fix: run python_multiprocessing_lock_works integration test on Mac and Linux (#2318)

The high-order bit on this PR is that it makes it so `sandbox.rs` tests
both Mac and Linux, as we introduce a general
`spawn_command_under_sandbox()` function with platform-specific
implementations for testing.

An important, and interesting, discovery in porting the test to Linux is
that (for reasons cited in the code comments), `/dev/shm` has to be
added to `writable_roots` on Linux in order for `multiprocessing.Lock`
to work there. Granting write access to `/dev/shm` comes with some
degree of risk, so we do not make this the default for Codex CLI.

Piggybacking on top of #2317, this moves the
`python_multiprocessing_lock_works` test yet again, moving
`codex-rs/core/tests/sandbox.rs` to `codex-rs/exec/tests/sandbox.rs`
because in `codex-rs/exec/tests` we can use `cargo_bin()` like so:

```
let codex_linux_sandbox_exe = assert_cmd::cargo::cargo_bin("codex-exec");
```

which is necessary so we can use `codex_linux_sandbox_exe` and therefore
`spawn_command_under_linux_sandbox` in an integration test.

This also moves `spawn_command_under_linux_sandbox()` out of `exec.rs`
and into `landlock.rs`, which makes things more consistent with
`seatbelt.rs` in `codex-core`.

For reference, https://github.com/openai/codex/pull/1808 is the PR that
made the change to Seatbelt to get this test to pass on Mac.

codex-rs/exec/tests/sandbox.rs

@@ -0,0 +1,92 @@
+#![cfg(unix)]
+#![expect(clippy::expect_used)]
+
+use codex_core::protocol::SandboxPolicy;
+use codex_core::spawn::StdioPolicy;
+use std::collections::HashMap;
+use std::path::PathBuf;
+use tokio::process::Child;
+
+#[cfg(target_os = "macos")]
+async fn spawn_command_under_sandbox(
+    command: Vec<String>,
+    sandbox_policy: &SandboxPolicy,
+    cwd: PathBuf,
+    stdio_policy: StdioPolicy,
+    env: HashMap<String, String>,
+) -> std::io::Result<Child> {
+    use codex_core::seatbelt::spawn_command_under_seatbelt;
+    spawn_command_under_seatbelt(command, sandbox_policy, cwd, stdio_policy, env).await
+}
+
+#[cfg(target_os = "linux")]
+async fn spawn_command_under_sandbox(
+    command: Vec<String>,
+    sandbox_policy: &SandboxPolicy,
+    cwd: PathBuf,
+    stdio_policy: StdioPolicy,
+    env: HashMap<String, String>,
+) -> std::io::Result<Child> {
+    use codex_core::landlock::spawn_command_under_linux_sandbox;
+    let codex_linux_sandbox_exe = assert_cmd::cargo::cargo_bin("codex-exec");
+    spawn_command_under_linux_sandbox(
+        codex_linux_sandbox_exe,
+        command,
+        sandbox_policy,
+        cwd,
+        stdio_policy,
+        env,
+    )
+    .await
+}
+
+#[tokio::test]
+async fn python_multiprocessing_lock_works_under_sandbox() {
+    #[cfg(target_os = "macos")]
+    let writable_roots = Vec::<PathBuf>::new();
+
+    // From https://man7.org/linux/man-pages/man7/sem_overview.7.html
+    //
+    // > On Linux, named semaphores are created in a virtual filesystem,
+    // > normally mounted under /dev/shm.
+    #[cfg(target_os = "linux")]
+    let writable_roots = vec![PathBuf::from("/dev/shm")];
+
+    let policy = SandboxPolicy::WorkspaceWrite {
+        writable_roots,
+        network_access: false,
+        exclude_tmpdir_env_var: false,
+        exclude_slash_tmp: false,
+    };
+
+    let python_code = r#"import multiprocessing
+from multiprocessing import Lock, Process
+
+def f(lock):
+    with lock:
+        print("Lock acquired in child process")
+
+if __name__ == '__main__':
+    lock = Lock()
+    p = Process(target=f, args=(lock,))
+    p.start()
+    p.join()
+"#;
+
+    let mut child = spawn_command_under_sandbox(
+        vec![
+            "python3".to_string(),
+            "-c".to_string(),
+            python_code.to_string(),
+        ],
+        &policy,
+        std::env::current_dir().expect("should be able to get current dir"),
+        StdioPolicy::Inherit,
+        HashMap::new(),
+    )
+    .await
+    .expect("should be able to spawn python under sandbox");
+
+    let status = child.wait().await.expect("should wait for child process");
+    assert!(status.success(), "python exited with {status:?}");
+}