[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3601: CI builds new images for test jobs even when Tahoe's dependencies haven't changed

[Tahoe-LAFS]

#3601: CI builds new images for test jobs even when Tahoe's dependencies haven't
changed
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 Reporter:  exarkun  |          Owner:
     Type:  defect   |         Status:  new
 Priority:  normal   |      Milestone:  undecided
Component:  unknown  |        Version:  n/a
 Keywords:           |  Launchpad Bug:
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 Currently on CircleCI we use pre-built Docker images populated with our
 dependencies in order to make build results more easily reproducible and
 to speed up individual test jobs.

 The Docker images are built and replaced nightly:

 https://github.com/tahoe-lafs/tahoe-
 lafs/blob/94716c078a5c9cbaf468b88206d911146ba6091e/.circleci/config.yml#L97-L129

 There are some downsides to replacing the images this way:

 * Often Tahoe's dependencies have not changed.  The introduction of a new
 image only allows for outside factors to lead to changes in behavior.
 This reduces reproducibility since it means the same Tahoe source tested
 on two different days might encounter two different environments and end
 up with two different results.
 * It's wasteful to build images when nothing has changed.  It's wasteful
 of CircleCI's resources (which we currently consume for free).  It's
 wasteful of our time since re-built images are less likely to be cached on
 build hosts and need to be downloaded which takes time.

 For a set of "stable" CI jobs which should only fail if Tahoe introduces a
 regression, not if the outside world changes in a way that breaks Tahoe,
 the ideal would be to only rebuild the images when Tahoe's dependencies
 change, only allow the new image to become the canonical image if the
 Tahoe works on the image, and only allow Tahoe dependencies to be resolved
 during image creation, not during test execution.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3601>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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