////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2016 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// Licensed under the Apache License, Version 2.0 (the "License");
/// you may not use this file except in compliance with the License.
/// You may obtain a copy of the License at
///
///     http://www.apache.org/licenses/LICENSE-2.0
///
/// Unless required by applicable law or agreed to in writing, software
/// distributed under the License is distributed on an "AS IS" BASIS,
/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
/// See the License for the specific language governing permissions and
/// limitations under the License.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Jan Steemann
////////////////////////////////////////////////////////////////////////////////

#include "CollectorThread.h"
#include "Basics/ConditionLocker.h"
#include "Basics/Exceptions.h"
#include "Basics/hashes.h"
#include "Logger/Logger.h"
#include "Basics/memory-map.h"
#include "Basics/MutexLocker.h"
#include "Basics/ReadLocker.h"
#include "Basics/VelocyPackHelper.h"
#include "Indexes/PrimaryIndex.h"
#include "Utils/CollectionGuard.h"
#include "Utils/DatabaseGuard.h"
#include "Utils/SingleCollectionTransaction.h"
#include "Utils/StandaloneTransactionContext.h"
#include "VocBase/DatafileHelper.h"
#include "VocBase/DatafileStatistics.h"
#include "VocBase/collection.h"
#include "VocBase/LogicalCollection.h"
#include "Wal/Logfile.h"
#include "Wal/LogfileManager.h"

#ifdef ARANGODB_ENABLE_ROCKSDB
#include "Indexes/RocksDBIndex.h"
#endif

using namespace arangodb;
using namespace arangodb::wal;

/// @brief return a reference to an existing datafile statistics struct
static inline DatafileStatisticsContainer& getDfi(CollectorCache* cache,
                                                  TRI_voc_fid_t fid) {
  return cache->dfi[fid];
}

/// @brief return a reference to an existing datafile statistics struct,
/// create it if it does not exist
static inline DatafileStatisticsContainer& createDfi(CollectorCache* cache,
                                                     TRI_voc_fid_t fid) {
  auto it = cache->dfi.find(fid);

  if (it != cache->dfi.end()) {
    return (*it).second;
  }

  cache->dfi.emplace(fid, DatafileStatisticsContainer());

  return cache->dfi[fid];
}

/// @brief state that is built up when scanning a WAL logfile
struct CollectorState {
  std::unordered_map<TRI_voc_cid_t, TRI_voc_tick_t> collections;
  std::unordered_map<TRI_voc_cid_t, int64_t> operationsCount;
  std::unordered_map<TRI_voc_cid_t, CollectorThread::OperationsType>
      structuralOperations;
  std::unordered_map<TRI_voc_cid_t, CollectorThread::DocumentOperationsType>
      documentOperations;
  std::unordered_set<TRI_voc_tid_t> failedTransactions;
  std::unordered_set<TRI_voc_tid_t> handledTransactions;
  std::unordered_set<TRI_voc_cid_t> droppedCollections;
  std::unordered_set<TRI_voc_tick_t> droppedDatabases;

  TRI_voc_tick_t lastDatabaseId;
  TRI_voc_cid_t lastCollectionId;

  CollectorState() : lastDatabaseId(0), lastCollectionId(0) {}

  void resetCollection() {
    return resetCollection(0, 0);
  }

  void resetCollection(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId) {
    lastDatabaseId = databaseId;
    lastCollectionId = collectionId;
  }
};

/// @brief whether or not a collection can be ignored in the gc
static bool ShouldIgnoreCollection(CollectorState const* state,
                                   TRI_voc_cid_t cid) {
  if (state->droppedCollections.find(cid) != state->droppedCollections.end()) {
    // collection was dropped
    return true;
  }

  // look up database id for collection
  auto it = state->collections.find(cid);
  if (it == state->collections.end()) {
    // no database found for collection - should not happen normally
    return true;
  }

  TRI_voc_tick_t databaseId = (*it).second;

  if (state->droppedDatabases.find(databaseId) !=
      state->droppedDatabases.end()) {
    // database of the collection was already dropped
    return true;
  }

  // collection not dropped, database not dropped
  return false;
}

/// @brief callback to handle one marker during collection
static bool ScanMarker(TRI_df_marker_t const* marker, void* data,
                       TRI_datafile_t* datafile) {
  CollectorState* state = static_cast<CollectorState*>(data);

