[bazel] Sketch out changes for Mac toolchain

Change-Id: Idae84d8d9538012e5cfb75a1a477dbc72a4da5bc
Bug: skia:13125
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/526264
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>

.bazelrc

@@ -1,8 +1,13 @@
+# All of our clang builds should use the hermetic toolchain
 build:clang --crosstool_top=//toolchain:clang_suite
-build:clang --compiler=clang
 
-# Use the default Bazel C++ toolchain to build the tools used during the build.
-build:clang --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+# We don't have a great way to test the host system in order to use the right
+# version of the toolchain. So we need to have the user specify what their host is.
+build:clang_linux --config=clang
+build:clang_linux --compiler=host_is_linux_amd64
+
+build:clang_mac --config=clang
+build:clang_mac --compiler=host_is_mac_M1
 
 # =============================================================================
 # Alias to build configurations below. This makes configuring things from

WORKSPACE.bazel

@@ -1,7 +1,10 @@
 workspace(name = "skia")
 
+load("//toolchain:download_toolchains.bzl", "download_toolchains_for_skia")
+
+download_toolchains_for_skia("clang_linux_amd64", "clang_mac_m1")
+
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-load("//toolchain:build_toolchain.bzl", "build_cpp_toolchain")
 
 # See https://github.com/emscripten-core/emsdk/tree/85d27a4a2a60d591613a305b14ae438c2bb3ce11/bazel#setup-instructions
 http_archive(
@@ -24,11 +27,6 @@ load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
 
 emsdk_emscripten_deps(emscripten_version = "3.1.4")
 
-build_cpp_toolchain(
-    # Meant to run on amd64 linux and compile for amd64 linux.
-    name = "clang_linux_amd64",
-)
-
 http_archive(
     name = "bazel_skylib",
     sha256 = "c6966ec828da198c5d9adbaa94c05e3a1c7f21bd012a0b29ba8ddbccb2c93b0d",

toolchain/BUILD.bazel

@@ -1,23 +1,27 @@
 load("@rules_cc//cc:defs.bzl", "cc_toolchain", "cc_toolchain_suite")
-load(":clang_toolchain_config.bzl", "provide_clang_toolchain_config")
+load(":linux_amd64_toolchain_config.bzl", "provide_linux_amd64_toolchain_config")
 
 package(default_visibility = ["//visibility:public"])
 
-# https://docs.bazel.build/versions/main/be/c-cpp.html#cc_toolchain_suite
+# https://bazel.build/reference/be/c-cpp#cc_toolchain_suite
 cc_toolchain_suite(
     name = "clang_suite",
     toolchains = {
         # The key is target_cpu|compiler
-        "k8|clang": ":clang_toolchain",
-        "x86_64|clang": ":clang_toolchain",
-        "k8": ":clang_toolchain",
+        # compiler appears to be a string we can choose arbitrarily
+        # https://bazel.build/reference/command-line-reference?hl=en#flag--compiler
+        "k8|host_is_linux_amd64": ":linux_amd64_host",
+        "x86_64|host_is_linux_amd64": ":linux_amd64_host",
+        "k8": ":linux_amd64_host",
+        # TODO(jmbetancourt)
+        # "arm64|host_is_mac_M1": ":mac_M1_host",
     },
 )
 
 filegroup(name = "not_implemented")
 
 filegroup(
-    name = "all-toolchain-files",
+    name = "all_linux_amd64_files",
     srcs = [
         "IWYU_mapping.imp",
         "ar_trampoline.sh",
@@ -27,20 +31,20 @@ filegroup(
     ],
 )
 
-provide_clang_toolchain_config(
-    name = "clang_toolchain_config",
+provide_linux_amd64_toolchain_config(
+    name = "linux_amd64_toolchain_config",
 )
 
+# https://bazel.build/reference/be/c-cpp#cc_toolchain
 cc_toolchain(
-    name = "clang_toolchain",
-    all_files = ":all-toolchain-files",
-    ar_files = ":all-toolchain-files",
-    compiler_files = ":all-toolchain-files",
+    name = "linux_amd64_host",
+    all_files = ":all_linux_amd64_files",
+    ar_files = ":all_linux_amd64_files",
+    compiler_files = ":all_linux_amd64_files",
     dwp_files = ":not_implemented",
-    linker_files = ":all-toolchain-files",
+    linker_files = ":all_linux_amd64_files",
     objcopy_files = ":not_implemented",
     strip_files = ":not_implemented",
-    supports_param_files = 0,
-    toolchain_config = ":clang_toolchain_config",
-    toolchain_identifier = "clang-toolchain",
+    supports_param_files = False,
+    toolchain_config = ":linux_amd64_toolchain_config",
 )

toolchain/download_linux_amd64_toolchain.bzl

@@ -1,7 +1,16 @@
 """
-This file assembles a toolchain for Linux using the Clang Compiler glibc.
+This file assembles a toolchain for an amd64 Linux host using the Clang Compiler and glibc.
 
