From 2255f824d44caf48f9c8b0e23ffaf8483f4e0afe Mon Sep 17 00:00:00 2001
From: Amirsaman Memaripour <amirsaman.memaripour@mongodb.com>
Date: Thu, 28 Apr 2022 15:55:44 +0000
Subject: SERVER-64191 Strictly order cancellation/scheduling of async
 operations on `Session`

---
 src/mongo/transport/session_asio.h | 60 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)

(limited to 'src/mongo/transport/session_asio.h')

diff --git a/src/mongo/transport/session_asio.h b/src/mongo/transport/session_asio.h
index aa747202945..5142d60202d 100644
--- a/src/mongo/transport/session_asio.h
+++ b/src/mongo/transport/session_asio.h
@@ -34,6 +34,7 @@
 #include "mongo/base/system_error.h"
 #include "mongo/config.h"
 #include "mongo/db/stats/counters.h"
+#include "mongo/stdx/mutex.h"
 #include "mongo/transport/asio_utils.h"
 #include "mongo/transport/baton.h"
 #include "mongo/transport/ssl_connection_context.h"
@@ -160,10 +161,57 @@ protected:
     void ensureAsync();
 
 private:
+    /**
+     * Provides the means to track and cancel async I/O operations scheduled through `Session`.
+     * Any I/O operation goes through the following steps:
+     * - `start()`: changes the state from `kNotStarted` to `kRunning`.
+     * - Before scheduling the async operation, checks for cancellation through `isCanceled()`.
+     * - `complete()`: clears the state, and prepares the session for future operations.
+     *
+     * This class is thread-safe.
+     */
+    class AsyncOperationState {
+    public:
+        void start() {
+            const auto prev = _state.swap(State::kRunning);
+            invariant(prev == State::kNotStarted, "Another operation was in progress");
+        }
+
+        bool isCanceled() const {
+            return _state.load() == State::kCanceled;
+        }
+
+        void complete() {
+            const auto prev = _state.swap(State::kNotStarted);
+            invariant(prev != State::kNotStarted, "No operation was running");
+        }
+
+        /**
+         * Instructs an active operation to cancel (if there is any). Otherwise, it does nothing.
+         * Cancellation is non-blocking and `cancel()` doesn't block for completion of ongoing
+         * operations.
+         */
+        void cancel() {
+            auto expected = State::kRunning;
+            _state.compareAndSwap(&expected, State::kCanceled);
+        }
+
+    private:
+        /**
+         * State transition diagram:
+         * -+-> [kNotStarted] --> [kRunning] --> [kCanceled]
+         *  |                          |              |
+         *  +--------------------------+--------------+
+         */
+        enum class State { kNotStarted, kRunning, kCanceled };
+        AtomicWord<State> _state{State::kNotStarted};
+    };
+
     GenericSocket& getSocket();
 
     ExecutorFuture<void> parseProxyProtocolHeader(const ReactorHandle& reactor);
     Future<Message> sourceMessageImpl(const BatonHandle& baton = nullptr);
+    Future<void> sinkMessageImpl(Message message, const BatonHandle& baton = nullptr);
 
     template <typename MutableBufferSequence>
     Future<void> read(const MutableBufferSequence& buffers, const BatonHandle& baton = nullptr);
@@ -257,6 +305,18 @@ private:
     bool _isFromLoadBalancer = false;
     boost::optional<SockAddr> _proxiedSrcEndpoint;
     boost::optional<SockAddr> _proxiedDstEndpoint;
+
+    AsyncOperationState _asyncOpState;
+
+    /**
+     * The following mutex strictly orders the start and cancellation of asynchronous operations:
+     * - Holding the mutex while starting asynchronous operations (e.g., adding the session to the
+     *   networking baton) ensures cancellation either happens before or after scheduling of the
+     *   operation.
+     * - Holding the mutex while canceling asynchronous operations guarantees no operation can start
+     *   while cancellation is in progress.
+     */
+    stdx::mutex _asyncOpMutex;  // NOLINT
 };
 
 }  // namespace mongo::transport
-- 
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