  TRI_ASSERT(marker != nullptr);
  TRI_df_marker_type_t const type = marker->getType();
  
  switch (type) {
    case TRI_DF_MARKER_PROLOGUE: {
      // simply note the last state
      TRI_voc_tick_t const databaseId = DatafileHelper::DatabaseId(marker);
      TRI_voc_cid_t const collectionId = DatafileHelper::CollectionId(marker);
      state->resetCollection(databaseId, collectionId);
      break;
    }

    case TRI_DF_MARKER_VPACK_DOCUMENT: 
    case TRI_DF_MARKER_VPACK_REMOVE: {
      TRI_voc_tick_t const databaseId = state->lastDatabaseId;
      TRI_voc_cid_t const collectionId = state->lastCollectionId;
      TRI_ASSERT(databaseId > 0);
      TRI_ASSERT(collectionId > 0);

      TRI_voc_tid_t transactionId = DatafileHelper::TransactionId(marker);

      state->collections[collectionId] = databaseId;

      if (state->failedTransactions.find(transactionId) !=
          state->failedTransactions.end()) {
        // transaction had failed
        state->operationsCount[collectionId]++;
        break;
      }

      if (ShouldIgnoreCollection(state, collectionId)) {
        break;
      }

      VPackSlice slice(reinterpret_cast<char const*>(marker) + DatafileHelper::VPackOffset(type));
      state->documentOperations[collectionId][Transaction::extractKeyFromDocument(slice).copyString()] = marker;
      state->operationsCount[collectionId]++;
      break;
    }

    case TRI_DF_MARKER_VPACK_BEGIN_TRANSACTION:
    case TRI_DF_MARKER_VPACK_COMMIT_TRANSACTION: {
      break;
    }

    case TRI_DF_MARKER_VPACK_ABORT_TRANSACTION: {
      TRI_voc_tid_t const tid = DatafileHelper::TransactionId(marker);

      // note which abort markers we found
      state->handledTransactions.emplace(tid);
      break;
    }

    case TRI_DF_MARKER_VPACK_CREATE_COLLECTION: {
      TRI_voc_cid_t const collectionId = DatafileHelper::CollectionId(marker);
      // note that the collection is now considered not dropped
      state->droppedCollections.erase(collectionId);
      break;
    }

    case TRI_DF_MARKER_VPACK_DROP_COLLECTION: {
      TRI_voc_cid_t const collectionId = DatafileHelper::CollectionId(marker);
      // note that the collection was dropped and doesn't need to be collected
      state->droppedCollections.emplace(collectionId);
      state->structuralOperations.erase(collectionId);
      state->documentOperations.erase(collectionId);
      state->operationsCount.erase(collectionId);
      state->collections.erase(collectionId);
      break;
    }

    case TRI_DF_MARKER_VPACK_CREATE_DATABASE: {
      TRI_voc_tick_t const database = DatafileHelper::DatabaseId(marker);
      // note that the database is now considered not dropped
      state->droppedDatabases.erase(database);
      break;
    }

    case TRI_DF_MARKER_VPACK_DROP_DATABASE: {
      TRI_voc_tick_t const database = DatafileHelper::DatabaseId(marker);
      // note that the database was dropped and doesn't need to be collected
      state->droppedDatabases.emplace(database);

      // find all collections for the same database and erase their state, too
      for (auto it = state->collections.begin(); it != state->collections.end();
           /* no hoisting */) {
        if ((*it).second == database) {
          state->droppedCollections.emplace((*it).first);
          state->structuralOperations.erase((*it).first);
          state->documentOperations.erase((*it).first);
          state->operationsCount.erase((*it).first);
          it = state->collections.erase(it);
        } else {
          ++it;
        }
      }
      break;
    }

    case TRI_DF_MARKER_HEADER: 
    case TRI_DF_MARKER_FOOTER: {
      // new datafile or end of datafile. forget state!
      state->resetCollection();
      break;
    }

    default: {
      // do nothing intentionally
    }
  }

  return true;
}

/// @brief wait interval for the collector thread when idle
uint64_t const CollectorThread::Interval = 1000000;

