SERVER-45815 Make observer for topologyVersion change

1 files changed, 141 insertions, 0 deletions

diff --git a/src/mongo/db/repl/topology_version_observer.h b/src/mongo/db/repl/topology_version_observer.h
new file mode 100644
index 00000000000..afc55321ab2
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+++ b/src/mongo/db/repl/topology_version_observer.h
@@ -0,0 +1,141 @@
+/**
+ *    Copyright (C) 2020-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <boost/none.hpp>
+#include <boost/optional.hpp>
+#include <memory>
+#include <string>
+
+#include "mongo/db/repl/is_master_response.h"
+#include "mongo/db/repl/replication_coordinator.h"
+#include "mongo/platform/atomic_word.h"
+#include "mongo/platform/mutex.h"
+#include "mongo/stdx/thread.h"
+#include "mongo/util/time_support.h"
+
+namespace mongo {
+
+class Client;
+
+namespace repl {
+
+constexpr auto kTopologyVersionObserverName = "TopologyVersionObserver";
+
+/**
+ * An utility to observe topology changes asynchronously and cache updates.
+ * `getCached()` is thread-safe (through `_mutex`), but its behavior is undefined during the
+ * execution of the constructor/destructor methods.
+ *
+ * The life-cycle for instances of this class (`_state`) is as follows:
+ * * Uninitialized: immediately after construction.
+ *   Call `init()` to initialize the instance and start the observer thread.
+ *   You may only call `init()` once -- otherwise, it will terminate the process.
+ * * Running: anytime after returning from `init()` and before calling `shutdown()`.
+ *   Note that if the observer thread stops due to an error, it will set the state to Shutdown.
+ * * Shutdown: the object is ready for destruction.
+ *   You must wait for `shutdown()` to return before deleting the object.
+ *   Multiple, multithreaded calls to `shutdown()` is safe, and only one will proceed.
+ *   After transitioning to shutdown, you can only call the destructor.
+ *
+ * constructor() -> init() -> getCached() ... getCached() -> shutdown() -> destructor()
+ */
+class TopologyVersionObserver final {
+public:
+    TopologyVersionObserver() {}
+
+    TopologyVersionObserver(const TopologyVersionObserver&) = delete;
+
+    TopologyVersionObserver(TopologyVersionObserver&&) noexcept = delete;
+
+    TopologyVersionObserver& operator=(const TopologyVersionObserver&) = delete;
+
+    TopologyVersionObserver& operator=(TopologyVersionObserver&&) = delete;
+
+    ~TopologyVersionObserver() {
+        auto state = _state.load();
+        invariant(state == State::kShutdown || state == State::kUninitialized);
+    }
+
+    void init(ReplicationCoordinator* replCoordinator) noexcept;
+
+    void shutdown() noexcept;
+
+    /**
+     * Returns a reference (shared pointer) to the cached version of `IsMasterResponse`.
+     * Note that the reference is initially set to `nullptr`.
+     * Also, the reference is set back to `nullptr` if the thread that updates `_cache` terminates
+     * due to an error (i.e., exception), or it receives an invalid response.
+     */
+    std::shared_ptr<const IsMasterResponse> getCached() noexcept;
+
+    std::string toString() const;
+
+private:
+    enum class State {
+        kUninitialized,
+        kRunning,
+        kShutdown,
+    };
+
+    std::shared_ptr<const IsMasterResponse> _getIsMasterResponse(boost::optional<TopologyVersion>,
+                                                                 bool*);
+
+    void _workerThreadBody() noexcept;
+
+    /**
+     * Protects shared accesses to `_cache`, `_observerClient`, and serializes calls to `init()`
+     * and `shutdown()` methods.
+     *
+     * Accessing the cached `IsMasterResponse` follows a single-producer, multi-consumer model:
+     * consumers are readers of `_cache` and the producer is the observer thread. The assumption
+     * is that the contention on this lock is insignificant.
+     */
+    mutable Mutex _mutex = MONGO_MAKE_LATCH(kTopologyVersionObserverName);
+
+    // The reference to the latest cached version of `IsMasterResponse`
+    std::shared_ptr<const IsMasterResponse> _cache;
+
+    // Holds a reference to the observer client to allow `shutdown()` to stop the observer thread.
+    Client* _observerClient;
+
+    /**
+     * Represents the current state of the observer.
+     * Note that the observer thread never updates the state.
+     */
+    AtomicWord<State> _state;
+
+    boost::optional<stdx::thread> _thread;
+
+    ReplicationCoordinator* _replCoordinator;
+};
+
+}  // namespace repl
+}  // namespace mongo