97 lines
3.7 KiB
Markdown
97 lines
3.7 KiB
Markdown
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Notes about Bazel Builds
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========================
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Skia cannot be built with Bazel yet.
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But you should be able to build and run the trivial `tools/bazel_test.cc`:
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$ bazel test ...
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Dependencies
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------------
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`WORKSPACE.bazel` acts like `DEPS`, listing external dependencies and how to
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fetch them. You can call `bazel sync`, or just let `bazel {build,test,run}`
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handle it as needed on its own. The easiest way to add a new dependency is to
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start using `tag="..."` or `branch="..."` and then follow the advice of Bazel
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to pin that to the `commit` and `shallow_since` it suggests.
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We must provide Bazel build configuration for dependencies like `libpng` that
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don't provide their own. For `libpng` that's `bazel/libpng.bazel`, linked by
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the `new_git_repository()` `build_file` argument, written relative to that
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fetched Git repo's root. Its resemblance to `third_party/libpng/BUILD.gn` is
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no coincidence... it's pretty much a 1:1 translation between GN and Bazel.
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Everything that's checked in builds external dependencies from source. I've
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not written an integrated system for substituting prebuilt versions of these
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dependencies (e.g. `/usr/include/png.h` and `/usr/lib/libpng.so`), instead
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leaving that up to users who want it. The process is not exactly trivial, but
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closer to tedious than difficult. Here's an example, overriding `libpng` to
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point to prebuilts from Homebrew in ~/brew:
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Each overridden dependency will need its own directory with a few files.
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$ find overrides
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overrides
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overrides/libpng
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overrides/libpng/include
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overrides/libpng/WORKSPACE.bazel
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overrides/libpng/BUILD.bazel
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`WORKSPACE.bazel` must be present, but in this case can be empty.
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$ cat overrides/libpng/WORKSPACE.bazel
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`BUILD.bazel` is where it all happens:
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$ cat overrides/libpng/BUILD.bazel
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cc_library(
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name = "libpng",
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hdrs = ["include/png.h"],
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srcs = ["include/pngconf.h", "include/pnglibconf.h"],
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includes = ["include"],
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linkopts = ["-lpng", "-L/Users/mtklein/brew/lib"],
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visibility = ["//visibility:public"],
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)
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`include` is a symlink I've made to `~/brew/include` because Bazel doesn't like
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absolute paths in `hdrs` or `includes`. On the other hand, a symlink to
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`~/brew/lib` doesn't work here, though `-L/Users/mtklein/brew/lib` works fine.
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$ readlink overrides/libpng/include
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/Users/mtklein/brew/include/
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Finally, we point Bazel at all that using `--override_repository`:
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$ bazel test ... --override_repository libpng=/Users/mtklein/overrides/libpng
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I expect building from source to be the most common use case, and it's more or
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less enough to simply know that we can substitute prebuilts this way. The most
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interesting part to me is that we don't need to provide this mechanism... it's
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all there in stock Bazel. This plan may all want some rethinking in the future
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if we want to add the option to trim the dependency entirely and make this
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tristate (build it, use it prebuilt, or trim).
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.bazelrc
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--------
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I have not (yet?) checked in a .bazelrc to the Skia repo, but have found it
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handy to write my own in ~/.bazelrc:
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$ cat ~/.bazelrc
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# Print more information on failures.
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build --verbose_failures
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test --test_output errors
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# Create an ASAN config, try `bazel test --config asan ...`.
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build:asan --copt -fsanitize=address
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build:asan --copt -Wno-macro-redefined # (_FORTIFY_SOURCE redefined.)
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build:asan --linkopt -fsanitize=address
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# Flip on and off prebuilt overrides easily.
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build --override_repository libpng=/Users/mtklein/overrides/libpng
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I'm impressed by how much you can configure via bazelrc, and I think this
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should let our Bazel build configuration stay mostly focused on the structure
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of the project, less cluttered by build settings.
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