[MacPorts] #54800: Cache built dependency packages on Travis CI to speed up testing flow

[MacPorts]

#54800: Cache built dependency packages on Travis CI to speed up testing flow
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  Reporter:  yan12125        |      Owner:  admin@…
      Type:  enhancement     |     Status:  new
  Priority:  Normal          |  Milestone:
 Component:  server/hosting  |    Version:
Resolution:                  |   Keywords:
      Port:                  |
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Comment (by l2dy):

 1. `$PREFIX` is not writable by the user (https://docs.travis-
 ci.com/user/caching/#caches-and-read-permissions). But this is easy to
 workaround by moving files over and fixing permissions.
 2. In order to cache dependencies across PRs, we have to enable builds on
 the master branch. To avoid timeouts, we can only build ports changed in
 the last commit pushed. (When many unrelated commits are pushed at once,
 only one Travis build will be triggered.)
 3. We should not cache distributable packages and purge outdated cached
 packages to avoid making the cache too big. Extracting the cache (and
 maybe generating it) costs precious build time.

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Ticket URL: <https://trac.macports.org/ticket/54800#comment:3>
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