https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch05s05.html it
 says that for /var/cache/, "The application must always be able to recover
 from manual deletion of these files (generally because of a disk space
 shortage)."

 So I think the answer is yes, we should support having the files in
 CacheDirectory disappear out from under us.

 So:

 * Step zero is to decide that this is what we want.

 * Step one is to learn whether Tor breaks currently in this situation.
 Like, we mmap some of those files. If they get deleted, does everything go
 smoothly? (For some Linux filesystems, the file can be deleted but Tor
 will still hold its copy in existence until it gives up the file
 descriptor, which would seem helpful in this situation. But I bet not all
 OSes and FSes work that way.)

 * And step two would be to change the man page to say what we do or don't
 promise here.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/31937>
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