Support exec platform different from host (#292)

Resolves #206.

Author: siddharthab

README.md

@@ -156,8 +156,9 @@ The following pairs have been tested to work for some hello-world binaries:
 A recommended approach would be to define two toolchains, one without sysroot
 for single-platform builds, and one with sysroot for cross-compilation builds.
 Then, when cross-compiling, explicitly specify the toolchain with the sysroot
-and the target platform. For example, see the [WORKSPACE](tests/WORKSPACE) file and
-the [test script](tests/scripts/run_xcompile_tests.sh) for cross-compilation.
+and the target platform. For example, see the [MODULE.bazel](tests/MODULE.bazel)
+file for `llvm_toolchain_with_sysroot` and the [test
+script](tests/scripts/run_xcompile_tests.sh) for cross-compilation.
 
 ```sh
 bazel build \
@@ -166,6 +167,25 @@ bazel build \
   //...
 ```
 
+### Multi-platform builds
+
+The toolchain supports multi-platform builds through the combination of the
+`exec_os`, `exec_arch` attribute pair, and either the `distribution` attribute,
+or the `urls` attribute. This allows one to run their builds on one platform
+(e.g. macOS) and their build actions to run on another (e.g. Linux), enabling
+remote build execution (RBE). For example, see the [MODULE.bazel](tests/MODULE.bazel)
+file for `llvm_toolchain_linux_exec` and the [test
+script](tests/scripts/run_docker_exec_test.sh) for running the build actions on
+Linux even if the build is being run from macOS.
+
+```sh
+bazel build \
+  --platforms=@toolchains_llvm//platforms:linux-x86_64 \
+  --extra_execution_platforms=@toolchains_llvm//platforms:linux-x86_64 \
+  --extra_toolchains=@llvm_toolchain_linux_exec//:cc-toolchain-x86_64-linux \
+  //...
+```
+
 ### Supporting New Target Platforms
 
 The following is a rough (untested) list of steps:

tests/MODULE.bazel

@@ -148,3 +148,17 @@ llvm.sysroot(
     label = "@@org_chromium_sysroot_linux_x64//:sysroot",
 )
 use_repo(llvm, "llvm_toolchain_with_sysroot")
+
+llvm.toolchain(
+    name = "llvm_toolchain_linux_exec",
+    llvm_version = "17.0.6",
+    # Option 1:
+    sha256 = {"": "884ee67d647d77e58740c1e645649e29ae9e8a6fe87c1376be0f3a30f3cc9ab3"},
+    strip_prefix = {"": "clang+llvm-17.0.6-x86_64-linux-gnu-ubuntu-22.04"},
+    urls = {"": ["https://github.com/llvm/llvm-project/releases/download/llvmorg-17.0.6/clang+llvm-17.0.6-x86_64-linux-gnu-ubuntu-22.04.tar.xz"]},
+    # Option 2:
+    # distribution = "clang+llvm-17.0.6-x86_64-linux-gnu-ubuntu-22.04.tar.xz",
+    exec_os = "linux",
+    exec_arch = "amd64",
+)
+use_repo(llvm, "llvm_toolchain_linux_exec")

tests/scripts/run_docker_exec_test.sh

@@ -0,0 +1,65 @@
+#!/bin/bash
+# Copyright 2024 The Bazel Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+set -euo pipefail
+
+os="$(uname -s)"
+if [[ ${os} != "Darwin" ]]; then
+  echo >&2 "Test, to be most effective, is meant to be run on Darwin."
+  exit 1
+fi
+
+if "${CI:-false}"; then
+  # macOS GitHub Action Runners do not have docker installed on them.
+  echo >&2 "Test can not be run on GitHub Actions"
+  exit 1
+fi
+
+scripts_dir="$(dirname "${BASH_SOURCE[0]}")"
+source "${scripts_dir}/bazel.sh"
+"${bazel}" version
+
+cd "${scripts_dir}"
+
+base_image="debian:stable-slim"
+binpath="$("${bazel}" info bazel-bin)/stdlib_test"
+
+docker build --platform=linux/amd64 --pull --tag=bazel-docker-sandbox - <<-EOF
+	FROM ${base_image}
+	ENV DEBIAN_FRONTEND=noninteractive
+	RUN apt-get -qq update && \
+		apt-get -qq -y install libtinfo5 zlib1g-dev libxml2
+EOF
+
+build_args=(
+  # Platforms
+  "--platforms=@toolchains_llvm//platforms:linux-x86_64"
+  "--extra_execution_platforms=@toolchains_llvm//platforms:linux-x86_64"
+  "--extra_toolchains=@llvm_toolchain_linux_exec//:cc-toolchain-x86_64-linux"
+  # Docker sandbox
+  "--experimental_enable_docker_sandbox"
+  "--experimental_docker_verbose"
+  "--experimental_docker_image=bazel-docker-sandbox"
+  "--spawn_strategy=docker"
+  # Verbosity of build actions
+  "--copt=-v"
+  "--linkopt=-v"
+  "--linkopt=-Wl,-v"
+)
+
+"${bazel}" --bazelrc=/dev/null build "${build_args[@]}" //:stdlib_test
+file "${binpath}" | tee /dev/stderr | grep -q ELF
+docker run --rm -it --platform=linux/amd64 \
+  --mount "type=bind,source=${binpath},target=/stdlib_test" "${base_image}" /stdlib_test

toolchain/internal/common.bzl

@@ -92,7 +92,7 @@ def os_version_arch(rctx):
     _os = os(rctx)
     _arch = arch(rctx)
 
-    if _os == "linux":
+    if _os == "linux" and not rctx.attr.exec_os:
         (distname, version) = _linux_dist(rctx)
         return distname, version, _arch
 
@@ -101,6 +101,13 @@ def os_version_arch(rctx):
 def os(rctx):
     # Less granular host OS name, e.g. linux.
 
+    name = rctx.attr.exec_os
+    if name:
+        if name in ("linux", "darwin"):
+            return name
+        else:
+            fail("Unsupported value for exec_os: %s" % name)
+
     name = rctx.os.name
     if name == "linux":
         return "linux"
@@ -115,6 +122,15 @@ def os_bzl(os):
     return {"darwin": "osx", "linux": "linux"}[os]
 
 def arch(rctx):
+    arch = rctx.attr.exec_arch
+    if arch:
+        if arch in ("arm64", "aarch64"):
+            return "aarch64"
+        elif arch in ("amd64", "x86_64"):
+            return "x86_64"
+        else:
+            fail("Unsupported value for exec_arch: %s" % arch)
+
     arch = rctx.os.arch
     if arch == "arm64":
         return "aarch64"

toolchain/internal/repo.bzl

@@ -35,6 +35,14 @@ common_attrs = {
                "in the list of known llvm_distributions using the provided version. " +
                "If unset, a default value is set from the `llvm_version` attribute."),
     ),
+    "exec_os": attr.string(
+        mandatory = False,
+        doc = "Execution platform OS, if different from host OS.",
+    ),
+    "exec_arch": attr.string(
+        mandatory = False,
+        doc = "Execution platform architecture, if different from host arch.",
+    ),
 }
 
 llvm_repo_attrs = dict(common_attrs)