-It downloads the necessary header files and pre-compiled static/shared libraries to
+It downloads the necessary headers, executables, and pre-compiled static/shared libraries to
+the external subfolder of the Bazel cache (the same place third party deps are downloaded with
+http_archive or similar functions in WORKSPACE.bazel). These will be able to be used via our
+custom c++ toolchain configuration (see //toolchain/clang_toolchain_config.bzl)
+
+Most files are downloaded as .deb files from packages.debian.org (with us acting as the dependency
+resolver) and extracted to
+  [outputRoot (aka Bazel cache)]/[outputUserRoot]/[outputBase]/external/clang_linux_amd64
+  (See https://bazel.build/docs/output_directories#layout-diagram)
+which will act as our sysroot.
 """
 
 # From https://github.com/llvm/llvm-project/releases/download/llvmorg-13.0.0/clang+llvm-13.0.0-x86_64-linux-gnu-ubuntu-20.04.tar.xz.sha256
@@ -166,7 +175,7 @@ def _download_and_extract_deb(ctx, deb, sha256, prefix, output = ""):
     # Clean up
     ctx.delete("tmp")
 
-def _build_cpp_toolchain_impl(ctx):
+def _download_linux_amd64_toolchain_impl(ctx):
     # Workaround for Bazel not yet supporting .ar files
     # See https://skia-review.googlesource.com/c/buildbot/+/524764
     # https://bazel.build/rules/lib/repository_ctx#download
@@ -220,8 +229,10 @@ def _mirror(arr):
         return [arr[1]]
     return arr
 
-build_cpp_toolchain = repository_rule(
-    implementation = _build_cpp_toolchain_impl,
+# https://bazel.build/rules/repository_rules
+download_linux_amd64_toolchain = repository_rule(
+    implementation = _download_linux_amd64_toolchain_impl,
     attrs = {},
-    doc = "",
+    doc = "Downloads clang, and all supporting headers, executables, " +
+          "and shared libraries required to build Skia on a Linux amd64 host",
 )

toolchain/download_mac_m1_toolchain.bzl

@@ -0,0 +1,20 @@
+"""
+This file assembles a toolchain for a Mac M1 host using the Clang Compiler and glibc.
+
+It downloads the necessary headers, executables, and pre-compiled static/shared libraries to
+the external subfolder of the Bazel cache (the same place third party deps are downloaded with
+http_archive or similar functions in WORKSPACE.bazel). These will be able to be used via our
+custom c++ toolchain configuration (see //toolchain/clang_toolchain_config.bzl)
+"""
+
+def _download_mac_m1_toolchain(ctx):
+    # TODO(jmbetancourt)
+    pass
+
+# https://bazel.build/rules/repository_rules
+download_mac_m1_toolchain = repository_rule(
+    implementation = _download_mac_m1_toolchain,
+    attrs = {},
+    doc = "Downloads clang, and all supporting headers, executables, " +
+          "and shared libraries required to build Skia on a Mac M1 host",
+)

toolchain/download_toolchains.bzl

@@ -0,0 +1,29 @@
+"""
+This file exports the various toolchains for the hosts that we support building Skia on.
+
+Supported:
+ - Linux amd64
+
+Planned:
+ - Mac M1
+ - Windows amd64
+
+"""
+
+load(":download_linux_amd64_toolchain.bzl", "download_linux_amd64_toolchain")
+load(":download_mac_m1_toolchain.bzl", "download_mac_m1_toolchain")
+
+# This key in this dictionary (and thus the name passed into the rule) controls what the subfolder
+# will be called in the external directory. It must match what we use in the appropriate
+# toolchain_config.bzl file or it will not be able to locate the sysroot to build with.
+name_toolchain = {
+    "clang_linux_amd64": download_linux_amd64_toolchain,
+    "clang_mac_m1": download_mac_m1_toolchain,
+}
+
+def download_toolchains_for_skia(*args):
+    for toolchain_name in args:
+        if toolchain_name not in name_toolchain:
+            fail("unrecognized toolchain name " + toolchain_name)
+        download_toolchain = name_toolchain[toolchain_name]
+        download_toolchain(name = toolchain_name)

toolchain/linux_amd64_toolchain_config.bzl

@@ -26,7 +26,7 @@ load("@bazel_tools//tools/build_defs/cc:action_names.bzl", "ACTION_NAMES")
 # The location of the created clang toolchain.
 EXTERNAL_TOOLCHAIN = "external/clang_linux_amd64"
 
-def _clang_impl(ctx):
+def _linux_amd64_toolchain_info(ctx):
     action_configs = _make_action_configs()
     features = []
     features += _make_default_flags()
@@ -59,10 +59,10 @@ def _clang_impl(ctx):
         toolchain_identifier = "clang-toolchain",
     )
 
-provide_clang_toolchain_config = rule(
+provide_linux_amd64_toolchain_config = rule(
     attrs = {},
     provides = [CcToolchainConfigInfo],
-    implementation = _clang_impl,
+    implementation = _linux_amd64_toolchain_info,
 )
 
 def _make_action_configs():