Fix for CI Tests failing from stack overflow (#17846)

### **Issue**
guardian_parallel_reviews_fork_from_last_committed_trunk_history was
failing on Windows/Bazel with a stack overflow:

`thread
'guardian::tests::guardian_parallel_reviews_fork_from_last_committed_trunk_history'
has overflowed its stack`

- This problem was a stack-headroom problem

### **Solution**

Reduced stack pressure in the guardian async path by boxing thin wrapper
futures, and run the affected test on a dedicated 2 MiB thread stack.

Concretely:
- added Box::pin(...) around thin async wrapper hops in the guardian
review/delegate path
- changed
guardian_parallel_reviews_fork_from_last_committed_trunk_history to run
inside an explicitly sized thread stack so it has enough headroom in
low-stack environments

codex-rs/core/src/codex_delegate.rs

@@ -75,7 +75,7 @@ pub(crate) async fn run_codex_thread_interactive(
     let (tx_sub, rx_sub) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
     let (tx_ops, rx_ops) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
 
-    let CodexSpawnOk { codex, .. } = Codex::spawn(CodexSpawnArgs {
+    let CodexSpawnOk { codex, .. } = Box::pin(Codex::spawn(CodexSpawnArgs {
         config,
         auth_manager,
         models_manager,
@@ -97,7 +97,7 @@ pub(crate) async fn run_codex_thread_interactive(
         inherited_exec_policy: Some(Arc::clone(&parent_session.services.exec_policy)),
         parent_trace: None,
         analytics_events_client: Some(parent_session.services.analytics_events_client.clone()),
-    })
+    }))
     .await?;
     if parent_session.enabled(codex_features::Feature::GeneralAnalytics) {
         let thread_config = codex.thread_config_snapshot().await;
@@ -172,7 +172,7 @@ pub(crate) async fn run_codex_thread_one_shot(
     // Use a child token so we can stop the delegate after completion without
     // requiring the caller to cancel the parent token.
     let child_cancel = cancel_token.child_token();
-    let io = run_codex_thread_interactive(
+    let io = Box::pin(run_codex_thread_interactive(
         config,
         auth_manager,
         models_manager,
@@ -181,7 +181,7 @@ pub(crate) async fn run_codex_thread_one_shot(
         child_cancel.clone(),
         subagent_source,
         initial_history,
-    )
+    ))
     .await?;
 
     // Send the initial input to kick off the one-shot turn.

codex-rs/core/src/guardian/review.rs

@@ -163,14 +163,14 @@ async fn run_guardian_review(
 
     let schema = guardian_output_schema();
     let terminal_action = action_summary.clone();
-    let outcome = run_guardian_review_session(
+    let outcome = Box::pin(run_guardian_review_session(
         session.clone(),
         turn.clone(),
         request,
         retry_reason,
         schema,
         external_cancel,
-    )
+    ))
     .await;
 
     let assessment = match outcome {
@@ -303,14 +303,16 @@ pub(crate) async fn review_approval_request(
     request: GuardianApprovalRequest,
     retry_reason: Option<String>,
 ) -> ReviewDecision {
-    run_guardian_review(
+    // Box the delegated review future so callers do not inline the entire
+    // guardian session state machine into their own async stack.
+    Box::pin(run_guardian_review(
         Arc::clone(session),
         Arc::clone(turn),
         review_id,
         request,
         retry_reason,
         /*external_cancel*/ None,
-    )
+    ))
     .await
 }
 
@@ -322,14 +324,14 @@ pub(crate) async fn review_approval_request_with_cancel(
     retry_reason: Option<String>,
     cancel_token: CancellationToken,
 ) -> ReviewDecision {
-    run_guardian_review(
+    Box::pin(run_guardian_review(
         Arc::clone(session),
         Arc::clone(turn),
         review_id,
         request,
         retry_reason,
         Some(cancel_token),
-    )
+    ))
     .await
 }
 
@@ -411,22 +413,24 @@ pub(super) async fn run_guardian_review_session(
         Err(err) => return GuardianReviewOutcome::Completed(Err(err)),
     };
 
