Currently, kaniko can only build a single image per container run, because the filesystem is full of the content of the first image.
When running kaniko in Jenkins, where we need to start the container "doing nothing" first (using the debug kaniko container), and then exec /kaniko/executor, this is a limitation because it means that if we want to build multiple images, we need to start multiple containers - see https://groups.google.com/forum/#!topic/kaniko-users/_7LivHdMdy0 for more details
A solution to fix this issue is to add a new flag to cleanup the filesystem at the end - the same way it is done between stages when building a multi-stages image. This way, the same (debug) container can be used to build multiple images.
To add layer caching to kaniko, I added two flags: --cache and
--use-cache.
If --use-cache is set, then the cache will be used, and if --cache is
specified then that repo will be used to store cached layers. If --cache
isn't set, a cache will be inferred from the destination provided.
Currently, caching only works for RUN commands. Before executing the
command, kaniko checks if the cached layer exists. If it does, it pulls
it and extracts it. It then adds those files to the snapshotter and
append a layer to the config history. If the cached layer does not exist, kaniko executes the command and
pushes the newly created layer to the cache.
All cached layers are tagged with a stable key, which is built based off
of:
1. The base image digest
2. The current state of the filesystem
3. The current command being run
4. The current config file (to account for metadata changes)
I also added two integration tests to make sure caching works
1. Dockerfile_test_cache runs 'date', which should be exactly the same
the second time the image is built
2. Dockerfile_test_cache_install makes sure apt-get install can be
reproduced
gometalinter is broken @ HEAD, and I looked into why that was. During
that process, I remembered that we took the linting scripts from
skaffold, and found that in skaffold gometalinter was replaced with
GolangCI-Lint:
https://github.com/GoogleContainerTools/skaffold/pull/619
The change made linting in skaffold faster, so I figured instead of
fixing gometalinter it made more sense to remove it and replace it with
GolangCI-Lint for kaniko as well.
I added a KanikoStage to hold each stage of the Dockerfile along with
information about each stage that would be useful later on.
The new KanikoStage type holds the stage itself, along with some
additional information:
1. FinalStage -- whether the current stage is the final stage
2. BaseImageStoredLocally/BaseImageIndex -- whether the base image for
this stage is stored locally, and if so what the index of the base image
is
3. SaveStage -- whether this stage needs to be saved for use in a future
stage
This is the first part of a larger refactor for building stages, which
will later make it easier to add layer caching.
Vincent Behar
Priya Wadhwa