How to use docker to test multiple compiler versions - c++

Question

What is the idiomatic way to write a docker file for building against many different versions of the same compiler?
I have a project which tests against a wide-range of versions of different compilers like gcc and clang as part of a CI job. At some point, the agents for the CI tasks were updated/changed, resulting in newer jobs failing -- and so I've started looking into dockerizing these builds to try to guarantee better reliability and stability.
However, I'm having some difficulty understanding what a proper and idiomatic approach is to producing build images like this without causing a large amount of duplication caused by layers.
For example, let's say I want to build using the following toolset:
gcc 4.8, 4.9, 5.1, ... (various versions)
cmake (latest)
ninja-build
I could write something like:
# syntax=docker/dockerfile:1.3-labs
# Parameterizing here possible, but would cause bloat from duplicated
# layers defined after this
FROM gcc:4.8
ENV DEBIAN_FRONTEND noninteractive
# Set the work directory
WORKDIR /home/dev
COPY . /home/dev/
# Install tools (cmake, ninja, etc)
# this will cause bloat if the FROM layer changes
RUN <<EOF
apt update
apt install -y cmake ninja-build
rm -rf /var/lib/apt/lists/*
EOF
# Default command is to use CMak
CMD ["cmake"]
However, the installation of tools like ninja-build and cmake occur after the base image, which changes per compiler version. Since these layers are built off of a different parent layer, this would (as far as I'm aware) result in layer duplication for each different compiler version that is used.
One alternative to avoid this duplication could hypothetically be using a smaller base image like alpine with separate installations of the compiler instead. The tools could be installed first so the layers remain shared, and only the compiler changes as the last layer -- however this presents its own difficulties, since it's often the case that certain compiler versions may require custom steps, such as installing certain keyrings.
What is the idiomatic way of accomplishing this? Would this typically be done through multiple docker files, or a single docker file with parameters? Any examples would be greatly appreciated.

Answer 1

I would separate the parts of preparing the compiler and doing the calculation, so the source doesn't become part of the docker container.
Prepare Compiler
For preparing the compiler I would take the ARG approach but without copying the data into the container. In case you wanna fast retry while having enough resources you could spin up multiple instances the same time.
ARG COMPILER=gcc:4.8
FROM ${COMPILER}
ENV DEBIAN_FRONTEND noninteractive
# Install tools (cmake, ninja, etc)
# this will cause bloat if the FROM layer changes
RUN <<EOF
apt update
apt install -y cmake ninja-build
rm -rf /var/lib/apt/lists/*
EOF
# Set the work directory
VOLUME /src
WORKDIR /src
CMD ["cmake"]
Build it
Here you have few options. You could either prepare a volume with the sources or use bind mounts together with docker exec like this:
#bash style
for compiler in gcc:4.9 gcc:4.8 gcc:5.1
do
docker build -t mytag-${compiler} --build-arg COMPILER=${compiler} .
# place to clean the target folder
docker run -v $(pwd)/src:/src mytag-${compiler}
done
And because the source is not part of the docker image you don't have bloat. You can also have two mounts, one for a readonly source tree and one for the output files.
Note: If you remove the CMake command you could also spin up the docker containers in parallel and use docker exec to start the build. The downside of this is that you have to take care of out of source builds to avoid clashes on the output folder.

Answer 2

put an ARG before the FROM and then invoke the ARG as the FROM
so:
ARG COMPILER=gcc:4.8
FROM ${COMPILER}
# rest goes here
then you
docker build . -t test/clang-8 --build-args COMPILER=clang-8
or similar.
If you want to automate just make a list of compilers and a bash script looping over the lines in your file, and paste the lines as inputs to the tag and COMPILER build args.
As for Cmake, I'd just do:
RUN wget -qO- "https://cmake.org/files/v3.23/cmake-3.23.1-linux-"$(uname -m)".tar.gz" | tar --strip-components=1 -xz -C /usr/local
When copying, I find it cleaner to do
WORKDIR /app/build
COPY . .
edit: formatting