/// @brief create the collector thread
CollectorThread::CollectorThread(LogfileManager* logfileManager)
    : Thread("WalCollector"),
      _logfileManager(logfileManager),
      _condition(),
      _operationsQueueLock(),
      _operationsQueue(),
      _operationsQueueInUse(false),
      _numPendingOperations(0),
      _collectorResultCondition(),
      _collectorResult(TRI_ERROR_NO_ERROR) {}

/// @brief wait for the collector result
int CollectorThread::waitForResult(uint64_t timeout) {
  CONDITION_LOCKER(guard, _collectorResultCondition);

  if (_collectorResult == TRI_ERROR_NO_ERROR) {
    if (guard.wait(timeout)) {
      return TRI_ERROR_LOCK_TIMEOUT;
    }
  }

  return _collectorResult;
}

/// @brief begin shutdown sequence
void CollectorThread::beginShutdown() {
  Thread::beginShutdown();

  CONDITION_LOCKER(guard, _condition);
  guard.signal();
}

/// @brief signal the thread that there is something to do
void CollectorThread::signal() {
  CONDITION_LOCKER(guard, _condition);
  guard.signal();
}

/// @brief main loop
void CollectorThread::run() {
  int counter = 0;

  while (true) {
    bool hasWorked = false;
    bool doDelay = false;

    try {
      // step 1: collect a logfile if any qualifies
      if (!isStopping()) {
        // don't collect additional logfiles in case we want to shut down
        bool worked;
        int res = this->collectLogfiles(worked);

        if (res == TRI_ERROR_NO_ERROR) {
          hasWorked |= worked;
        } else if (res == TRI_ERROR_ARANGO_FILESYSTEM_FULL) {
          doDelay = true;
        }
      }

      // step 2: update master pointers
      try {
        bool worked;
        int res = this->processQueuedOperations(worked);

        if (res == TRI_ERROR_NO_ERROR) {
          hasWorked |= worked;
        } else if (res == TRI_ERROR_ARANGO_FILESYSTEM_FULL) {
          doDelay = true;
        }
      } catch (...) {
        // re-activate the queue
        MUTEX_LOCKER(mutexLocker, _operationsQueueLock);
        _operationsQueueInUse = false;
        throw;
      }
    } catch (arangodb::basics::Exception const& ex) {
      int res = ex.code();
      LOG_TOPIC(ERR, Logger::COLLECTOR) << "got unexpected error in collectorThread::run: "
               << TRI_errno_string(res);
    } catch (...) {
      LOG_TOPIC(ERR, Logger::COLLECTOR) << "got unspecific error in collectorThread::run";
    }

    uint64_t interval = Interval;

    if (doDelay) {
      hasWorked = false;
      // wait longer before retrying in case disk is full
      interval *= 2;
    }

    CONDITION_LOCKER(guard, _condition);

    if (!isStopping() && !hasWorked) {
      // sleep only if there was nothing to do

      if (!guard.wait(interval)) {
        if (++counter > 10) {
          LOG_TOPIC(TRACE, Logger::COLLECTOR) << "wal collector has queued operations: "
                     << numQueuedOperations();
          counter = 0;
        }
      }
    } else if (isStopping() && !hasQueuedOperations()) {
      // no operations left to execute, we can exit
      break;
    }
  }

  // all queues are empty, so we can exit
  TRI_ASSERT(!hasQueuedOperations());
}

/// @brief check whether there are queued operations left
bool CollectorThread::hasQueuedOperations() {
  MUTEX_LOCKER(mutexLocker, _operationsQueueLock);

  return !_operationsQueue.empty();
}

/// @brief check whether there are queued operations left
bool CollectorThread::hasQueuedOperations(TRI_voc_cid_t cid) {
  MUTEX_LOCKER(mutexLocker, _operationsQueueLock);

  return (_operationsQueue.find(cid) != _operationsQueue.end());
}

/// @brief step 1: perform collection of a logfile (if any)
int CollectorThread::collectLogfiles(bool& worked) {
  // always init result variable
  worked = false;

