common/async: add service template for execution_context shutdown

Signed-off-by: Casey Bodley <cbodley@redhat.com>

diff --git a/src/common/async/service.h b/src/common/async/service.h
new file mode 100644 (file)
index 0000000..f611e7c
--- /dev/null
+++ b/src/common/async/service.h
@@ -0,0 +1,75 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright contributors to the Ceph project
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ *
+ */
+
+#pragma once
+
+#include <mutex>
+#include <boost/asio/execution_context.hpp>
+#include <boost/intrusive/list.hpp>
+
+namespace ceph::async {
+
+struct service_tag {};
+using service_list_base_hook = boost::intrusive::list_base_hook<
+    boost::intrusive::tag<service_tag>>;
+
+/// Service for two-phase execution_context shutdown, which breaks ownership
+/// cycles between completion handlers and their io objects. Tracks objects
+/// which may have outstanding completion handlers, and calls their member
+/// function service_shutdown() when the execution_context is shutting down.
+/// This member function should destroy any memory associated with its
+/// outstanding completion handlers.
+///
+/// Requirements for IoObject:
+/// * Inherits publicly from service_list_base_hook
+/// * Has public member function service_shutdown()
+/// * Calls add(*this) on construction and remove(*this) on destruction.
+template <typename IoObject>
+class service : public boost::asio::execution_context::service {
+  using base_hook = boost::intrusive::base_hook<service_list_base_hook>;
+  boost::intrusive::list<IoObject, base_hook> entries;
+  std::mutex mutex;
+
+  /// Called by the execution_context on shutdown
+  void shutdown() override {
+    while (!entries.empty()) {
+      auto& entry = entries.front();
+      entries.pop_front();
+      entry.service_shutdown();
+    }
+  }
+ public:
+  using key_type = service;
+  static inline boost::asio::execution_context::id id;
+
+  explicit service(boost::asio::execution_context& ctx)
+      : boost::asio::execution_context::service(ctx) {}
+
+  /// Register an io object for notification of service_shutdown()
+  void add(IoObject& entry) {
+    auto lock = std::scoped_lock{mutex};
+    entries.push_back(entry);
+  }
+  /// Unregister an object
+  void remove(IoObject& entry) {
+    auto lock = std::scoped_lock{mutex};
+    if (entries.empty()) {
+      // already shut down
+    } else {
+      entries.erase(entries.iterator_to(entry));
+    }
+  }
+};
+
+} // namespace ceph::async