[tbb-bugs] #26050 [Applications/Tor Browser]: achieve update "watershed" for ESR60-based Tor Browser

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#26050: achieve update "watershed" for ESR60-based Tor Browser
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     Reporter:  mcs                       |      Owner:  tbb-team
         Type:  defect                    |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:  ff60-esr
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 Once Tor Browser is based on the ESR60 code, because of the switch to xz
 compression and SHA384-based signing for the MAR files, we will need to
 have an update "watershed" event. The purpose of this ticket is to track
 the things we need to do to make the watershed happen.

 Let's suppose that for our stable channel the first release that uses the
 new MAR format is 8.0. This means:
 * When an older version of Tor Browser (< 8.0) requests an update, it must
 receive an old format MAR file that will update it to 8.0. We must keep
 these MAR files around as long as we care about updating older browsers
 (probably a long time).
 * Newer versions of Tor Browser (>= 8.0) must receive new format MAR files
 (of course).

 Of course we will need to do this sooner for our alpha channel.

 My guess is that it will make things easier if we use a new location for
 the new update requests, e.g., from
 https://aus1.torproject.org/torbrowser/update_4.

 One useful thing to know is that setting `MAR_OLD_FORMAT=1` in the
 environment before running the `make_full_update.sh` or
 `make_incremental_update.sh` script causes the old bzip2 compression to be
 used when creating the MAR file.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26050>
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