SERVER-39146 Refactor Baton

Refactor the baton into regular and networking batons while also
cleaning up the basic baton implementation.

1 files changed, 107 insertions, 0 deletions

diff --git a/src/mongo/db/baton.h b/src/mongo/db/baton.h
new file mode 100644
index 00000000000..6e1d1319d2b
--- /dev/null
+++ b/src/mongo/db/baton.h
@@ -0,0 +1,107 @@
+/**
+ *    Copyright (C) 2019-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 <memory>
+
+#include "mongo/util/functional.h"
+#include "mongo/util/future.h"
+#include "mongo/util/time_support.h"
+#include "mongo/util/waitable.h"
+
+namespace mongo {
+
+class OperationContext;
+
+namespace transport {
+
+class NetworkingBaton;
+
+}  // namespace transport
+
+/**
+ * A Baton is lightweight executor, with parallel forward progress guarantees.  Rather than
+ * asynchronously running tasks through one, the baton records the intent of those tasks and defers
+ * waiting and execution to a later call to run().
+ *
+ * Note: This occurs automatically when opCtx waiting on a condition variable.
+ */
+class Baton : public Waitable, public std::enable_shared_from_this<Baton> {
+public:
+    virtual ~Baton() = default;
+
+    /**
+     * Detaches a baton from an associated opCtx.
+     *
+     * This invokes all callbacks currently inside the baton (from the detaching thread) with a void
+     * opCtx.  Also, any calls to schedule after this point will immediately invoke their callback
+     * with a null opCtx.
+     */
+    void detach() noexcept {
+        // We make this anchor so that deleting the shared_ptr inside opCtx doesn't remove the last
+        // reference to this type until we return from detach.
+        const auto anchor = shared_from_this();
+
+        detachImpl();
+    }
+
+    /**
+     * Schedules a callback to run on the baton.
+     *
+     * The function will be invoked in one of 3 ways:
+     *   1. With an opCtx inside run() (I.e. opCtx->waitForConditionOrInterrupt)
+     *   2. Without an opCtx inside detach()
+     *   3. Without an opCtx inside schedule() after detach()
+     *
+     * Note that the latter two cases are indistinguishable, so the callback should be safe to run
+     * inline if passed a nullptr.  Examples of such work are logging, simple cleanup and
+     * rescheduling the task on another executor.
+     */
+    virtual void schedule(unique_function<void(OperationContext*)> func) noexcept = 0;
+
+    /**
+     * Returns a networking view of the baton, if this baton supports networking functionality
+     */
+    virtual transport::NetworkingBaton* networking() noexcept {
+        return nullptr;
+    }
+
+    /**
+     * Marks the baton to wake up on client socket disconnect
+     */
+    virtual void markKillOnClientDisconnect() noexcept = 0;
+
+private:
+    virtual void detachImpl() noexcept = 0;
+};
+
+using BatonHandle = std::shared_ptr<Baton>;
+
+}  // namespace mongo