[tbb-bugs] #13012 [Tor Browser]: Reviewing Bug #3229: Make content pref service memory-only + clearable

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#13012: Reviewing Bug #3229: Make content pref service memory-only + clearable
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 Reporter:  arthuredelstein  |          Owner:  tbb-team
     Type:  defect           |         Status:  new
 Priority:  normal           |      Milestone:
Component:  Tor Browser      |        Version:
 Keywords:  tbb-disk-leak    |  Actual Points:
Parent ID:                   |         Points:
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 I noticed that nsContentPrefService.js can be expected to store prefs in
 memory, providing that any provided "loading context" has
 "usePrivateBrowsing" set to true, an assumption that may or may not hold
 for Firefox's Private Browsing (PB) mode. The patch for #3229 in addition
 applies to non-PB mode. Since Tor Browser uses PB mode by default, it's
 not entirely clear whether or not #3229 is needed.

 To complicate matters, nsContentPrefService.js has been deprecated in
 favor of [http://dxr.mozilla.org/mozilla-
 central/source/toolkit/components/contentprefs/ContentPrefService2.jsm
 ContentPrefService2.jsm], at least in ESR31. In this new implementation,
 it looks like PB mode will also use an in-memory store, provided we make
 the same possibly dangerous assumption that loading contexts will always
 have "usePrivateBrowsing" set to true.

 So my question is: should we drop the #3229 patch (assuming Firefox gets
 the loading contexts right), or should we be extra defensive and write a
 similar patch to apply to ContentPrefService2.jsm? Perhaps Mike has some
 insight here.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13012>
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