diff --git a/src/pid_watcher.cc b/src/pid_watcher.cc
new file mode 100644
index 00000000..cc3d5b1e
--- /dev/null
+++ b/src/pid_watcher.cc
@@ -0,0 +1,20 @@
+#include "config.h"
+
+#include "pid_watcher.h"
+
+#include <cassert>
+#include <torrent/exceptions.h>
+
+PidWatcher::PidWatcher(std::thread::id thread_id)
+  : m_thread_id(thread_id) {
+}
+
+PidWatcher::~PidWatcher() = default;
+
+void
+PidWatcher::prepare_spawn() {
+  assert(std::this_thread::get_id() == m_thread_id);
+
+  if (m_spawn_in_progress.exchange(true, std::memory_order_acquire))
+    throw torrent::internal_error("PidWatcher::prepare_spawn() called while spawn already in progress.");
+}
diff --git a/src/pid_watcher.h b/src/pid_watcher.h
new file mode 100644
index 00000000..93ea70d3
--- /dev/null
+++ b/src/pid_watcher.h
@@ -0,0 +1,27 @@
+#ifndef RTORRENT_PID_WATCHER_H
+#define RTORRENT_PID_WATCHER_H
+
+#include <torrent/common.h>
+
+struct atomic_queue {
+  std::array<std::atomic<pid_t>, 16> queue{};
+
+
+class PidWatcher {
+public:
+  PidWatcher(std::thread::id thread_id);
+  ~PidWatcher();
+
+  void                prepare_spawn();
+  void                completed_spawn(pid_t pid);
+
+private:
+  std::thread::id     m_thread_id;
+
+  align_cacheline
+
+  std::atomic<bool>   m_spawn_in_progress{};
+
+};
+
+#endif