Fixed a data race when using UV loop data

src/merge_mining_client_json_rpc.h

@@ -76,7 +76,7 @@ private:
 
 		uv_close(reinterpret_cast<uv_handle_t*>(&client->m_shutdownAsync), nullptr);
 
-		delete GetLoopUserData(&client->m_loop, false);
+		DeleteLoopUserData(&client->m_loop);
 	}
 
 	void on_shutdown();

src/p2pool.cpp

@@ -769,9 +769,7 @@ void p2pool::on_stop(uv_async_t* async)
 
 	init_signals(pool, false);
 
-	uv_loop_t* loop = uv_default_loop_checked();
-	delete GetLoopUserData(loop, false);
-	loop->data = nullptr;
+	DeleteLoopUserData(uv_default_loop_checked());
 }
 
 void p2pool::submit_block() const

src/pow_hash.h

@@ -121,7 +121,7 @@ private:
 		RandomX_Hasher_RPC* server = reinterpret_cast<RandomX_Hasher_RPC*>(async->data);
 		uv_close(reinterpret_cast<uv_handle_t*>(&server->m_shutdownAsync), nullptr);
 
-		delete GetLoopUserData(&server->m_loop, false);
+		DeleteLoopUserData(&server->m_loop);
 	}
 };
 

src/tcp_server.cpp

@@ -879,7 +879,7 @@ void TCPServer::on_shutdown(uv_async_t* async)
 	uv_close(reinterpret_cast<uv_handle_t*>(&s->m_dropConnectionsAsync), nullptr);
 	uv_close(reinterpret_cast<uv_handle_t*>(&s->m_shutdownAsync), nullptr);
 
-	delete GetLoopUserData(&s->m_loop, false);
+	DeleteLoopUserData(&s->m_loop);
 
 	s->m_numHandles = 0;
 	uv_walk(&s->m_loop, [](uv_handle_t*, void* n) { (*reinterpret_cast<uint32_t*>(n))++; }, &s->m_numHandles);

src/util.cpp

@@ -667,14 +667,30 @@ bool is_localhost(const std::string& host)
 
 UV_LoopUserData* GetLoopUserData(uv_loop_t* loop, bool create)
 {
-	UV_LoopUserData* data = reinterpret_cast<UV_LoopUserData*>(loop->data);
+	static_assert(sizeof(std::atomic<UV_LoopUserData*>) <= sizeof(void*), "loop->data size mismatch");
+	static_assert(alignof(std::atomic<UV_LoopUserData*>) <= alignof(void*), "loop->data alignment mismatch");
 
-	if (!data && create) {
-		data = new UV_LoopUserData(loop);
-		loop->data = data;
+	std::atomic<UV_LoopUserData*>& data = reinterpret_cast<std::atomic<UV_LoopUserData*>&>(loop->data);
+
+	UV_LoopUserData* result = data.load();
+
+	if (!result && create) {
+		UV_LoopUserData* new_data = new UV_LoopUserData(loop);
+
+		if (data.compare_exchange_strong(result, new_data)) {
+			result = new_data;
+		} else {
+			delete new_data;
+		}
 	}
 
-	return data;
+	return result;
+}
+
+void DeleteLoopUserData(uv_loop_t* loop)
+{
+	std::atomic<UV_LoopUserData*>& data = reinterpret_cast<std::atomic<UV_LoopUserData*>&>(loop->data);
+	delete data.exchange(nullptr);
 }
 
 #ifdef WITH_UPNP

src/uv_util.h

@@ -94,7 +94,6 @@ struct UV_LoopUserData
 
 	~UV_LoopUserData()
 	{
-		m_loop->data = nullptr;
 		uv_mutex_destroy(&m_callbacksLock);
 		uv_close(reinterpret_cast<uv_handle_t*>(m_async), [](uv_handle_t* h) { delete reinterpret_cast<uv_async_t*>(h); });
 		for (const UV_LoopCallbackBase* cb : m_callbacks) {
@@ -123,6 +122,7 @@ struct UV_LoopUserData
 };
 
 UV_LoopUserData* GetLoopUserData(uv_loop_t* loop, bool create = true);
+void DeleteLoopUserData(uv_loop_t* loop);
 
 template<typename T>
 bool CallOnLoop(uv_loop_t* loop, T&& callback)