From 3928517d3755fe5cff303f8098ebc2e3648161cd Mon Sep 17 00:00:00 2001
From: Tolya Korniltsev <anatoly.korniltsev@grafana.com>
Date: Tue, 14 Apr 2026 11:51:35 +1000
Subject: [PATCH] fix: make ProfilerSignalManager::_handler atomic to prevent
 data race
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_handler is written on a normal thread in RegisterHandler and read inside
a signal handler in CallCustomHandler. Without atomics this is undefined
behavior — the C++ standard requires lock-free atomics for signal-safe
access ([support.signal]/3).

Also fixes the double-checked locking in RegisterHandler: the second
check after acquiring the mutex was comparing a stale local copy instead
of re-loading from the (now atomic) _handler.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 .../ProfilerSignalManager.cpp                            | 9 +++++----
 .../ProfilerSignalManager.h                              | 4 ++--
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.cpp b/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.cpp
index 3f4fd1feb982..cb9e61fb24e8 100644
--- a/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.cpp
+++ b/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.cpp
@@ -28,7 +28,7 @@ ProfilerSignalManager::~ProfilerSignalManager() noexcept
         _isHandlerInPlace = false;
         sigaction(_signalToSend, &_previousAction, nullptr);
     }
-    _handler = nullptr;
+    _handler.store(nullptr, std::memory_order_relaxed);
 }
 
 ProfilerSignalManager* ProfilerSignalManager::Get(int signal)
@@ -49,7 +49,7 @@ ProfilerSignalManager* ProfilerSignalManager::Get(int signal)
 
 bool ProfilerSignalManager::RegisterHandler(HandlerFn_t handler)
 {
-    HandlerFn_t current = _handler;
+    HandlerFn_t current = _handler.load(std::memory_order_acquire);
 
     if (current != nullptr)
     {
@@ -58,6 +58,7 @@ bool ProfilerSignalManager::RegisterHandler(HandlerFn_t handler)
     }
     std::unique_lock<std::mutex> lock(_handlerRegisterMutex);
 
+    current = _handler.load(std::memory_order_acquire);
     if (current != nullptr)
     {
         assert(current == handler);
@@ -68,7 +69,7 @@ bool ProfilerSignalManager::RegisterHandler(HandlerFn_t handler)
 
     if (_isHandlerInPlace)
     {
-        _handler = handler;
+        _handler.store(handler, std::memory_order_release);
     }
 
     return _isHandlerInPlace;
@@ -204,7 +205,7 @@ void ProfilerSignalManager::SignalHandler(int signal, siginfo_t* info, void* con
 
 bool ProfilerSignalManager::CallCustomHandler(int32_t signal, siginfo_t* info, void* context)
 {
-    HandlerFn_t handler = _handler;
+    HandlerFn_t handler = _handler.load(std::memory_order_acquire);
     return handler != nullptr && handler(signal, info, context);
 }
 
diff --git a/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.h b/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.h
index a4f4dcc50b62..8593e616ca90 100644
--- a/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.h
+++ b/profiler/src/ProfilerEngine/Datadog.Profiler.Native.Linux/ProfilerSignalManager.h
@@ -27,7 +27,7 @@ class ProfilerSignalManager
 #ifdef DD_TEST
     void Reset()
     {
-        _handler = nullptr;
+        _handler.store(nullptr, std::memory_order_relaxed);
         sigaction(_signalToSend, &_previousAction, nullptr);
         _isHandlerInPlace = false;
         _canReplaceSignalHandler = true;
@@ -56,7 +56,7 @@ class ProfilerSignalManager
 private:
     bool _canReplaceSignalHandler;
     int32_t _signalToSend;
-    HandlerFn_t _handler;
+    std::atomic<HandlerFn_t> _handler;
     pid_t _processId;
     bool _isHandlerInPlace;
     struct sigaction _previousAction;