text: /**
   *    Copyright (C) 2013-2014 MongoDB 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 "mongo/base/owned_pointer_vector.h"
  #include "mongo/db/exec/collection_scan.h"
  #include "mongo/db/exec/plan_stage.h"
  #include "mongo/db/matcher/expression.h"
  #include "mongo/db/record_id.h"
  #include "mongo/util/timer.h"
  
  namespace mongo {
  
      /**
       * OplogStart walks a collection backwards to find the first object in the collection that
       * matches the query.  It's used by replication to efficiently find where the oplog should be
       * replayed from.
       *
       * The oplog is always a capped collection.  In capped collections, documents are oriented on
       * disk according to insertion order.  The oplog inserts documents with increasing timestamps.
       * Queries on the oplog look for entries that are after a certain time.  Therefore if we
       * navigate backwards, the last document we encounter that satisfies our query (over the
       * timestamp) is the first document we must scan from to answer the query.
       *
       * Why isn't this a normal reverse table scan, you may ask?  We could be correct if we used a
       * normal reverse collection scan.  However, that's not fast enough.  Since we know all
       * documents are oriented on disk in insertion order, we know all documents in one extent were
       * inserted before documents in a subsequent extent.  As such we can skip through entire extents
       * looking only at the first document.
       *
       * Why is this a stage?  Because we want to yield, and we want to be notified of RecordId
       * invalidations.  :(
       */
      class OplogStart : public PlanStage {
      public:
          // Does not take ownership.
          OplogStart(OperationContext* txn,
                     const Collection* collection,
                     MatchExpression* filter,
                     WorkingSet* ws);
          virtual ~OplogStart();
  
          virtual StageState work(WorkingSetID* out);
          virtual bool isEOF();
  
          virtual void invalidate(OperationContext* txn, const RecordId& dl, InvalidationType type);
          virtual void saveState();
          virtual void restoreState(OperationContext* opCtx);
  
          virtual std::vector<PlanStage*> getChildren() const;
  
          //
          // Exec stats -- do not call these for the oplog start stage.
          //
  
          virtual PlanStageStats* getStats() { return NULL; }
  
          virtual const CommonStats* getCommonStats() { return NULL; }
  
          virtual const SpecificStats* getSpecificStats() { return NULL; }
  
          virtual StageType stageType() const { return STAGE_OPLOG_START; }
  
          // For testing only.
          void setBackwardsScanTime(int newTime) { _backwardsScanTime = newTime; }
          bool isExtentHopping() { return _extentHopping; }
          bool isBackwardsScanning() { return _backwardsScanning; }
      private:
          StageState workBackwardsScan(WorkingSetID* out);
  
          void switchToExtentHopping();
  
          StageState workExtentHopping(WorkingSetID* out);
  
          // transactional context for read locks. Not owned by us
          OperationContext* _txn;
  
          // If we're backwards scanning we just punt to a collscan.
          scoped_ptr<CollectionScan> _cs;
  
          // This is only used for the extent hopping scan.
          typedef OwnedPointerVector<RecordIterator> SubIterators;
          SubIterators _subIterators;
  
          // Have we done our heavy init yet?
          bool _needInit;
  
          // Our first state: going backwards via a collscan.
          bool _backwardsScanning;
  
          // Our second state: hopping backwards extent by extent.
          bool _extentHopping;
  
          // Our final state: done.
          bool _done;
  
          const Collection* _collection;
  
          // We only go backwards via a collscan for a few seconds.
          Timer _timer;
  
          // WorkingSet is not owned by us.
          WorkingSet* _workingSet;
  
          std::string _ns;
  
          MatchExpression* _filter;
  
          static int _backwardsScanTime;
      };
  
  }  // namespace mongo
  