[tor-bugs] #33283 [Applications/rbm]: Add caching for the exec function in rbm

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#33283: Add caching for the exec function in rbm
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     Reporter:  boklm             |      Owner:  boklm
         Type:  task              |     Status:  new
     Priority:  Medium            |  Milestone:
    Component:  Applications/rbm  |    Version:
     Severity:  Normal            |   Keywords:  tbb-rbm
Actual Points:                    |  Parent ID:
       Points:                    |   Reviewer:
      Sponsor:                    |
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 In a mail to tbb-dev, dcf mentioned that while building tor-browser, a lot
 of time is spent in rbm:
 https://lists.torproject.org/pipermail/tbb-dev/2020-February/001045.html

 I think an easy way to improve this is by caching the result from the
 `exec()` template function. We use it for example with the line `version:
 '[% c("abbrev") %]'` which runs something like `exec("git log -1
 --format=%h")`, which then runs a `git checkout` before running the `git
 log` command. The `version` value is used in filenames, and build_id,
 which are then used in a lot of places, but with no caching we end up
 doing a lot of calls the same `git checkout` and `git log` commands. The
 switch to `pion` which added a lot of go projects each with their git
 repository probably made this issue more visible.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/33283>
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