Status

This is the Bazel PR-CI cross-compilation follow-up to #20485. It is intentionally split from the Cargo/cargo-xwin release-build PoC so #20485 can stay as the historical release-build exploration. The unrelated async-utils test cleanup has been moved to #20686, so this PR is focused on the Windows Bazel CI path.

The intended tradeoff is now explicit in .github/workflows/bazel.yml: pull requests get the fast Windows cross-compiled Bazel test leg, while post-merge pushes to main run both that fast cross leg and a fully native Windows Bazel test leg. The native main-only job keeps full V8/code-mode coverage and gets a 40-minute timeout because it is less latency-sensitive than PR CI. All other Bazel jobs remain at 30 minutes.

Why

Windows Bazel PR CI currently does the expensive part of the build on Windows. A native Windows Bazel test job on main completed in about 28m12s, leaving very little headroom under the 30-minute job timeout and making Windows the slowest PR signal.

#20485 showed that Windows cross-compilation can be materially faster for Cargo release builds, but PR CI needs Bazel because Bazel owns our test sharding, flaky-test retries, and integration-test layout. This PR applies the same high-level shape we already use for macOS Bazel CI: compile with remote Linux execution, then run platform-specific tests on the platform runner.

The compromise is deliberately signal-aware: code-mode/V8 changes are rare enough that PR CI can accept losing the direct V8/code-mode smoke-test signal temporarily, while main still runs the native Windows job post-merge to catch that class of regression. A follow-up PR should investigate making the cross-built Windows gnullvm V8 archive pass the direct V8/code-mode tests so this tradeoff can eventually go away.

What Changed

• Adds a ci-windows-cross Bazel config that targets x86_64-pc-windows-gnullvm, uses Linux RBE for build actions, and keeps TestRunner actions local on the Windows runner.
• Adds explicit Windows platform definitions for windows_x86_64_gnullvm, windows_x86_64_msvc, and a bridge toolchain that lets gnullvm test targets execute under the Windows MSVC host platform.
• Updates the Windows Bazel PR test leg to opt into the cross-compile path via --windows-cross-compile and --remote-download-toplevel.
• Adds a test-windows-native-main job that runs only for push events on refs/heads/main, uses the native Windows Bazel path, includes V8/code-mode smoke tests, and has timeout-minutes: 40.
• Keeps fork/community PRs without BUILDBUDDY_API_KEY on the previous local Windows MSVC-host fallback, including --host_platform=//:local_windows_msvc and --jobs=8.
• Preserves the existing integration-test shape on non-gnullvm platforms, while generating Windows-cross wrapper targets only for windows_gnullvm.
• Resolves CARGO_BIN_EXE_* values from runfiles at test runtime, avoiding hard-coded Cargo paths and duplicate test runfiles.
• Extends the V8 Bazel patches enough for the x86_64-pc-windows-gnullvm target and Linux remote execution path.
• Makes the Windows sandbox test cwd derive from INSTA_WORKSPACE_ROOT at runtime when Bazel provides it, because cross-compiled binaries may contain Linux compile-time paths.
• Keeps the direct V8/code-mode unit smoke tests out of the Windows cross PR path for now while native Windows CI continues to cover them post-merge.

Command Shape

The fast Windows PR test leg invokes the normal Bazel CI wrapper like this:

./.github/scripts/run-bazel-ci.sh \
  --print-failed-action-summary \
  --print-failed-test-logs \
  --windows-cross-compile \
  --remote-download-toplevel \
  -- \
  test \
  --test_tag_filters=-argument-comment-lint \
  --test_verbose_timeout_warnings \
  --build_metadata=COMMIT_SHA=${GITHUB_SHA} \
  -- \
  //... \
  -//third_party/v8:all \
  -//codex-rs/code-mode:code-mode-unit-tests \
  -//codex-rs/v8-poc:v8-poc-unit-tests

With the BuildBuddy secret available on Windows, the wrapper selects --config=ci-windows-cross and appends the important Windows-cross overrides after rc expansion:

--host_platform=//:rbe
--shell_executable=/bin/bash
--action_env=PATH=/usr/bin:/bin
--host_action_env=PATH=/usr/bin:/bin
--test_env=PATH=${CODEX_BAZEL_WINDOWS_PATH}

