active_migrations_registry.h

/**
 *    Copyright (C) 2018-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/optional.hpp>
#include <memory>

#include "mongo/db/s/migration_session_id.h"
#include "mongo/platform/mutex.h"
#include "mongo/s/request_types/move_chunk_request.h"
#include "mongo/util/concurrency/notification.h"

namespace mongo {

class OperationContext;
class ScopedDonateChunk;
class ScopedReceiveChunk;
template <typename T>
class StatusWith;

/**
 * Thread-safe object that keeps track of the active migrations running on a node and limits them
 * to only one per shard. There is only one instance of this object per shard.
 */
class ActiveMigrationsRegistry {
    ActiveMigrationsRegistry(const ActiveMigrationsRegistry&) = delete;
    ActiveMigrationsRegistry& operator=(const ActiveMigrationsRegistry&) = delete;

public:
    ActiveMigrationsRegistry();
    ~ActiveMigrationsRegistry();

    static ActiveMigrationsRegistry& get(ServiceContext* service);
    static ActiveMigrationsRegistry& get(OperationContext* opCtx);

    /**
     * These methods can be used to block migrations temporarily. The lock() method will block if
     * there is a migration operation in progress and will return once it is completed. Any
     * subsequent migration operations will return ConflictingOperationInProgress until the unlock()
     * method is called.
     */
    void lock(OperationContext* opCtx, StringData reason);
    void unlock(StringData reason);

    /**
     * If there are no migrations running on this shard, registers an active migration with the
     * specified arguments. Returns a ScopedDonateChunk, which must be signaled by the
     * caller before it goes out of scope.
     *
     * If there is an active migration already running on this shard and it has the exact same
     * arguments, returns a ScopedDonateChunk. The ScopedDonateChunk can be used to join the
     * already running migration.
     *
     * Otherwise returns a ConflictingOperationInProgress error.
     */
    StatusWith<ScopedDonateChunk> registerDonateChunk(OperationContext* opCtx,
                                                      const MoveChunkRequest& args);

    /**
     * If there are no migrations running on this shard, registers an active receive operation with
     * the specified session id and returns a ScopedReceiveChunk. The ScopedReceiveChunk will
     * unregister the migration when the ScopedReceiveChunk goes out of scope.
     *
     * Otherwise returns a ConflictingOperationInProgress error.
     */
    StatusWith<ScopedReceiveChunk> registerReceiveChunk(OperationContext* opCtx,
                                                        const NamespaceString& nss,
                                                        const ChunkRange& chunkRange,
                                                        const ShardId& fromShardId);

    /**
     * If a migration has been previously registered through a call to registerDonateChunk, returns
     * that namespace. Otherwise returns boost::none.
     */
    boost::optional<NamespaceString> getActiveDonateChunkNss();

    /**
     * Returns a report on the active migration if there currently is one. Otherwise, returns an
     * empty BSONObj.
     *
     * Takes an IS lock on the namespace of the active migration, if one is active.
     */
    BSONObj getActiveMigrationStatusReport(OperationContext* opCtx);

private:
    friend class ScopedDonateChunk;
    friend class ScopedReceiveChunk;

    // Describes the state of a currently active moveChunk operation
    struct ActiveMoveChunkState {
        ActiveMoveChunkState(MoveChunkRequest inArgs)
            : args(std::move(inArgs)), notification(std::make_shared<Notification<Status>>()) {}

        /**
         * Constructs an error status to return in the case of conflicting operations.
         */
        Status constructErrorStatus() const;

        // Exact arguments of the currently active operation
        MoveChunkRequest args;

        // Notification event that will be signaled when the currently active operation completes
        std::shared_ptr<Notification<Status>> notification;
    };

    // Describes the state of a currently active receive chunk operation
    struct ActiveReceiveChunkState {
        ActiveReceiveChunkState(NamespaceString inNss, ChunkRange inRange, ShardId inFromShardId)
            : nss(std::move(inNss)), range(std::move(inRange)), fromShardId(inFromShardId) {}

        /**
         * Constructs an error status to return in the case of conflicting operations.
         */
        Status constructErrorStatus() const;

        // Namespace for which a chunk is being received
        NamespaceString nss;

        // Bounds of the chunk being migrated
        ChunkRange range;