-    match session
-        .guardian_review_session
-        .run_review(GuardianReviewSessionParams {
-            parent_session: Arc::clone(&session),
-            parent_turn: turn.clone(),
-            spawn_config: guardian_config,
-            request,
-            retry_reason,
-            schema,
-            model: guardian_model,
-            reasoning_effort: guardian_reasoning_effort,
-            reasoning_summary: turn.reasoning_summary,
-            personality: turn.personality,
-            external_cancel,
-        })
-        .await
+    match Box::pin(
+        session
+            .guardian_review_session
+            .run_review(GuardianReviewSessionParams {
+                parent_session: Arc::clone(&session),
+                parent_turn: turn.clone(),
+                spawn_config: guardian_config,
+                request,
+                retry_reason,
+                schema,
+                model: guardian_model,
+                reasoning_effort: guardian_reasoning_effort,
+                reasoning_summary: turn.reasoning_summary,
+                personality: turn.personality,
+                external_cancel,
+            }),
+    )
+    .await
     {
         GuardianReviewSessionOutcome::Completed(Ok(last_agent_message)) => {
             GuardianReviewOutcome::Completed(parse_guardian_assessment(

codex-rs/core/src/guardian/review_session.rs

@@ -327,32 +327,30 @@ impl GuardianReviewSessionManager {
         };
 
         if trunk.reuse_key != next_reuse_key {
-            return self
-                .run_ephemeral_review(
-                    params,
-                    next_reuse_key,
-                    deadline,
-                    /*fork_snapshot*/ None,
-                )
-                .await;
+            return Box::pin(self.run_ephemeral_review(
+                params,
+                next_reuse_key,
+                deadline,
+                /*fork_snapshot*/ None,
+            ))
+            .await;
         }
 
         let trunk_guard = match trunk.review_lock.try_lock() {
             Ok(trunk_guard) => trunk_guard,
             Err(_) => {
-                return self
-                    .run_ephemeral_review(
-                        params,
-                        next_reuse_key,
-                        deadline,
-                        trunk.fork_snapshot().await,
-                    )
-                    .await;
+                return Box::pin(self.run_ephemeral_review(
+                    params,
+                    next_reuse_key,
+                    deadline,
+                    trunk.fork_snapshot().await,
+                ))
+                .await;
             }
         };
 
         let (outcome, keep_review_session) =
-            run_review_on_session(trunk.as_ref(), &params, deadline).await;
+            Box::pin(run_review_on_session(trunk.as_ref(), &params, deadline)).await;
         if keep_review_session && matches!(outcome, GuardianReviewSessionOutcome::Completed(_)) {
             trunk.refresh_last_committed_fork_snapshot().await;
         }
@@ -488,7 +486,12 @@ impl GuardianReviewSessionManager {
         let mut cleanup =
             EphemeralReviewCleanup::new(Arc::clone(&self.state), Arc::clone(&review_session));
 
-        let (outcome, _) = run_review_on_session(review_session.as_ref(), &params, deadline).await;
+        let (outcome, _) = Box::pin(run_review_on_session(
+            review_session.as_ref(),
+            &params,
+            deadline,
+        ))
+        .await;
         if let Some(review_session) = self.take_active_ephemeral(&review_session).await {
             cleanup.disarm();
             review_session.shutdown_in_background();
@@ -512,7 +515,7 @@ async fn spawn_guardian_review_session(
         ),
         None => (None, 0, None),
     };
-    let codex = run_codex_thread_interactive(
+    let codex = Box::pin(run_codex_thread_interactive(
         spawn_config,
         params.parent_session.services.auth_manager.clone(),
         params.parent_session.services.models_manager.clone(),
@@ -521,7 +524,7 @@ async fn spawn_guardian_review_session(
         cancel_token.clone(),
         SubAgentSource::Other(GUARDIAN_REVIEWER_NAME.to_string()),
         initial_history,
-    )
+    ))
     .await?;
 
