Added WaitpidQueue to handle background process reaping.

src/utils/waitpid_queue.h

@@ -0,0 +1,44 @@
+#ifndef RTORRENT_UTILS_WAITPID_QUEUE_H
+#define RTORRENT_UTILS_WAITPID_QUEUE_H
+
+#include <future>
+#include <set>
+#include <torrent/system/common.h>
+
+namespace utils {
+
+class WaitpidQueue {
+public:
+  WaitpidQueue();
+  ~WaitpidQueue();
+
+  uint32_t            size() const;
+
+  void                close_pid(int pid);
+
+  void                wait_for(uint32_t max_remaining);
+
+private:
+  WaitpidQueue(const WaitpidQueue&) = delete;
+  WaitpidQueue& operator=(const WaitpidQueue&) = delete;
+
+  std::future<void>   m_worker;
+
+  align_cacheline
+
+  std::mutex          m_mutex;
+  std::set<pid_t>     m_queue;
+
+  bool                m_should_shutdown{};
+
+  align_cacheline
+
+  std::atomic<bool>     m_wakeup_worker{};
+  std::atomic<uint32_t> m_remaining{};
+};
+
+inline uint32_t WaitpidQueue::size() const { return m_remaining.load(std::memory_order_acquire); }
+
+} // namespace utils
+
+#endif