        // Id of the shard from which the chunk is being received
        ShardId fromShardId;
    };

    /**
     * Unregisters a previously registered namespace with an ongoing migration. Must only be called
     * if a previous call to registerDonateChunk has succeeded.
     */
    void _clearDonateChunk();

    /**
     * Unregisters a previously registered incoming migration. Must only be called if a previous
     * call to registerReceiveChunk has succeeded.
     */
    void _clearReceiveChunk();

    // Protects the state below
    Mutex _mutex = MONGO_MAKE_LATCH("ActiveMigrationsRegistry::_mutex");
    stdx::condition_variable _lockCond;

    bool _migrationsBlocked{false};

    // If there is an active moveChunk operation, this field contains the original request
    boost::optional<ActiveMoveChunkState> _activeMoveChunkState;

    // If there is an active chunk receive operation, this field contains the original session id
    boost::optional<ActiveReceiveChunkState> _activeReceiveChunkState;
};

class MigrationBlockingGuard {
public:
    MigrationBlockingGuard(OperationContext* opCtx, std::string reason)
        : _registry(ActiveMigrationsRegistry::get(opCtx)), _reason(std::move(reason)) {
        // Ensure any thread attempting to use a MigrationBlockingGuard will be interrupted by
        // a stepdown.
        invariant(opCtx->lockState()->wasGlobalLockTakenInModeConflictingWithWrites());
        _registry.lock(opCtx, _reason);
    }

    ~MigrationBlockingGuard() {
        _registry.unlock(_reason);
    }

private:
    ActiveMigrationsRegistry& _registry;
    std::string _reason;
};

/**
 * Object of this class is returned from the registerDonateChunk call of the active migrations
 * registry. It can exist in two modes - 'execute' and 'join'. See the comments for
 * registerDonateChunk method for more details.
 */
class ScopedDonateChunk {
    ScopedDonateChunk(const ScopedDonateChunk&) = delete;
    ScopedDonateChunk& operator=(const ScopedDonateChunk&) = delete;

public:
    ScopedDonateChunk(ActiveMigrationsRegistry* registry,
                      bool shouldExecute,
                      std::shared_ptr<Notification<Status>> completionNotification);
    ~ScopedDonateChunk();

    ScopedDonateChunk(ScopedDonateChunk&&);
    ScopedDonateChunk& operator=(ScopedDonateChunk&&);

    /**
     * Returns true if the migration object is in the 'execute' mode. This means that the migration
     * object holder is next in line to execute the moveChunk command. The holder must execute the
     * command and call signalComplete with a status.
     */
    bool mustExecute() const {
        return _shouldExecute;
    }

    /**
     * Must only be called if the object is in the 'execute' mode when the moveChunk command was
     * invoked (the command immediately executed). Signals any callers that might be blocked in
     * waitForCompletion.
     */
    void signalComplete(Status status);

    /**
     * Must only be called if the object is in the 'join' mode. Blocks until the main executor of
     * the moveChunk command calls signalComplete.
     */
    Status waitForCompletion(OperationContext* opCtx);

private:
    // Registry from which to unregister the migration. Not owned.
    ActiveMigrationsRegistry* _registry;

    /**
     * Whether the holder is the first in line for a newly started migration (in which case the
     * destructor must unregister) or the caller is joining on an already-running migration
     * (in which case the caller must block and wait for completion).
     */
    bool _shouldExecute;

    // This is the future, which will be signaled at the end of a migration
    std::shared_ptr<Notification<Status>> _completionNotification;
};

/**
 * Object of this class is returned from the registerReceiveChunk call of the active migrations
 * registry.
 */
class ScopedReceiveChunk {
    ScopedReceiveChunk(const ScopedReceiveChunk&) = delete;
    ScopedReceiveChunk& operator=(const ScopedReceiveChunk&) = delete;

public:
    ScopedReceiveChunk(ActiveMigrationsRegistry* registry);
    ~ScopedReceiveChunk();

    ScopedReceiveChunk(ScopedReceiveChunk&&);
    ScopedReceiveChunk& operator=(ScopedReceiveChunk&&);

private:
    // Registry from which to unregister the migration. Not owned.
    ActiveMigrationsRegistry* _registry;
};

}  // namespace mongo