test: faster test execution in codex-core (#2633)

this dramatically improves time to run `cargo test -p codex-core` (~25x
speedup).

before:
```
cargo test -p codex-core  35.96s user 68.63s system 19% cpu 8:49.80 total
```

after:
```
cargo test -p codex-core  5.51s user 8.16s system 63% cpu 21.407 total
```

both tests measured "hot", i.e. on a 2nd run with no filesystem changes,
to exclude compile times.

approach inspired by [Delete Cargo Integration
Tests](https://matklad.github.io/2021/02/27/delete-cargo-integration-tests.html),
we move all test cases in tests/ into a single suite in order to have a
single binary, as there is significant overhead for each test binary
executed, and because test execution is only parallelized with a single
binary.

codex-rs/exec/tests/suite/sandbox.rs

@@ -0,0 +1,219 @@
+#![cfg(unix)]
+use codex_core::protocol::SandboxPolicy;
+use codex_core::spawn::StdioPolicy;
+use std::collections::HashMap;
+use std::future::Future;
+use std::io;
+use std::path::PathBuf;
+use std::process::ExitStatus;
+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:?}");
+}
+
+fn unix_sock_body() {
+    unsafe {
+        let mut fds = [0i32; 2];
+        let r = libc::socketpair(libc::AF_UNIX, libc::SOCK_DGRAM, 0, fds.as_mut_ptr());
+        assert_eq!(
+            r,
+            0,
+            "socketpair(AF_UNIX, SOCK_DGRAM) failed: {}",
+            io::Error::last_os_error()
+        );
+
+        let msg = b"hello_unix";
+        // write() from one end (generic write is allowed)
+        let sent = libc::write(fds[0], msg.as_ptr() as *const libc::c_void, msg.len());
+        assert!(sent >= 0, "write() failed: {}", io::Error::last_os_error());
+
+        // recvfrom() on the other end. We don’t need the address for socketpair,
+        // so we pass null pointers for src address.
+        let mut buf = [0u8; 64];
+        let recvd = libc::recvfrom(
+            fds[1],
+            buf.as_mut_ptr() as *mut libc::c_void,
+            buf.len(),
+            0,
+            std::ptr::null_mut(),
+            std::ptr::null_mut(),
+        );
+        assert!(
+            recvd >= 0,
+            "recvfrom() failed: {}",
+            io::Error::last_os_error()
+        );
+
+        let recvd_slice = &buf[..(recvd as usize)];
+        assert_eq!(
+            recvd_slice,
+            &msg[..],
+            "payload mismatch: sent {} bytes, got {} bytes",
+            msg.len(),
+            recvd
+        );
+
+        // Also exercise AF_UNIX stream socketpair quickly to ensure AF_UNIX in general works.
+        let mut sfds = [0i32; 2];
+        let sr = libc::socketpair(libc::AF_UNIX, libc::SOCK_STREAM, 0, sfds.as_mut_ptr());
+        assert_eq!(
+            sr,
+            0,
+            "socketpair(AF_UNIX, SOCK_STREAM) failed: {}",
+            io::Error::last_os_error()
+        );
+        let snt2 = libc::write(sfds[0], msg.as_ptr() as *const libc::c_void, msg.len());
+        assert!(
+            snt2 >= 0,
+            "write(stream) failed: {}",
+            io::Error::last_os_error()
+        );
+        let mut b2 = [0u8; 64];
+        let rcv2 = libc::recv(sfds[1], b2.as_mut_ptr() as *mut libc::c_void, b2.len(), 0);
+        assert!(
+            rcv2 >= 0,
+            "recv(stream) failed: {}",
+            io::Error::last_os_error()
+        );
+
+        // Clean up
+        let _ = libc::close(sfds[0]);
+        let _ = libc::close(sfds[1]);
+        let _ = libc::close(fds[0]);
+        let _ = libc::close(fds[1]);
+    }
+}
+
+#[tokio::test]
+async fn allow_unix_socketpair_recvfrom() {
+    run_code_under_sandbox(
+        "allow_unix_socketpair_recvfrom",
+        &SandboxPolicy::ReadOnly,
+        || async { unix_sock_body() },
+    )
+    .await
+    .expect("should be able to reexec");
+}
+
+const IN_SANDBOX_ENV_VAR: &str = "IN_SANDBOX";
+
+#[expect(clippy::expect_used)]
+pub async fn run_code_under_sandbox<F, Fut>(
+    test_selector: &str,
+    policy: &SandboxPolicy,
+    child_body: F,
+) -> io::Result<Option<ExitStatus>>
+where
+    F: FnOnce() -> Fut + Send + 'static,
+    Fut: Future<Output = ()> + Send + 'static,
+{
+    if std::env::var(IN_SANDBOX_ENV_VAR).is_err() {
+        let exe = std::env::current_exe()?;
+        let mut cmds = vec![exe.to_string_lossy().into_owned(), "--exact".into()];
+        let mut stdio_policy = StdioPolicy::RedirectForShellTool;
+        // Allow for us to pass forward --nocapture / use the right stdio policy.
+        if std::env::args().any(|a| a == "--nocapture") {
+            cmds.push("--nocapture".into());
+            stdio_policy = StdioPolicy::Inherit;
+        }
+        cmds.push(test_selector.into());
+
+        // Your existing launcher:
+        let mut child = spawn_command_under_sandbox(
+            cmds,
+            policy,
+            std::env::current_dir().expect("should be able to get current dir"),
+            stdio_policy,
+            HashMap::from([("IN_SANDBOX".into(), "1".into())]),
+        )
+        .await?;
+
+        let status = child.wait().await?;
+        Ok(Some(status))
+    } else {
+        // Child branch: run the provided body.
+        child_body().await;
+        Ok(None)
+    }
+}