     Ok(GuardianReviewSession {

codex-rs/core/src/guardian/tests.rs

@@ -1332,222 +1332,242 @@ async fn guardian_review_surfaces_responses_api_errors_in_rejection_reason() ->
     Ok(())
 }
 
-#[tokio::test(flavor = "current_thread")]
-async fn guardian_parallel_reviews_fork_from_last_committed_trunk_history() -> anyhow::Result<()> {
-    let first_assessment = serde_json::json!({
-        "risk_level": "low",
-        "user_authorization": "high",
-        "outcome": "allow",
-        "rationale": "first guardian rationale",
-    })
-    .to_string();
-    let second_assessment = serde_json::json!({
-        "risk_level": "low",
-        "user_authorization": "high",
-        "outcome": "allow",
-        "rationale": "second guardian rationale",
-    })
-    .to_string();
-    let third_assessment = serde_json::json!({
-        "risk_level": "low",
-        "user_authorization": "high",
-        "outcome": "allow",
-        "rationale": "third guardian rationale",
-    })
-    .to_string();
-    let (gate_tx, gate_rx) = tokio::sync::oneshot::channel();
-    let (server, _) = start_streaming_sse_server(vec![
-        vec![StreamingSseChunk {
-            gate: None,
-            body: sse(vec![
-                ev_response_created("resp-guardian-1"),
-                ev_assistant_message("msg-guardian-1", &first_assessment),
-                ev_completed("resp-guardian-1"),
-            ]),
-        }],
-        vec![
-            StreamingSseChunk {
+#[test]
+fn guardian_parallel_reviews_fork_from_last_committed_trunk_history() -> anyhow::Result<()> {
+    const TEST_STACK_SIZE_BYTES: usize = 2 * 1024 * 1024;
+
+    let handle =
+        std::thread::Builder::new()
+            .name("guardian_parallel_reviews_fork_from_last_committed_trunk_history".to_string())
+            .stack_size(TEST_STACK_SIZE_BYTES)
+            .spawn(|| -> anyhow::Result<()> {
+                let runtime = tokio::runtime::Builder::new_current_thread()
+                    .enable_all()
+                    .build()?;
+                runtime.block_on(Box::pin(async {
+        let first_assessment = serde_json::json!({
+            "risk_level": "low",
+            "user_authorization": "high",
+            "outcome": "allow",
+            "rationale": "first guardian rationale",
+        })
+        .to_string();
+        let second_assessment = serde_json::json!({
+            "risk_level": "low",
+            "user_authorization": "high",
+            "outcome": "allow",
+            "rationale": "second guardian rationale",
+        })
+        .to_string();
+        let third_assessment = serde_json::json!({
+            "risk_level": "low",
+            "user_authorization": "high",
+            "outcome": "allow",
+            "rationale": "third guardian rationale",
+        })
+        .to_string();
+        let (gate_tx, gate_rx) = tokio::sync::oneshot::channel();
+        let (server, _) = start_streaming_sse_server(vec![
+            vec![StreamingSseChunk {
                 gate: None,
-                body: sse(vec![ev_response_created("resp-guardian-2")]),
-            },
-            StreamingSseChunk {
-                gate: Some(gate_rx),
                 body: sse(vec![
-                    ev_assistant_message("msg-guardian-2", &second_assessment),
-                    ev_completed("resp-guardian-2"),
+                    ev_response_created("resp-guardian-1"),
+                    ev_assistant_message("msg-guardian-1", &first_assessment),
+                    ev_completed("resp-guardian-1"),
                 ]),
-            },
-        ],
-        vec![StreamingSseChunk {
-            gate: None,
-            body: sse(vec![
-                ev_response_created("resp-guardian-3"),
-                ev_assistant_message("msg-guardian-3", &third_assessment),
-                ev_completed("resp-guardian-3"),
-            ]),
-        }],
-    ])
-    .await;
+            }],
+            vec![
+                StreamingSseChunk {
+                    gate: None,
+                    body: sse(vec![ev_response_created("resp-guardian-2")]),
+                },
+                StreamingSseChunk {
+                    gate: Some(gate_rx),
+                    body: sse(vec![
+                        ev_assistant_message("msg-guardian-2", &second_assessment),
+                        ev_completed("resp-guardian-2"),
+                    ]),
+                },
+            ],
+            vec![StreamingSseChunk {
+                gate: None,
+                body: sse(vec![
+                    ev_response_created("resp-guardian-3"),
+                    ev_assistant_message("msg-guardian-3", &third_assessment),
+                    ev_completed("resp-guardian-3"),
+                ]),
+            }],
+        ])
+        .await;
 
