Added WaitpidQueue to handle background process reaping.

src/utils/waitpid_queue.cc

@@ -0,0 +1,115 @@
+#include "config.h"
+
+#include "utils/waitpid_queue.h"
+
+#include <sys/wait.h>
+#include <torrent/exceptions.h>
+
+namespace utils {
+
+WaitpidQueue::WaitpidQueue() {
+  m_worker = std::async(std::launch::async, [this]() {
+      bool is_running = true;
+      auto wait_time  = 50ms;
+
+      while (is_running) {
+        if (!m_queue.empty()) {
+          auto start_time = std::chrono::steady_clock::now();
+
+          while (!m_wakeup_worker.load(std::memory_order_acquire) && !m_should_shutdown) {
+            auto elapsed = std::chrono::steady_clock::now() - start_time;
+
+            if (elapsed >= wait_time)
+              break;
+
+            std::this_thread::sleep_for(50ms);
+          }
+
+        } else {
+          m_wakeup_worker.wait(false, std::memory_order_acquire);
+          std::this_thread::sleep_for(50ms);
+        }
+
+        std::set<pid_t> queue;
+
+        {
+          std::lock_guard<std::mutex> guard(m_mutex);
+
+          if (m_should_shutdown) {
+            if (m_queue.empty())
+              return;
+
+            is_running = false;
+          }
+
+          if (m_queue.empty())
+            throw torrent::internal_error("WaitpidQueue worker thread woke up but queue is empty.");
+
+          queue = m_queue;
+
+          m_wakeup_worker.store(false, std::memory_order_release);
+        }
+
+        wait_time = std::min(10 * 1000ms, wait_time * 2);
+
+        for (auto pid : queue) {
+          if (::waitpid(pid, nullptr, WNOHANG) == 0)
+            continue;
+
+          {
+            std::lock_guard<std::mutex> guard(m_mutex);
+
+            if (m_queue.erase(pid) != 1)
+              throw torrent::internal_error("WaitpidQueue worker thread could not find pid in queue.");
+          }
+
+          wait_time = std::max(50ms, wait_time / 2);
+
+          m_remaining.fetch_sub(1, std::memory_order_release);
+          m_remaining.notify_all();
+        }
+      }
+    });
+}
+
+WaitpidQueue::~WaitpidQueue() {
+  {
+    std::lock_guard<std::mutex> guard(m_mutex);
+    m_should_shutdown = true;
+  }
+
+  m_wakeup_worker.store(true, std::memory_order_release);
+  m_wakeup_worker.notify_all();
+
+  // m_worker.wait();
+}
+
+void
+WaitpidQueue::close_pid(pid_t pid) {
+  if (pid < 0)
+    throw torrent::internal_error("WaitpidQueue::close_pid() called with invalid pid.");
+
+  m_remaining.fetch_add(1, std::memory_order_acquire);
+
+  {
+    std::lock_guard<std::mutex> guard(m_mutex);
+
+    // if (!m_queue.empty()) {
+    //   m_queue.push_back(pid);
+    //   return;
+    // }
+
+    m_queue.insert(pid);
+  }
+
+  m_wakeup_worker.store(true, std::memory_order_release);
+  m_wakeup_worker.notify_all();
+}
+
+void
+WaitpidQueue::wait_for(uint32_t max_remaining) {
+  while (m_remaining.load(std::memory_order_acquire) > max_remaining)
+    m_remaining.wait(max_remaining, std::memory_order_acquire);
+}
+
+} // namespace torrent::utils