From b731fe28cc492cb36c51c89866f5b63a3ffae2aa Mon Sep 17 00:00:00 2001
From: Christian Van <113378434+cvan20191@users.noreply.github.com>
Date: Wed, 17 Dec 2025 06:22:26 -0500
Subject: [PATCH] Improve cyclic thenable detection in ReactFlightReplyServer
 (#35369)

## Summary

This PR improves cyclic thenable detection in
`ReactFlightReplyServer.js`. Fixes #35368.
The previous fix only detected direct self-references (`inspectedValue
=== chunk`) and relied on the `cycleProtection` counter to eventually
bail out of longer cycles. This change keeps the existing
MAX_THENABLE_CYCLE_DEPTH ($1000$) `cycleProtection` cap as a hard
guardrail and adds a visited set so that we can detect self-cycles and
multi-node cycles as soon as any `ReactPromise` is revisited and while
still bounding the amount of work we do for deep acyclic chains via
`cycleProtection`.

## How did you test this change?

- Ran the existing test suite for the server renderer:

  ```bash
  yarn test react-server
  yarn test --prod react-server
  yarn flow dom-node
  yarn linc
  ```

---------

Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
---
 packages/react-server/src/ReactFlightReplyServer.js | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/packages/react-server/src/ReactFlightReplyServer.js b/packages/react-server/src/ReactFlightReplyServer.js
index 4d5a6da8b8..88781cc6eb 100644
--- a/packages/react-server/src/ReactFlightReplyServer.js
+++ b/packages/react-server/src/ReactFlightReplyServer.js
@@ -133,14 +133,20 @@ ReactPromise.prototype.then = function <T>(
         // Recursively check if the value is itself a ReactPromise and if so if it points
         // back to itself. This helps catch recursive thenables early error.
         let cycleProtection = 0;
+        const visited = new Set<typeof ReactPromise>();
         while (inspectedValue instanceof ReactPromise) {
           cycleProtection++;
-          if (inspectedValue === chunk || cycleProtection > 1000) {
+          if (
+            inspectedValue === chunk ||
+            visited.has(inspectedValue) ||
+            cycleProtection > 1000
+          ) {
             if (typeof reject === 'function') {
               reject(new Error('Cannot have cyclic thenables.'));
             }
             return;
           }
+          visited.add(inspectedValue);
           if (inspectedValue.status === INITIALIZED) {
             inspectedValue = inspectedValue.value;
           } else {