-    let (session, turn) = guardian_test_session_and_turn_with_base_url(server.uri()).await;
-    seed_guardian_parent_history(&session, &turn).await;
+        let (session, turn) = guardian_test_session_and_turn_with_base_url(server.uri()).await;
+        seed_guardian_parent_history(&session, &turn).await;
 
-    let initial_request = GuardianApprovalRequest::Shell {
-        id: "shell-guardian-1".to_string(),
-        command: vec!["git".to_string(), "status".to_string()],
-        cwd: test_path_buf("/repo/codex-rs/core").abs(),
-        sandbox_permissions: crate::sandboxing::SandboxPermissions::UseDefault,
-        additional_permissions: None,
-        justification: Some("Inspect repo state before proceeding.".to_string()),
-    };
-    assert_eq!(
-        review_approval_request(
+        let initial_request = GuardianApprovalRequest::Shell {
+            id: "shell-guardian-1".to_string(),
+            command: vec!["git".to_string(), "status".to_string()],
+            cwd: test_path_buf("/repo/codex-rs/core").abs(),
+            sandbox_permissions: crate::sandboxing::SandboxPermissions::UseDefault,
+            additional_permissions: None,
+            justification: Some("Inspect repo state before proceeding.".to_string()),
+        };
+        assert_eq!(
+            review_approval_request(
+                &session,
+                &turn,
+                "review-shell-guardian-1".to_string(),
+                initial_request,
+                /*retry_reason*/ None
+            )
+            .await,
+            ReviewDecision::Approved
+        );
+        session
+            .record_into_history(
+                &[
+                    ResponseItem::Message {
+                        id: None,
+                        role: "user".to_string(),
+                        content: vec![ContentItem::InputText {
+                            text: "Please inspect pending changes before pushing.".to_string(),
+                        }],
+                        end_turn: None,
+                        phase: None,
+                    },
+                    ResponseItem::Message {
+                        id: None,
+                        role: "assistant".to_string(),
+                        content: vec![ContentItem::OutputText {
+                            text: "I need approval to run git diff.".to_string(),
+                        }],
+                        end_turn: None,
+                        phase: None,
+                    },
+                ],
+                turn.as_ref(),
+            )
+            .await;
+
+        let second_request = GuardianApprovalRequest::Shell {
+            id: "shell-guardian-2".to_string(),
+            command: vec!["git".to_string(), "diff".to_string()],
+            cwd: test_path_buf("/repo/codex-rs/core").abs(),
+            sandbox_permissions: crate::sandboxing::SandboxPermissions::UseDefault,
+            additional_permissions: None,
+            justification: Some("Inspect pending changes before proceeding.".to_string()),
+        };
+        let third_request = GuardianApprovalRequest::Shell {
+            id: "shell-guardian-3".to_string(),
+            command: vec!["git".to_string(), "push".to_string()],
+            cwd: test_path_buf("/repo/codex-rs/core").abs(),
+            sandbox_permissions: crate::sandboxing::SandboxPermissions::UseDefault,
+            additional_permissions: None,
+            justification: Some("Inspect whether pushing is safe before proceeding.".to_string()),
+        };
+
+        let session_for_second = Arc::clone(&session);
+        let turn_for_second = Arc::clone(&turn);
+        let mut second_review = tokio::spawn(async move {
+            review_approval_request(
+                &session_for_second,
+                &turn_for_second,
+                "review-shell-guardian-2".to_string(),
+                second_request,
+                Some("trunk follow-up".to_string()),
+            )
+            .await
+        });
+
+        let second_request_observed = tokio::time::timeout(Duration::from_secs(5), async {
+            loop {
+                if server.requests().await.len() >= 2 {
+                    break;
+                }
+                tokio::task::yield_now().await;
+            }
+        })
+        .await;
+        assert!(
+            second_request_observed.is_ok(),
+            "second guardian request was not observed"
+        );
+        session
+            .record_into_history(
+                &[
+                    ResponseItem::Message {
+                        id: None,
+                        role: "user".to_string(),
