Allow setting platform constraints with --exec_constraints

[coeuvre]

By default, clang is detected for cpp configs. While --cpp_env_json can be used to override the default environment variables during toolchain generation, there isn't a way to override platform constraints.

This PR adds flag --exec_constraints which accept a comma separated list of constraint values e.g. --exec_constraints=@bazel_tools//platforms:linux,@bazel_tools//platforms:x86_64,@bazel_tools//tools/cpp:gcc to override the default values.

[crackcomm]

I would be delighted to have this feature.

[coeuvre]

I have rebased the PR. Please review and let me know whether it's feasible to merge!

[jjmaestro]

@coeuvre I'm trying your PR and it's not overriding any of the env flags :-? When I run with

--exec_constraints=@bazel_tools//platforms:linux,@bazel_tools//platforms:x86_64,@bazel_tools//tools/cpp:gcc

I still see x86 and clang everywhere:

(...)
2024/10/03 10:54:23 Running: 'docker exec -w /workdir/cpp_configs_project \
-e USE_BAZEL_VERSION=7.3.1 \
-e ABI_LIBC_VERSION=glibc_2.19 \
-e BAZEL_TARGET_LIBC=glibc_2.19 \
-e BAZEL_TARGET_SYSTEM=x86_64-unknown-linux-gnu \
-e CC_TOOLCHAIN_NAME=linux_gnu_x86 \
-e ABI_VERSION=clang \
-e BAZEL_COMPILER=clang \
-e BAZEL_HOST_SYSTEM=i686-unknown-linux-gnu \
-e BAZEL_TARGET_CPU=k8 \
-e CC=clang \
cf841a5922ed2a0fe4772303e04280a3dff41c9eeb25facd00d05635c373afd0 \
/workdir/bazelisk build @local_config_cc//...'

Also, I can't get your PR and bazel-toolchains to work with another arch

--exec_constraints=@bazel_tools//platforms:linux,@bazel_tools//platforms:arm64,@bazel_tools//tools/cpp:gcc

it keeps trying to force x86:

(...)
2024/10/03 11:41:22 Output: 2024/10/03 10:41:08 Downloading https://releases.bazel.build/7.3.1/release/bazel-7.3.1-linux-x86_64...
rosetta error: failed to open elf at /lib64/ld-linux-x86-64.so.2

I checked the code and, if I'm not mistaken, we still need to use --cpp_env_json and make sure "everything matches" (e.g. compiler, architecture, etc)?

Maybe there's something I'm doing wrong... :-? Anyway, it would be great if you have a working example that uses --exec_constraints.

Thanks!

[jjmaestro]

OK, I checked further and... there's so many things hardcoded that I don't think this PR helps, sorry :-/

Here's how I got bazel-toolchains to work with my image:

diff --git a/pkg/rbeconfigsgen/options.go b/pkg/rbeconfigsgen/options.go
index b32965a..be90748 100644
--- a/pkg/rbeconfigsgen/options.go
+++ b/pkg/rbeconfigsgen/options.go
@@ -134,12 +134,12 @@ var (
 			PlatformParams: PlatformToolchainsTemplateParams{
 				ExecConstraints: []string{
 					"@platforms//os:linux",
-					"@platforms//cpu:x86_64",
-					"@bazel_tools//tools/cpp:clang",
+					"@platforms//cpu:arm64",
+					"@bazel_tools//tools/cpp:gcc",
 				},
 				TargetConstraints: []string{
 					"@platforms//os:linux",
-					"@platforms//cpu:x86_64",
+					"@platforms//cpu:arm64",
 				},
 				OSFamily: "Linux",
 			},
@@ -147,15 +147,15 @@ var (
 			CPPConfigRepo:    "local_config_cc",
 			CppBazelCmd:      "build",
 			CppGenEnv: map[string]string{
-				"ABI_LIBC_VERSION":    "glibc_2.19",
-				"ABI_VERSION":         "clang",
-				"BAZEL_COMPILER":      "clang",
+				"ABI_LIBC_VERSION":    "glibc_2.36",
+				"ABI_VERSION":         "gcc",
+				"BAZEL_COMPILER":      "gcc",
 				"BAZEL_HOST_SYSTEM":   "i686-unknown-linux-gnu",
 				"BAZEL_TARGET_CPU":    "k8",
-				"BAZEL_TARGET_LIBC":   "glibc_2.19",
+				"BAZEL_TARGET_LIBC":   "glibc_2.36",
 				"BAZEL_TARGET_SYSTEM": "x86_64-unknown-linux-gnu",
-				"CC":                  "clang",
-				"CC_TOOLCHAIN_NAME":   "linux_gnu_x86",
+				"CC":                  "gcc",
+				"CC_TOOLCHAIN_NAME":   "linux_gnu_arm64",
 			},
 			CPPToolchainTargetName: "cc-compiler-k8",
 		},
diff --git a/pkg/rbeconfigsgen/rbeconfigsgen.go b/pkg/rbeconfigsgen/rbeconfigsgen.go
index b7815f9..1c9799b 100644
--- a/pkg/rbeconfigsgen/rbeconfigsgen.go
+++ b/pkg/rbeconfigsgen/rbeconfigsgen.go
@@ -262,7 +262,7 @@ func workdir(os string) string {
 func BazeliskDownloadInfo(os string) (string, string, error) {
 	switch os {
 	case OSLinux:
-		return "https://github.com/bazelbuild/bazelisk/releases/download/v1.19.0/bazelisk-linux-amd64", "bazelisk", nil
+		return "https://github.com/bazelbuild/bazelisk/releases/download/v1.19.0/bazelisk-linux-arm64", "bazelisk", nil
 	case OSWindows:
 		return "https://github.com/bazelbuild/bazelisk/releases/download/v1.19.0/bazelisk-windows-amd64.exe", "bazelisk.exe", nil
 	}

