From bb529d03175fc988a9d7b593bf60e21b6803cbb7 Mon Sep 17 00:00:00 2001
From: Jingchen <a1q123456@users.noreply.github.com>
Date: Mon, 26 Jan 2026 21:39:28 +0000
Subject: [PATCH] fix: Stop embedded tests from hanging on ARM by using
 `atomic_flag` (#6248)

This change replaces the mutex `stoppingMutex_`, the `atomic_bool` variable `isTimeToStop`, and the conditional variable `stoppingCondition_` with an `atomic_flag` variable.

When `xrpld` is running the embedded tests as a child process, it has a control thread (the app bundle thread) that starts the application, and an application thread (the thread that executes `app_->run()`). Due to the relaxed memory ordering on ARM, it's not guaranteed that the application thread can see the change of the value resulting from the `isTimeToStop.exchange(true)` call before it is notified by `stoppingCondition_.notify_all()`, even though they do happen in the right order in the app bundle thread in `ApplicationImp::signalStop`. We therefore often get into the situation where `isTimeToStop` is `true`, but the application thread is waiting for `stoppingCondition_` to notify, because the app bundle thread may have already notified before the application thread actually starts waiting.

Switching to a single `atomic_flag` variable makes sure that there's only one synchronisation object and then the memory order guarantee provided by c++ can make sure that `notify_all` gets synchronised after `test_and_set` does.

Fixing this issue will stop the unit tests hanging forever and then we should see less (or hopefully no) time out errors in daily github action runs
---
 src/xrpld/app/main/Application.cpp | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/src/xrpld/app/main/Application.cpp b/src/xrpld/app/main/Application.cpp
index 15abff9b14..785b02dbb8 100644
--- a/src/xrpld/app/main/Application.cpp
+++ b/src/xrpld/app/main/Application.cpp
@@ -203,11 +203,7 @@ public:
 
     boost::asio::signal_set m_signals;
 
-    // Once we get C++20, we could use `std::atomic_flag` for `isTimeToStop`
-    // and eliminate the need for the condition variable and the mutex.
-    std::condition_variable stoppingCondition_;
-    mutable std::mutex stoppingMutex_;
-    std::atomic<bool> isTimeToStop = false;
+    std::atomic_flag isTimeToStop;
 
     std::atomic<bool> checkSigs_;
 
@@ -1539,10 +1535,7 @@ ApplicationImp::run()
         getLoadManager().activateStallDetector();
     }
 
-    {
-        std::unique_lock<std::mutex> lk{stoppingMutex_};
-        stoppingCondition_.wait(lk, [this] { return isTimeToStop.load(); });
-    }
+    isTimeToStop.wait(false, std::memory_order_relaxed);
 
     JLOG(m_journal.debug()) << "Application stopping";
 
@@ -1629,14 +1622,14 @@ ApplicationImp::run()
 void
 ApplicationImp::signalStop(std::string msg)
 {
-    if (!isTimeToStop.exchange(true))
+    if (!isTimeToStop.test_and_set(std::memory_order_acquire))
     {
         if (msg.empty())
             JLOG(m_journal.warn()) << "Server stopping";
         else
             JLOG(m_journal.warn()) << "Server stopping: " << msg;
 
-        stoppingCondition_.notify_all();
+        isTimeToStop.notify_all();
     }
 }
 
@@ -1655,7 +1648,7 @@ ApplicationImp::checkSigs(bool check)
 bool
 ApplicationImp::isStopping() const
 {
-    return isTimeToStop.load();
+    return isTimeToStop.test(std::memory_order_relaxed);
 }
 
 int