The native post-merge Windows job intentionally omits --windows-cross-compile and does not exclude the V8/code-mode unit targets:

./.github/scripts/run-bazel-ci.sh \
  --print-failed-action-summary \
  --print-failed-test-logs \
  -- \
  test \
  --test_tag_filters=-argument-comment-lint \
  --test_verbose_timeout_warnings \
  --build_metadata=COMMIT_SHA=${GITHUB_SHA} \
  --build_metadata=TAG_windows_native_main=true \
  -- \
  //... \
  -//third_party/v8:all

Research Notes

The existing macOS Bazel CI config already uses the model we want here: build actions run remotely with --strategy=remote, but TestRunner actions execute on the macOS runner. This PR mirrors that pattern for Windows with --strategy=TestRunner=local.

The important Bazel detail is that rules_rs is already targeting x86_64-pc-windows-gnullvm for Windows Bazel PR tests. This PR changes where the build actions execute; it does not switch the Bazel PR test target to Cargo, cargo-nextest, or the MSVC release target.

Cargo release builds differ from this Bazel path for V8: the normal Windows Cargo release target is MSVC, and rusty_v8 publishes prebuilt Windows MSVC .lib.gz archives. The Bazel PR path targets windows-gnullvm; rusty_v8 does not publish a prebuilt Windows GNU/gnullvm archive, so this PR builds that archive in-tree. That Linux-RBE-built gnullvm archive currently crashes in direct V8/code-mode smoke tests, which is why the workflow keeps native Windows coverage on main.

The less obvious Bazel detail is test wrapper selection. Bazel chooses the Windows test wrapper (tw.exe) from the test action execution platform, not merely from the Rust target triple. The outer workspace_root_test therefore declares the default test toolchain and uses the bridge toolchain above so the test action executes on Windows while its inner Rust binary is built for gnullvm.

The V8 investigation exposed a Windows-client gotcha: even when an action execution platform is Linux RBE, Bazel can still derive the genrule shell path from the Windows client. That produced remote commands trying to run C:\Program Files\Git\usr\bin\bash.exe on Linux workers. The wrapper now passes --shell_executable=/bin/bash with --host_platform=//:rbe for the Windows cross path.

The same Windows-client/Linux-RBE boundary also affected third_party/v8:binding_cc: a multiline genrule command can carry CRLF line endings into Linux remote bash, which failed as $'\r'. That genrule now keeps the sed command on one physical shell line while using an explicit Starlark join so the shell arguments stay readable.

Verification

Local checks included:

bash -n .github/scripts/run-bazel-ci.sh
bash -n workspace_root_test_launcher.sh.tpl
ruby -e "require %q{yaml}; YAML.load_file(%q{.github/workflows/bazel.yml}); puts %q{ok}"
RUNNER_OS=Linux ./scripts/list-bazel-clippy-targets.sh
RUNNER_OS=Windows ./scripts/list-bazel-clippy-targets.sh
RUNNER_OS=Linux ./tools/argument-comment-lint/list-bazel-targets.sh
RUNNER_OS=Windows ./tools/argument-comment-lint/list-bazel-targets.sh

The Linux clippy and argument-comment target lists contain zero *-windows-cross-bin labels, while the Windows lists still include 47 Windows-cross internal test binaries.

CI evidence:

• Baseline native Windows Bazel test on main: success in about 28m12s, https://github.com/openai/codex/actions/runs/25206257208/job/73907325959
• Green Windows-cross Bazel run on the split PR before adding the main-only native leg: Windows test 9m16s, Windows release verify 5m10s, Windows clippy 4m43s, https://github.com/openai/codex/actions/runs/25231890068
• The latest SHA adds the explicit PR-vs-main tradeoff in bazel.yml; CI is rerunning on that focused diff.

Follow-Up

A subsequent PR should investigate making a cross-built Windows binary work with V8/code-mode enabled. Likely options are either making the Linux-RBE-built windows-gnullvm V8 archive correct at runtime, or evaluating whether a Bazel MSVC target/toolchain can reuse the same prebuilt MSVC rusty_v8 archive shape that Cargo release builds already use.