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.
Kaniko uses mtime (as well as file contents and other attributes) to
determine if files have changed. COPY and ADD commands should _always_
update the mtime, because they actually overwrite the files. However it
turns out that the mtime can lag, so kaniko would sometimes add a new
layer when using COPY or ADD on a file, and sometimes would not. This
leads to a non-deterministic number of layers.
To fix this, we have updated the kaniko commands to be more
authoritative in declaring when they have changed a file (e.g. WORKDIR
will now only create the directory when it doesn't exist) and we will
trust those files and _always_ add them, instead of only adding them if
they haven't changed.
It is possible for RUN commands to also change the filesystem, in which
case kaniko has no choice but to look at the filesystem to determine
what has changed. For this case we have added a call to `sync` however
we still cannot guarantee that sometimes the mtime will not lag, causing the
number of layers to be non-deterministic. However when I tried to cause
this behaviour with the RUN command, I couldn't.
This changes the snapshotting logic a bit; before this change, the last
command of the last stage in a Dockerfile would always scan the whole
file system and ignore the files returned by the kaniko command. Instead
we will now trust those files and assume that the snapshotting
performed by previous commands will be adequate.
Docker itself seems to rely on the storage driver to determine when
files have changed and so doesn't have to deal with these problems
directly.
An alternative implementation would use `inotify` to track which files
have changed. However that would mean watching every file in the
filesystem, and adding new watches as files are added. Not only is there
a limit on the number of files that can be watched, but according to the
man pages a) this can take a significant amount of time b) there is
complication around when events arrive (e.g. by the time they arrive,
the files may have changed) and lastly c) events can be lost, which
would mean we'd run into this non-deterministic behaviour again anyway.
Fixes#251
In this refactor I:
1. Created KanikoOptions to make it easier to pass around arguments
passed in through the command line
2. Reorganized executor.go by putting the logic for pushing the image in
a new file push.go
3. Made some error messages clearer
4. Fixed a mistake in the README for pushing to AWS
5. Marked the --bucket flag as hidden since we want people to use
--context instead, and marked an aws flag as hidden which is set in a
vendored directorya
Priya Wadhwa
Christie Wilson