+                        content: vec![ContentItem::InputText {
+                            text: "Now inspect whether pushing is safe.".to_string(),
+                        }],
+                        end_turn: None,
+                        phase: None,
+                    },
+                    ResponseItem::Message {
+                        id: None,
+                        role: "assistant".to_string(),
+                        content: vec![ContentItem::OutputText {
+                            text: "I need approval to push after the diff check.".to_string(),
+                        }],
+                        end_turn: None,
+                        phase: None,
+                    },
+                ],
+                turn.as_ref(),
+            )
+            .await;
+
+        let third_decision = review_approval_request(
             &session,
             &turn,
-            "review-shell-guardian-1".to_string(),
-            initial_request,
-            /*retry_reason*/ None
-        )
-        .await,
-        ReviewDecision::Approved
-    );
-    session
-        .record_into_history(
-            &[
-                ResponseItem::Message {
-                    id: None,
-                    role: "user".to_string(),
-                    content: vec![ContentItem::InputText {
-                        text: "Please inspect pending changes before pushing.".to_string(),
-                    }],
-                    end_turn: None,
-                    phase: None,
-                },
-                ResponseItem::Message {
-                    id: None,
-                    role: "assistant".to_string(),
-                    content: vec![ContentItem::OutputText {
-                        text: "I need approval to run git diff.".to_string(),
-                    }],
-                    end_turn: None,
-                    phase: None,
-                },
-            ],
-            turn.as_ref(),
+            "review-shell-guardian-3".to_string(),
+            third_request,
+            Some("parallel follow-up".to_string()),
         )
         .await;
+        assert_eq!(third_decision, ReviewDecision::Approved);
+        let requests = server.requests().await;
+        assert_eq!(requests.len(), 3);
+        let third_request_body = serde_json::from_slice::<serde_json::Value>(&requests[2])?;
+        let third_request_body_text = third_request_body.to_string();
+        assert!(
+            third_request_body_text.contains("first guardian rationale"),
+            "forked guardian review should include the last committed trunk assessment"
+        );
+        let third_user_message = last_user_message_text_from_body(&third_request_body);
+        assert!(third_user_message.contains(">>> TRANSCRIPT DELTA START\n"));
+        assert!(
+            third_user_message.contains("[5] user: Please inspect pending changes before pushing.")
+        );
+        assert!(third_user_message.contains("[7] user: Now inspect whether pushing is safe."));
+        assert!(!third_user_message.contains("[1] user: Please check the repo visibility"));
+        assert!(
+            !third_request_body_text.contains("second guardian rationale"),
+            "forked guardian review should not include the still in-flight trunk assessment"
+        );
+        assert!(
+            tokio::time::timeout(Duration::from_millis(100), &mut second_review)
+                .await
+                .is_err(),
+            "the trunk guardian review should still be blocked on its gated response"
+        );
 
-    let second_request = GuardianApprovalRequest::Shell {
-        id: "shell-guardian-2".to_string(),
-        command: vec!["git".to_string(), "diff".to_string()],
-        cwd: test_path_buf("/repo/codex-rs/core").abs(),
-        sandbox_permissions: crate::sandboxing::SandboxPermissions::UseDefault,
-        additional_permissions: None,
-        justification: Some("Inspect pending changes before proceeding.".to_string()),
-    };
-    let third_request = GuardianApprovalRequest::Shell {
-        id: "shell-guardian-3".to_string(),
-        command: vec!["git".to_string(), "push".to_string()],
-        cwd: test_path_buf("/repo/codex-rs/core").abs(),
-        sandbox_permissions: crate::sandboxing::SandboxPermissions::UseDefault,
-        additional_permissions: None,
-        justification: Some("Inspect whether pushing is safe before proceeding.".to_string()),
-    };
+        gate_tx
+            .send(())
+            .expect("second guardian review gate should still be open");
+        assert_eq!(second_review.await?, ReviewDecision::Approved);
+        server.shutdown().await;
 
