Introduce WaitGroup and StoppableWaitGroup

lib/base/CMakeLists.txt

@@ -87,6 +87,7 @@ set(base_SOURCES
   unixsocket.cpp unixsocket.hpp
   utility.cpp utility.hpp
   value.cpp value.hpp value-operators.cpp
+  wait-group.cpp wait-group.hpp
   win32.hpp
   workqueue.cpp workqueue.hpp
 )

lib/base/wait-group.cpp

@@ -0,0 +1,38 @@
+/* Icinga 2 | (c) 2025 Icinga GmbH | GPLv2+ */
+
+#include "base/wait-group.hpp"
+
+using namespace icinga;
+
+bool StoppableWaitGroup::try_lock_shared()
+{
+	std::unique_lock lock (m_Mutex);
+
+	if (m_Stopped) {
+		return false;
+	}
+
+	++m_SharedLocks;
+	return true;
+}
+
+void StoppableWaitGroup::unlock_shared()
+{
+	std::unique_lock lock (m_Mutex);
+
+	if (!--m_SharedLocks && m_Stopped) {
+		lock.unlock();
+		m_CV.notify_all();
+	}
+}
+
+/**
+ * Disallow new shared locks, wait for all existing ones.
+ */
+void StoppableWaitGroup::Join()
+{
+	std::unique_lock lock (m_Mutex);
+
+	m_Stopped = true;
+	m_CV.wait(lock, [this] { return !m_SharedLocks; });
+}

lib/base/wait-group.hpp

@@ -0,0 +1,54 @@
+/* Icinga 2 | (c) 2025 Icinga GmbH | GPLv2+ */
+
+#pragma once
+
+#include "base/object.hpp"
+#include <condition_variable>
+#include <cstdint>
+#include <mutex>
+
+namespace icinga
+{
+
+/**
+ * A synchronization interface that allows concurrent shared locking.
+ *
+ * @ingroup base
+ */
+class WaitGroup : public Object
+{
+public:
+	DECLARE_PTR_TYPEDEFS(WaitGroup);
+
+	virtual bool try_lock_shared() = 0;
+	virtual void unlock_shared() = 0;
+};
+
+/**
+ * A thread-safe wait group that can be stopped to prevent further shared locking.
+ *
+ * @ingroup base
+ */
+class StoppableWaitGroup : public WaitGroup
+{
+public:
+	DECLARE_PTR_TYPEDEFS(StoppableWaitGroup);
+
+	StoppableWaitGroup() = default;
+	StoppableWaitGroup(const StoppableWaitGroup&) = delete;
+	StoppableWaitGroup(StoppableWaitGroup&&) = delete;
+	StoppableWaitGroup& operator=(const StoppableWaitGroup&) = delete;
+	StoppableWaitGroup& operator=(StoppableWaitGroup&&) = delete;
+
+	bool try_lock_shared() override;
+	void unlock_shared() override;
+	void Join();
+
+private:
+	std::mutex m_Mutex;
+	std::condition_variable m_CV;
+	uint_fast32_t m_SharedLocks = 0;
+	bool m_Stopped = false;
+};
+
+}