path: root/src/mongo/db/clientcursor.h

/**
 *    Copyright (C) 2008 10gen Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    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
 *    GNU Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    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 GNU Affero General 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/noncopyable.hpp>

#include "mongo/db/jsobj.h"
#include "mongo/db/query/plan_executor.h"
#include "mongo/db/record_id.h"
#include "mongo/s/collection_metadata.h"
#include "mongo/util/net/message.h"

namespace mongo {

    class ClientCursor;
    class Collection;
    class CursorManager;
    class RecoveryUnit;

    typedef long long CursorId; /* passed to the client so it can send back on getMore */
    static const CursorId INVALID_CURSOR_ID = -1; // But see SERVER-5726.

    /**
     * ClientCursor is a wrapper that represents a cursorid from our database application's
     * perspective.
     */
    class ClientCursor : private boost::noncopyable {
    public:
        /**
         * This ClientCursor constructor creates a cursorid that can be used with getMore and
         * killCursors.  "cursorManager" is the object that will manage the lifetime of this
         * cursor, and "ns" is the namespace string that should be associated with this cursor (e.g.
         * "test.foo", "test.$cmd.listCollections", etc).
         */
        ClientCursor(CursorManager* cursorManager,
                     PlanExecutor* exec,
                     const std::string& ns,
                     int qopts = 0,
                     const BSONObj query = BSONObj(),
                     bool isAggCursor = false);

        /**
         * This ClientCursor is used to track sharding state for the given collection.
         */
        explicit ClientCursor(const Collection* collection);

        //
        // Basic accessors
        //

        CursorId cursorid() const { return _cursorid; }
        std::string ns() const { return _ns; }
        CursorManager* cursorManager() const { return _cursorManager; }
        bool isAggCursor() const { return _isAggCursor; }

        //
        // Pinning functionality.
        //

        /**
         * Marks this ClientCursor as in use.  unsetPinned() must be called before the destructor of
         * this ClientCursor is invoked.
         */
        void setPinned() { _isPinned = true; }

        /**
         * Marks this ClientCursor as no longer in use.
         */
        void unsetPinned() { _isPinned = false; }

        bool isPinned() const { return _isPinned; }

        /**
         * This is called when someone is dropping a collection or something else that
         * goes through killing cursors.
         * It removes the responsiilibty of de-registering from ClientCursor.
         * Responsibility for deleting the ClientCursor doesn't change from this call
         * see PlanExecutor::kill.
         */
        void kill();

        //
        // Timing and timeouts
        //

        /**
         * @param millis amount of idle passed time since last call
         * note called outside of locks (other than ccmutex) so care must be exercised
         */
        bool shouldTimeout( int millis );
        void setIdleTime( int millis );
        int idleTime() const { return _idleAgeMillis; }

        uint64_t getLeftoverMaxTimeMicros() const { return _leftoverMaxTimeMicros; }
        void setLeftoverMaxTimeMicros( uint64_t leftoverMaxTimeMicros ) {
            _leftoverMaxTimeMicros = leftoverMaxTimeMicros;
        }

        //
        // Replication-related stuff.  TODO: Document and clean.
        //

        // Used to report replication position only in master-slave,
        // so we keep them as TimeStamp rather than OpTime.
        void updateSlaveLocation(OperationContext* txn);
        void slaveReadTill( const Timestamp& t ) { _slaveReadTill = t; }
        /** Just for testing. */
        Timestamp getSlaveReadTill() const { return _slaveReadTill; }

        //
        // Query-specific functionality that may be adapted for the PlanExecutor.
        //

        PlanExecutor* getExecutor() const { return _exec.get(); }
        int queryOptions() const { return _queryOptions; }
        const BSONObj& getQuery() const { return _query; }

        // Used by ops/query.cpp to stash how many results have been returned by a query.
        int pos() const { return _pos; }
        void incPos(int n) { _pos += n; }
        void setPos(int n) { _pos = n; }

        static long long totalOpen();

        //
        // Storage engine state for getMore.
        //

        bool hasRecoveryUnit() const { return _ownedRU.get() || _unownedRU; }

        /**
         * 
         * If a ClientCursor is created via DBDirectClient, it uses the same storage engine
         * context as the DBDirectClient caller.  We store this context in _unownedRU.  We use
         * this to verify that all further callers use the same RecoveryUnit.
         *
         * Once a ClientCursor has an unowned RecoveryUnit, it will always have one.
         *
         * Sets the unowned RecoveryUnit to 'ru'.  Does NOT take ownership of the pointer.
         */
        void setUnownedRecoveryUnit(RecoveryUnit* ru);

        /**
         * Return the unowned RecoveryUnit.  'this' does not own pointer and therefore cannot
         * transfer ownership.
         */
        RecoveryUnit* getUnownedRecoveryUnit() const;

        /**
         * If a ClientCursor is created via a client request, we bind its lifetime to the
         * ClientCursor's by storing it un _ownedRU.  In order to execute the query over repeated
         * network requests, we have to keep the execution state around.
         */

        /**
         * Set the owned recovery unit to 'ru'.  Takes ownership of it.  If there is a previous
         * owned recovery unit, it is deleted.
         */
        void setOwnedRecoveryUnit(RecoveryUnit* ru);