-    let session_for_second = Arc::clone(&session);
-    let turn_for_second = Arc::clone(&turn);
-    let mut second_review = tokio::spawn(async move {
-        review_approval_request(
-            &session_for_second,
-            &turn_for_second,
-            "review-shell-guardian-2".to_string(),
-            second_request,
-            Some("trunk follow-up".to_string()),
-        )
-        .await
-    });
+        Ok(())
+                }))
+            })?;
 
-    let second_request_observed = tokio::time::timeout(Duration::from_secs(5), async {
-        loop {
-            if server.requests().await.len() >= 2 {
-                break;
-            }
-            tokio::task::yield_now().await;
-        }
-    })
-    .await;
-    assert!(
-        second_request_observed.is_ok(),
-        "second guardian request was not observed"
-    );
-    session
-        .record_into_history(
-            &[
-                ResponseItem::Message {
-                    id: None,
-                    role: "user".to_string(),
-                    content: vec![ContentItem::InputText {
-                        text: "Now inspect whether pushing is safe.".to_string(),
-                    }],
-                    end_turn: None,
-                    phase: None,
-                },
-                ResponseItem::Message {
-                    id: None,
-                    role: "assistant".to_string(),
-                    content: vec![ContentItem::OutputText {
-                        text: "I need approval to push after the diff check.".to_string(),
-                    }],
-                    end_turn: None,
-                    phase: None,
-                },
-            ],
-            turn.as_ref(),
-        )
-        .await;
-
-    let third_decision = review_approval_request(
-        &session,
-        &turn,
-        "review-shell-guardian-3".to_string(),
-        third_request,
-        Some("parallel follow-up".to_string()),
-    )
-    .await;
-    assert_eq!(third_decision, ReviewDecision::Approved);
-    let requests = server.requests().await;
-    assert_eq!(requests.len(), 3);
-    let third_request_body = serde_json::from_slice::<serde_json::Value>(&requests[2])?;
-    let third_request_body_text = third_request_body.to_string();
-    assert!(
-        third_request_body_text.contains("first guardian rationale"),
-        "forked guardian review should include the last committed trunk assessment"
-    );
-    let third_user_message = last_user_message_text_from_body(&third_request_body);
-    assert!(third_user_message.contains(">>> TRANSCRIPT DELTA START\n"));
-    assert!(
-        third_user_message.contains("[5] user: Please inspect pending changes before pushing.")
-    );
-    assert!(third_user_message.contains("[7] user: Now inspect whether pushing is safe."));
-    assert!(!third_user_message.contains("[1] user: Please check the repo visibility"));
-    assert!(
-        !third_request_body_text.contains("second guardian rationale"),
-        "forked guardian review should not include the still in-flight trunk assessment"
-    );
-    assert!(
-        tokio::time::timeout(Duration::from_millis(100), &mut second_review)
-            .await
-            .is_err(),
-        "the trunk guardian review should still be blocked on its gated response"
-    );
-
-    gate_tx
-        .send(())
-        .expect("second guardian review gate should still be open");
-    assert_eq!(second_review.await?, ReviewDecision::Approved);
-    server.shutdown().await;
-
-    Ok(())
+    match handle.join() {
+        Ok(result) => result,
+        Err(_) => Err(anyhow::anyhow!(
+            "guardian_parallel_reviews_fork_from_last_committed_trunk_history thread panicked"
+        )),
+    }
 }
 #[test]
 fn guardian_review_session_config_preserves_parent_network_proxy() {