[tor-bugs] #21090 [Applications/Tor Browser Sandbox]: Integrate the update check/download/apply steps with torbutton.

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#21090: Integrate the update check/download/apply steps with torbutton.
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     Reporter:  yawning                           |      Owner:  yawning
         Type:  enhancement                       |     Status:  new
     Priority:  Medium                            |  Milestone:
    Component:  Applications/Tor Browser Sandbox  |    Version:
     Severity:  Normal                            |   Keywords:
Actual Points:                                    |  Parent ID:
       Points:                                    |   Reviewer:
      Sponsor:                                    |
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 Followup from #20778.  Instead of hiding/disabling the UI, torbutton
 should detect the presence of a sandbox, and integrate the UI with the
 sandbox's update mechanism.

 My current initial plan is to do something like:
  * Introduce a new env var `TBB_SANDBOX` that my implementation will set
 to `linux-v1`.
  * Modify the `sandboxed-tor-browser` control port surrogate to add:
    *`_TBB_CHECKUPDATE`, a command which will trigger (or display if
 already in progress) the sandboxed-tor-browser update check.
    * `_TBB_APPLYUPDATE`, a command which will inform the sandbox that
 firefox will exit imminently due to user requesting the pending update to
 apply.
    * `_TBB_UPDATE`, an event that will fire if there is a browser update
 pending, and optionally inform the browser that the user requested the
 update be applied via the sandbox's notification mechanism.
  * Modify torbutton to (all actions assuming the new env var is set
 appropriately):
    * Send the `_TBB_CHECKUPDATE` command from torbutton_check_for_update()
 instead of calling into the prompter.
    * Register for the `_TBB_UPDATE` event, and set
 extensions.torbutton.updateNeeded to true if it fires (or restart the
 browser).

 nb: Since the new command and event are entirely synthetic and never hit
 the actual tor daemon, I'm not going to bother with a control-spec.txt
 patch (and since the surrogate doesn't allow events/names as a `GETINFO`
 query, the right way to check if this behavior should happen is "is the
 env var set to what I said I will set it to".

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21090>
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