## Summary
- avoid generating host proxy bridge Unix socket paths that exceed
Linux's `sockaddr_un.sun_path` limit
- fall back from a long `$CODEX_HOME/tmp` path to the system temp
directory, then `/tmp`
- add focused unit coverage for short and overlong parent paths
## Root cause
With a sufficiently long `CODEX_HOME`, the generated
`proxy-route-*.sock` path exceeds Linux's 107-byte pathname limit. The
host bridge child exits before writing its readiness byte, so the parent
reports the indirect error `failed to prepare host proxy routing bridge:
failed to fill whole buffer`.
## Validation
- reproduced the original error with a long `CODEX_HOME` using
`codex-cli 0.138.0-alpha.4`
- `cargo clippy -p codex-linux-sandbox --all-targets`
- `just fix -p codex-linux-sandbox`
- `just fmt`
The Linux-only unit test could not execute locally: the arm64 Docker
build was repeatedly OOM-killed by `rustc` while compiling an unrelated
`codex-app-server-protocol` dependency, before reaching the test.
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Co-authored-by: Codex <noreply@openai.com>
