| # Hermetic Android NDK C++ toolchain |
| |
| This directory defines a hermetic C++ toolchain suite to compile with the Android NDK. |
| |
| ## Motivation |
| |
| Bazel has a built-in |
| [`android_ndk_repository`](https://bazel.build/reference/be/android#android_ndk_repository) rule, |
| which generates C++ toolchains based on a local NDK installation provided via the |
| `ANDROID_NDK_HOME` environment variable. However, this rule breaks |
| [hermeticity](https://bazel.build/concepts/hermeticity), and requires the user to provide an NDK |
| installation. |
| |
| This directory provides a `download_android_ndk` repository rule, which downloads the Android NDK |
| under `external/ndk_linux_amd64`, and a C++ toolchain suite that targets 32- and 64-bit ARM. |
| |
| ## Design |
| |
| The C++ toolchain suite is based on the C++ toolchain generated by the `android_ndk_repository` |
| rule. |
| |
| Steps taken: |
| |
| - Build SkCMS with the `android_ndk_repository` rule at |
| [this revision](https://skia.googlesource.com/skcms/+/30c8e303800c256febb03a09fdcda7f75d119b1b/WORKSPACE#22). |
| - Inspect the contents of `bazel-skcms/external/androidndk/BUILD.bazel` and |
| `bazel-skcms/external/androidndk/cc_toolchain_config.bzl`, which are generated by said rule. |
| - Extract the useful parts into the C++ toolchain defined in this directory. |
| |
| ### Trampoline scripts |
| |
| The |
| [`cc_common.create_cc_toolchain_config_info`](https://bazel.build/rules/lib/cc_common#create_cc_toolchain_config_info) |
| function expects tool paths to point to files under the directory in which it is invoked. This |
| means we cannot directly reference tools under `external/ndk_linux_amd64`. The solution is to use |
| "trampoline" scripts that pass through any command-line arguments to the NDK binaries under |
| `external/android_sdk`. |