I think we should have a good way to override all of the parameters, including the arch, and avoid hardcoding it / using bazelisk if e.g. there's already Bazel in the image, etc.

[jjmaestro]

OK, I've managed to use your PR and avoid the hardcoding of x86 by having Bazel in the image and specifying bazel_path, thus avoiding the installation of Bazelisk which is hardcoded to x86.

The following works with this PR:

#!/bin/bash

build() {
    go build \
        -o rbe_configs_gen \
        ./cmd/rbe_configs_gen/rbe_configs_gen.go
}

generate() {
    local docker_image="$1"
    local bazel_version="$2"
    local target_arch="$3"
    local repo_path="$4"

    local exec_constraints=(
        "@bazel_tools//platforms:linux"
        "@bazel_tools//platforms:$target_arch"
        "@bazel_tools//tools/cpp:gcc"
    )

    GLIBC_VERSION="2.36"  # ldd --version

    cat <<- EOF >/tmp/cpp_env.json
{
    "ABI_LIBC_VERSION": "glibc_$GLIBC_VERSION",
    "ABI_VERSION": "gcc",
    "BAZEL_COMPILER": "gcc",
    "BAZEL_HOST_SYSTEM": "i686-unknown-linux-gnu",
    "BAZEL_TARGET_CPU": "k8",
    "BAZEL_TARGET_LIBC": "glibc_$GLIBC_VERSION",
    "BAZEL_TARGET_SYSTEM": "${target_arch}-unknown-linux-gnu",
    "CC": "gcc",
    "CC_TOOLCHAIN_NAME": "linux_gnu_$target_arch"
}
EOF

    # NOTE: bazel_path is needed to avoid downloading Bazelisk inside the
    # image because the tool HARDCODES THE ARCH to x86 regardless of the
    # docker platform we use...

    IFS=','
    ./rbe_configs_gen \
        --generate_cpp_configs=true \
        --generate_java_configs=false \
        --exec_os="linux" \
        --target_os="linux" \
        --toolchain_container="$docker_image" \
        --docker_platform="linux/$target_arch" \
        --bazel_version="$bazel_version" \
        --bazel_path="/usr/bin/bazel" \
        --exec_constraints="${exec_constraints[*]}" \
        --cpp_env_json=/tmp/cpp_env.json \
        --output_src_root="$repo_path"
}

"$@"

Then, using the script, I (1) build the tool locally (in my case, MacOS arm64) and (2) run rbe_configs_gen with --cpp_env_json and --exec_constraints:

$ ./runner.sh build
$ ./runner.sh generate MY_CUSTOM_IMAGE 7.3.2 arm64 /tmp/repo

[jjmaestro]

@coeuvre Also, it would be awesome if you could rebase your branch, I've tried and it rebases cleanly on top of the current HEAD :) Tkanks!!

[jjmaestro]

I've also noticed that TargetConstraints is hardcoded to x86_64... so I guess that would also have to be changed to truly support platform selection, e.g. via a similar flag --target_constraints :-?