Kevin Lubick fed97e8f40 [bazel] Add RBE support using hermetic Linux Clang toolchain

A new RBE worker-pool called gce_linux was created in
conjunction with this CL. See
https://docs.google.com/document/d/14xMZCKews69SSTfULhE8HDUzT5XvPwZ4CvRufEvcZ74/edit#
for some details on that.

Note: everything under bazel/rbe/gce_linux was autogenerated
and can be ignored from manual review. It basically specifies
what files are on the RBE image that are necessary for running
Bazel.

Testing it out can be done by authenticating for RBE
gcloud auth application-default login --no-browser

Then, run make -C bazel rbe_known_good_builds
to test it out.

On my 4 core laptop with an empty local cache, but a
warm remote cache, the build took <2 min instead of the
10+ minutes it would have [1].

The folder structure in //bazel/rbe is meant to let us
have multiple remote configurations there, e.g.
//bazel/rbe/gce_windows.

Suggested Review Order:
- bazel/rbe/README.md
- bazel/rbe/gce_linux_container/Dockerfile to see the
bare-bones RBE image.
- bazel/rbe/BUILD.bazel to see a custom platform defined.
It is nearly identical to the autogenerated one
in bazel/rbe/gce_linux/config/BUILD, with one extra
field to force the gce_linux pool to be used.
- .bazelrc to see the settings needed to make
--config=linux-rbe work. The naming convention was
inspired by SkCMS's setup [2], and allows us to have
some common RBE settings (i.e. config:remote) and
some specialized ones for the given host machine
(e.g. config:linux-rbe) A very important, but subtle
configuration, is on line 86 of .bazelrc where we say
to use our hermetic toolchain and not whatever C++
compiler and headers are on the host machine (aka
the RBE container).
- toolchain/build_toolchain.bzl to see some additional
dependencies needed in the toolchain (to run IWYU) which
I had installed locally but didn't realize were important.
- third_party/BUILD.bazel to see an example of how failing
to specify all files can result in something that works
locally, but fails remotely.
--execution_log_json_file=/tmp/execlog.json helped debug
these issues.
- All other files.

[1] http://go/scrcast/NjM1ODE4MDI0NzM3MTc3Nnw3ODViZmFkMi1iOA
[2] https://skia.googlesource.com/skcms/+/30c8e303800c256febb03a09fdcda7f75d119b1b/.bazelrc#20

Change-Id: Ia0a9e6a06c1a13071949ab402dc5d897df6b12e1
Bug: skia:12541
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/524359
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>

2022-03-28 13:56:16 +00:00

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RBE configurations

Some subdirectories of this folder are generated. For example, gce_linux was generated by running make generate_linux_config. Those generated files describe the C++ and Java toolchain that are in the RBE Docker image; these toolchains are required to run Bazel, but are not the toolchains that we use to compile our code.

We build our own, bare-bones, Docker image to use on RBE. We intend to use a hermetic toolchain (see //toolchain) that specifies everything necessary to compile and link Skia. Use of the hermetic toolchain on and off RBE makes the build reproducible and consistent across machines, and not require internet access (assuming the toolchain has been cached at least once). This setup has the desirable property of not needing to change and upload RBE Docker images if we need to change a small detail of our toolchain.

The only requirement we have of our Docker image (beyond the minimum requirements to run Bazel) are that it have sufficient runtime libraries to run our toolchain. For example, this means that the Linux RBE image has at least glibc 2.32, which is the current minimum requirement of the Linux binaries in our toolchain. This is the same requirement of any developer who tries to build Skia using Bazel locally.

Getting rbe_configs_gen

It is suggested to download a prebuilt binary from GitHub and put that onto your PATH.

Creating/Updating the RBE image

In accordance with SLSA level 1, we want to be able to have a scripted way of building our image and specify exactly what artifacts are in it. To accommodate this, we specify the exact sha256 hash of the base Docker image we use and the exact versions of the packages we install on top of that. If we need to add a package or update things, it is best build the image without these qualifiers to see what was actually used, and then respecify them so if someone were to build the docker image again, they are likely to get the same image.

This process is:

Modify the appropriate Dockerfile (e.g. gce_linux_container/Dockerfile) to not have the version or hash qualifiers. Also increment the appropriate VERSION variable in Makefile.
Add any new packages or make any changes.
Run make build_linux_container to build the image locally. One may verify it works by running something like docker run -it gcr.io/skia-public/rbe_linux:v2 /bin/bash.
Note the versions and base image hash that were used. Modify the Dockerfile to use these.
Run make push_linux_container to rebuild the container and push it to GCS where it can be used by our RBE workers. Note the sha256 hash of this created container
Modify the appropriate generate step in Makefile (e.g. generate_linux_config) to refer to the correct toolchain_container. Then, run that step.
Modify the RBE platform in ./BUILD.bazel to refer to the new container_image.

We chose not to use Bazel rules for this container step, as that could be difficult to bootstrap without Bazel already setup. Additionally, Make is a simple and sufficient way to script the steps for SLSA purposes.

Defining our own Bazel RBE platforms

While the generated files do have a platform we can use (e.g. //bazel/rbe/gce_linux/config:platform), we do not use it because we cannot easily customize it without a risk that the changes will be lost when we update the image. Thankfully, we can specify our own platforms, which we do in ./BUILD.bazel.

More details

https://docs.google.com/document/d/14xMZCKews69SSTfULhE8HDUzT5XvPwZ4CvRufEvcZ74/edit

RBE Metrics

http://go/skia-rbe-metrics

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