[tor-bugs] #22424 [Core Tor/Tor]: Always check for usage underflows in the storage API

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#22424: Always check for usage underflows in the storage API
--------------------------------------------+------------------------------
 Reporter:  teor                            |          Owner:
     Type:  defect                          |         Status:  needs_review
 Priority:  Medium                          |      Milestone:  Tor:
                                            |  0.3.1.x-final
Component:  Core Tor/Tor                    |        Version:  Tor:
                                            |  0.3.1.1-alpha
 Severity:  Normal                          |     Resolution:
 Keywords:  integer-safety, race-condition  |  Actual Points:  0.2
Parent ID:                                  |         Points:  0.2
 Reviewer:                                  |        Sponsor:
--------------------------------------------+------------------------------
Changes (by teor):

 * keywords:   => integer-safety, race-condition
 * status:  new => needs_review
 * actualpoints:   => 0.2


Comment:

 Please see my branch bug22424, which fixes two different ways the usage
 could underflow, and does something sensible when it does underflow. (For
 example, the usage can underflow when we cache the usage, then an external
 process makes a file bigger, then we delete that file.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22424#comment:1>
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