Using ccache in automated builds on Docker cloud - docker

Question

I am using automated builds on Docker cloud to compile a C++ app and provide it in an image.
Compilation is quite long (range 2-3 hours) and commits on github are frequent (~10 to 30 per day).
Is there a way to keep the building cache (using ccache) somehow?
As far as I understand it, docker caching is useless since the compilation layer producing the ccache will not be used due to the source code changes.
Or can we tweak to bring some data back to first layer?
Any other solution? Pushing it somewhere?
Here is the Dockerfile:
# CACHE_TAG is provided by Docker cloud
# see https://docs.docker.com/docker-cloud/builds/advanced/
# using ARG in FROM requires min v17.05.0-ce
ARG CACHE_TAG=latest
FROM qgis/qgis3-build-deps:${CACHE_TAG}
MAINTAINER Denis Rouzaud <denis.rouzaud#gmail.com>
ENV CC=/usr/lib/ccache/clang
ENV CXX=/usr/lib/ccache/clang++
ENV QT_SELECT=5
COPY . /usr/src/QGIS
WORKDIR /usr/src/QGIS/build
RUN cmake \
-GNinja \
-DCMAKE_INSTALL_PREFIX=/usr \
-DBINDINGS_GLOBAL_INSTALL=ON \
-DWITH_STAGED_PLUGINS=ON \
-DWITH_GRASS=ON \
-DSUPPRESS_QT_WARNINGS=ON \
-DENABLE_TESTS=OFF \
-DWITH_QSPATIALITE=ON \
-DWITH_QWTPOLAR=OFF \
-DWITH_APIDOC=OFF \
-DWITH_ASTYLE=OFF \
-DWITH_DESKTOP=ON \
-DWITH_BINDINGS=ON \
-DDISABLE_DEPRECATED=ON \
.. \
&& ninja install \
&& rm -rf /usr/src/QGIS
WORKDIR /

Answer 1

You should try saving and restoring your cache data from a third party service:
- an online object storage like Amazon S3
- a simple FTP server
- an Internet available machine with ssh to make a scp
I'm assuming that your cache data is stored inside the ´~/.ccache´ directory
Using Docker multistage build
From some time, Docker supports Multi-stage builds and you can try using it to implement the solution with a single Dockerfile:
Warning: I've not tested it
# STAGE 1 - YOUR ORIGINAL DOCKER FILE CUSTOMIZED
# CACHE_TAG is provided by Docker cloud
# see https://docs.docker.com/docker-cloud/builds/advanced/
# using ARG in FROM requires min v17.05.0-ce
ARG CACHE_TAG=latest
FROM qgis/qgis3-build-deps:${CACHE_TAG} as builder
MAINTAINER Denis Rouzaud <denis.rouzaud#gmail.com>
ENV CC=/usr/lib/ccache/clang
ENV CXX=/usr/lib/ccache/clang++
ENV QT_SELECT=5
COPY . /usr/src/QGIS
WORKDIR /usr/src/QGIS/build
# restore cache
RUN curl -o ccache.tar.bz2 http://my-object-storage/ccache.tar.bz2
RUN tar -xjvf ccache.tar.bz2
COPY --from=downloader /.ccache ~/.ccache
RUN cmake \
-GNinja \
-DCMAKE_INSTALL_PREFIX=/usr \
-DBINDINGS_GLOBAL_INSTALL=ON \
-DWITH_STAGED_PLUGINS=ON \
-DWITH_GRASS=ON \
-DSUPPRESS_QT_WARNINGS=ON \
-DENABLE_TESTS=OFF \
-DWITH_QSPATIALITE=ON \
-DWITH_QWTPOLAR=OFF \
-DWITH_APIDOC=OFF \
-DWITH_ASTYLE=OFF \
-DWITH_DESKTOP=ON \
-DWITH_BINDINGS=ON \
-DDISABLE_DEPRECATED=ON \
.. \
&& ninja install
# save the current cache online
WORKDIR ~/
RUN tar -cvjSf ccache.tar.bz2 .ccache
RUN curl -T ccache.tar.bz2 -X PUT http://my-object-storage/ccache.tar.bz2
# STAGE 2
FROM alpine:latest
# YOUR CUSTOM LOGIC TO CREATE THE FINAL IMAGE WITH ONLY REQUIRED BINARIES
# USE THE FROM IMAGE YOU NEED, this is only an example
# E.g.:
# COPY --from=builder /usr/src/QGIS/build/YOUR_EXECUTABLE /usr/bin
# ...
In the stage 2 you will build the final image that will be pushed to your repository.
 Using Docker cloud hooks
Another, but less clear, approach could be using a Docker Cloud pre_build hook file to download cache data:
#!/bin/bash
echo "=> Downloading build cache data"
curl -o ccache.tar.bz2 http://my-object-storage/ccache.tar.bz2 # e.g. Amazon S3 like service
cd /
tar -xjvf ccache.tar.bz2
Obviously you can use dedicate docker images to run curl or tar mounting the local directory as a volume in this script.
Then, copy the .ccache extracted folder inside your container during the build, using a COPY command before your cmake call:
WORKDIR /usr/src/QGIS/build
COPY /.ccache ~/.ccache
RUN cmake ...
In order to make this you should find a way to upload your cache data after the build and you could make this easily using a post_build hook file:
#!/bin/bash
echo "=> Uploading build cache data"
tar -cvjSf ccache.tar.bz2 ~/.ccache
curl -T ccache.tar.bz2 -X PUT http://my-object-storage/ccache.tar.bz2
But your compilation data aren't available from the outside, because they live inside the container. So you should upload the cache after the cmake command inside your main Dockerfile:
RUN cmake...
&& tar ...
&& curl ...
&& ninja ...
&& rm ...
If curl or tar aren't available, just add them to your container using the package manager (qgis/qgis3-build-deps is based on Ubuntu 16.04, so they should be available).