arg0: keep dispatch aliases alive during async main

[bolinfest]

Why

The Ubuntu GNU remote Cargo run has been regularly failing sandboxed suite::remote_env filesystem tests with No such file or directory, while the same cases pass under Bazel. The Cargo remote-env setup starts target/debug/codex exec-server inside Docker via scripts/test-remote-env.sh. That CLI builds codex-linux-sandbox and other arg0 helper aliases in a temporary directory, then passes those alias paths into the exec-server runtime.

arg0_dispatch_or_else constructed Arg0DispatchPaths from that temporary alias guard, but then awaited the async CLI entry point without otherwise keeping the guard live. That allowed the guard to be dropped while the exec-server was still running, removing the helper alias directory. Later sandboxed filesystem calls tried to spawn the now-deleted codex-linux-sandbox path and surfaced as ENOENT.

The relevant distinction I found is that core/tests/common stores the result of arg0_dispatch() in a process-lifetime OnceLock<Option<Arg0PathEntryGuard>> for test binaries. The Cargo remote-env setup exercises a real codex exec-server process instead, so it depends on the normal CLI lifetime behavior fixed here.

What Changed

• Keep the arg0 tempdir guard alive until main_fn(paths).await completes.
• Keep the helper on the real arg0_dispatch() shape, where alias setup can fail and return None in production.
• Add a regression test that uses an explicit guard, yields once, and verifies the generated helper alias path still exists while the async entry point is running.

Verification

• cargo test -p codex-arg0
• just argument-comment-lint -p codex-arg0
• just fix -p codex-arg0

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Serialize environment mutation in this test

This test mutates CODEX_HOME/PATH via unsafe std::env::set_var/remove_var without any serialization, but Rust tests run in parallel by default. If any other test thread reads environment variables concurrently, this violates std::env's safety contract and can cause undefined behavior or flaky CI failures. Please wrap these mutations in a process-wide lock (or mark the test serial) so environment access is synchronized.

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[bolinfest]

I'm not sure why Bazel tests appear "frozen" on the PR page, but I see them running here: https://github.com/openai/codex/actions/runs/24792741756/job/72554548095?pr=18999