Answer 3

As far as I know, there is no way to do that easily and safely. You could use a RUN --mount=type=cache, but the documentation clearly says that:
Contents of the cache directories persist between builder invocations without invalidating the instruction cache. Cache mounts should only be used for better performance. Your build should work with any contents of the cache directory as another build may overwrite the files or GC may clean it if more storage space is needed.
I have not tried it but I guess the layers are duplicated anyway, you just save time, assuming the cache is not emptied.
The other possible solution you have is similar to the one you mention in the question: starting with the tools installation and then customizing it with the gcc image. Instead of starting with an alpine image, you could start FROM scratch. scratch is basically the empty image, you could COPY the files generated by
RUN <<EOF
apt update
apt install -y cmake ninja-build
rm -rf /var/lib/apt/lists/*
EOF
Then you COPY the entire gcc filesystem. However, I am not sure it will work because the order of the initial layers is now reversed. This means that some files that were in the upper layer (coming from tools) now are in the lower layer and could be overwritten. In the comments, I asked you for a working Dockerfile because I wanted to try this out before answering. If you want, you can try this method and let us know. Anyway, the first step is extracting the files created from the tools layer.
How to extract changes from a layer?
Let's consider this Dockerfile and build it with docker build -t test .:
FROM debian:10
RUN apt update && apt install -y cmake && ( echo "test" > test.txt )
RUN echo "new test" > test.txt
Now that we have built the test image, we should find 3 new layers. You mainly have 2 ways to extract the changes from each layer:
the first is docker inspecting the image and then find the ids of the layers in the /var/lib/docker folder, assuming you are on Linux. Each layer has a diff subfolder containing the changes. Actually, I think it is more complex than this, that is why I would opt for...
skopeo: you can install it with apt install skopeo and it is a very useful tool to operate on docker images. The command you are interested in is copy, that extracts the layers of an image and export them as .tar:
skopeo copy docker-daemon:{image_name}:latest "dir:/home/test_img"
where image_name is test in this case.
Extracting layer content with Skopeo
In the specified folder, you should find some tar files and a configuration file (look at the skopeo copy command output and you will know which one is that). Then extract each {layer}.tar in a different folder and you are done.
Note: to find the layer containing your tools just open the configuration file (maybe using jq because it is json) and take the diff_id that corresponds to the RUN instruction you find in the history property. You should understand it once you open the JSON configuration. This is unnecessary if you have a small image that has, for example, debian as parent image and a single RUN instruction containing the tools you want to install.
Get GCC image content
Now that we have the tool layer content, we need to extract the gcc filesystem. we don't need skopeo for this one, but docker export is enough:
create a container from gcc (with the tag you need):
docker create --name gcc4.8 gcc:4.8
export it as tar:
docker export -o gcc4.8.tar gcc4.8
finally extract the tar file.
Putting all together
The final Dockerfile could be something like:
FROM scratch
COPY ./tools_layer/ /
COPY ./gcc_4.x/ /
In this way, the tools layer is always reused (unless you change the content of that folder, of course), but you can parameterize the gcc_4.x with the ARG instruction for example.
Read carefully: all of this is not tested but you might encounter 2 issues:
the gcc image overwrites some files you have changed in the tools layer. You could check if this happens by computing the diff between the gcc layer folder and the tools layer folder. If it happens, you can only keep track of that file/s and add it/them in the dockerfile after the COPY ./gcc ... with another COPY.
When in the upper layer a file is removed, docker marks that file with a .wh extension (not sure if it is different with skopeo). If in the tools layer you delete a file that exists in the gcc layer, then that file will not be deleted using the above Dockerfile (the COPY ./gcc ... instruction would overwrite the .wh). In this case too, you would need to add an additional RUN rm ... instruction.
This is probably not the correct approach if you have a more complex image that the one you are showing us. In my opinion, you could give this a try and just see if this works out with a single Dockerfile. Obviously, if you have many compilers, each one having its own tools set, the maintainability of this approach could be a real burden. Instead, if the Dockerfile is more or less linear for all the compilers, this might be good (after all, you do not do this every day).
Now the question is: is avoiding layer replication so important that you are willing to complicate the image-building process this much?