        /**
         * Returns the owned recovery unit.  Ownership is transferred to the caller.
         */
        RecoveryUnit* releaseOwnedRecoveryUnit();

    private:
        friend class CursorManager;
        friend class ClientCursorPin;

        /**
         * Only friends are allowed to destroy ClientCursor objects.
         */
        ~ClientCursor();

        /**
         * Initialization common between both constructors for the ClientCursor. The database must
         * be stable when this is called, because cursors hang off the collection.
         */
        void init();

        //
        // ClientCursor-specific data, independent of the underlying execution type.
        //

        // The ID of the ClientCursor.
        CursorId _cursorid;

        // The namespace we're operating on.
        std::string _ns;

        CursorManager* _cursorManager;

        // if we've added it to the total open counter yet
        bool _countedYet;

        // How many objects have been returned by the find() so far?
        int _pos;

        // If this cursor was created by a find operation, '_query' holds the query predicate for
        // the find. If this cursor was created by a command (e.g. the aggregate command), then
        // '_query' holds the command specification received from the client.
        BSONObj _query;

        // See the QueryOptions enum in dbclient.h
        int _queryOptions;

        // Is this ClientCursor backed by an aggregation pipeline?  Defaults to false.
        //
        // Agg executors differ from others in that they manage their own locking internally and
        // should not be killed or destroyed when the underlying collection is deleted.
        //
        // Note: This should *not* be set for the internal cursor used as input to an aggregation.
        bool _isAggCursor;

        // Is this cursor in use?  Defaults to false.
        bool _isPinned;

        // Is the "no timeout" flag set on this cursor?  If false, this cursor may be targeted for
        // deletion after an interval of inactivity.  Defaults to false.
        bool _isNoTimeout;

        // The replication position only used in master-slave.
        Timestamp _slaveReadTill;

        // How long has the cursor been idle?
        int _idleAgeMillis;

        // TODO: Document.
        uint64_t _leftoverMaxTimeMicros;

        // For chunks that are being migrated, there is a period of time when that chunks data is in
        // two shards, the donor and the receiver one. That data is picked up by a cursor on the
        // receiver side, even before the migration was decided.  The CollectionMetadata allow one
        // to inquiry if any given document of the collection belongs indeed to this shard or if it
        // is coming from (or a vestige of) an ongoing migration.
        CollectionMetadataPtr _collMetadata;

        // Only one of these is not-NULL.
        RecoveryUnit* _unownedRU;
        std::unique_ptr<RecoveryUnit> _ownedRU;
        // NOTE: _ownedRU must come before _exec, because _ownedRU must outlive _exec.
        // The storage engine can have resources in the PlanExecutor that rely on
        // the RecoveryUnit being alive.

        //
        // The underlying execution machinery.
        //
        std::unique_ptr<PlanExecutor> _exec;
    };

    /**
     * ClientCursorPin is an RAII class that manages the pinned state of a ClientCursor.
     * ClientCursorPin objects pin the given cursor upon construction, and release the pin upon
     * destruction.
     *
     * A pin extends the lifetime of a ClientCursor object until the pin's release.  Pinned
     * ClientCursor objects cannot not be killed due to inactivity, and cannot be killed by user
     * kill requests.  When a CursorManager is destroyed (e.g. by a collection drop), ownership of
     * any still-pinned ClientCursor objects is transferred to their managing ClientCursorPin
     * objects.
     *
     * Example usage:
     * {
     *     ClientCursorPin pin(cursorManager, cursorid);
     *     ClientCursor* cursor = pin.c();
     *     if (cursor) {
     *         // Use cursor.
     *     }
     *     // Pin automatically released on block exit.
     * }
     *
     * Clients that wish to access ClientCursor objects owned by collection cursor managers must
     * hold the collection lock during pin acquisition and pin release.  This guards from a
     * collection drop (which requires an exclusive lock on the collection) occurring concurrently
     * with the pin request or unpin request.
     *
     * Clients that wish to access ClientCursor objects owned by the global cursor manager need not
     * hold any locks; the global cursor manager can only be destroyed by a process exit.
     */
    class ClientCursorPin {
        MONGO_DISALLOW_COPYING(ClientCursorPin);
    public:
        /**
         * Asks "cursorManager" to set a pin on the ClientCursor associated with "cursorid".  If no
         * such cursor exists, does nothing.  If the cursor is already pinned, throws a
         * UserException.
         */
        ClientCursorPin( CursorManager* cursorManager, long long cursorid );

        /**
         * Calls release().
         */
        ~ClientCursorPin();

        /**
         * Releases the pin.  It does not delete the underlying cursor unless ownership has passed
         * to us after kill.  Turns into a no-op if release() or deleteUnderlying() have already
         * been called on this pin.
         */
        void release();

        /**
         * Deletes the underlying cursor.  Cannot be called if release() or deleteUnderlying() have
         * already been called on this pin.
         */
        void deleteUnderlying();

        ClientCursor *c() const;

    private:
        ClientCursor* _cursor;
    };

    void startClientCursorMonitor();

} // namespace mongo