skia2/site/dev/contrib/bazel.md

Notes about Bazel Builds

Skia cannot be built with Bazel yet.

But you should be able to build and run the trivial tools/bazel_test.cc:

$ bazel test ...

Dependencies

WORKSPACE.bazel acts like DEPS, listing external dependencies and how to fetch them. You can call bazel sync, or just let bazel {build,test,run} handle it as needed on its own. The easiest way to add a new dependency is to start using tag="..." or branch="..." and then follow the advice of Bazel to pin that to the commit and shallow_since it suggests.

We must provide Bazel build configuration for dependencies like libpng that don't provide their own. For libpng that's bazel/libpng.bazel, linked by the new_git_repository() build_file argument, written relative to that fetched Git repo's root. Its resemblance to third_party/libpng/BUILD.gn is no coincidence... it's pretty much a 1:1 translation between GN and Bazel.

Everything that's checked in builds external dependencies from source. I've not written an integrated system for substituting prebuilt versions of these dependencies (e.g. /usr/include/png.h and /usr/lib/libpng.so), instead leaving that up to users who want it. The process is not exactly trivial, but closer to tedious than difficult. Here's an example, overriding libpng to point to prebuilts from Homebrew in ~/brew:

Each overridden dependency will need its own directory with a few files.

$ find overrides
overrides
overrides/libpng
overrides/libpng/include
overrides/libpng/WORKSPACE.bazel
overrides/libpng/BUILD.bazel

WORKSPACE.bazel must be present, but in this case can be empty.

$ cat overrides/libpng/WORKSPACE.bazel

BUILD.bazel is where it all happens:

$ cat overrides/libpng/BUILD.bazel
cc_library(
    name = "libpng",
    hdrs = ["include/png.h"],
    srcs = ["include/pngconf.h", "include/pnglibconf.h"],
    includes = ["include"],
    linkopts = ["-lpng", "-L/Users/mtklein/brew/lib"],
    visibility = ["//visibility:public"],
)

include is a symlink I've made to ~/brew/include because Bazel doesn't like absolute paths in hdrs or includes. On the other hand, a symlink to ~/brew/lib doesn't work here, though -L/Users/mtklein/brew/lib works fine.

$ readlink overrides/libpng/include
/Users/mtklein/brew/include/

Finally, we point Bazel at all that using --override_repository:

$ bazel test ... --override_repository libpng=/Users/mtklein/overrides/libpng

I expect building from source to be the most common use case, and it's more or less enough to simply know that we can substitute prebuilts this way. The most interesting part to me is that we don't need to provide this mechanism... it's all there in stock Bazel. This plan may all want some rethinking in the future if we want to add the option to trim the dependency entirely and make this tristate (build it, use it prebuilt, or trim).

.bazelrc

I have not (yet?) checked in a .bazelrc to the Skia repo, but have found it handy to write my own in ~/.bazelrc:

$ cat ~/.bazelrc
# Print more information on failures.
build --verbose_failures
test --test_output errors

# Create an ASAN config, try `bazel test --config asan ...`.
build:asan --copt -fsanitize=address
build:asan --copt -Wno-macro-redefined   # (_FORTIFY_SOURCE redefined.)
build:asan --linkopt -fsanitize=address

# Flip on and off prebuilt overrides easily.
build --override_repository libpng=/Users/mtklein/overrides/libpng

I'm impressed by how much you can configure via bazelrc, and I think this should let our Bazel build configuration stay mostly focused on the structure of the project, less cluttered by build settings.