  TRI_IF_FAILURE("CollectorThreadCollect") { return TRI_ERROR_NO_ERROR; }

  Logfile* logfile = _logfileManager->getCollectableLogfile();

  if (logfile == nullptr) {
    return TRI_ERROR_NO_ERROR;
  }

  worked = true;
  _logfileManager->setCollectionRequested(logfile);

  try {
    int res = collect(logfile);
    // LOG_TOPIC(TRACE, Logger::COLLECTOR) << "collected logfile: " << // logfile->id() << ". result: "
    // << res;

    if (res == TRI_ERROR_NO_ERROR) {
      // reset collector status
      {
        CONDITION_LOCKER(guard, _collectorResultCondition);
        _collectorResult = TRI_ERROR_NO_ERROR;
      }

#ifdef ARANGODB_ENABLE_ROCKSDB
      RocksDBFeature::syncWal();
#endif

      _logfileManager->setCollectionDone(logfile);
    } else {
      // return the logfile to the logfile manager in case of errors
      _logfileManager->forceStatus(logfile, Logfile::StatusType::SEALED);

      // set error in collector
      {
        CONDITION_LOCKER(guard, _collectorResultCondition);
        _collectorResult = res;
        _collectorResultCondition.broadcast();
      }
    }

    return res;
  } catch (arangodb::basics::Exception const& ex) {
    _logfileManager->forceStatus(logfile, Logfile::StatusType::SEALED);

    int res = ex.code();

    LOG_TOPIC(DEBUG, Logger::COLLECTOR) << "collecting logfile " << logfile->id()
               << " failed: " << TRI_errno_string(res);

    return res;
  } catch (...) {
    _logfileManager->forceStatus(logfile, Logfile::StatusType::SEALED);

    LOG_TOPIC(DEBUG, Logger::COLLECTOR) << "collecting logfile " << logfile->id() << " failed";

    return TRI_ERROR_INTERNAL;
  }
}

/// @brief step 2: process all still-queued collection operations
int CollectorThread::processQueuedOperations(bool& worked) {
  // always init result variable
  worked = false;

  TRI_IF_FAILURE("CollectorThreadProcessQueuedOperations") {
    return TRI_ERROR_NO_ERROR;
  }

  {
    MUTEX_LOCKER(mutexLocker, _operationsQueueLock);
    TRI_ASSERT(!_operationsQueueInUse);

    if (_operationsQueue.empty()) {
      // nothing to do
      return TRI_ERROR_NO_ERROR;
    }

    // this flag indicates that no one else must write to the queue
    _operationsQueueInUse = true;
  }

  // go on without the mutex!

  // process operations for each collection
  for (auto it = _operationsQueue.begin(); it != _operationsQueue.end(); ++it) {
    auto& operations = (*it).second;
    TRI_ASSERT(!operations.empty());

    for (auto it2 = operations.begin(); it2 != operations.end();
         /* no hoisting */) {
      Logfile* logfile = (*it2)->logfile;

      int res = TRI_ERROR_INTERNAL;

      try {
        res = processCollectionOperations((*it2));
      } catch (arangodb::basics::Exception const& ex) {
        res = ex.code();
      }

      if (res == TRI_ERROR_LOCK_TIMEOUT) {
        // could not acquire write-lock for collection in time
        // do not delete the operations
        ++it2;
        continue;
      }

      if (res == TRI_ERROR_NO_ERROR) {
        LOG_TOPIC(TRACE, Logger::COLLECTOR) << "queued operations applied successfully";
      } else if (res == TRI_ERROR_ARANGO_DATABASE_NOT_FOUND ||
                 res == TRI_ERROR_ARANGO_COLLECTION_NOT_FOUND) {
        // these are expected errors
        LOG_TOPIC(TRACE, Logger::COLLECTOR)
            << "removing queued operations for already deleted collection";
        res = TRI_ERROR_NO_ERROR;
      } else {
        LOG_TOPIC(WARN, Logger::COLLECTOR)
            << "got unexpected error code while applying queued operations: "
            << TRI_errno_string(res);
      }

      if (res == TRI_ERROR_NO_ERROR) {
        uint64_t numOperations = (*it2)->operations->size();
        uint64_t maxNumPendingOperations =
            _logfileManager->throttleWhenPending();

        if (maxNumPendingOperations > 0 &&
            _numPendingOperations >= maxNumPendingOperations &&
            (_numPendingOperations - numOperations) < maxNumPendingOperations) {
          // write-throttling was active, but can be turned off now
          _logfileManager->deactivateWriteThrottling();
          LOG_TOPIC(INFO, Logger::COLLECTOR) << "deactivating write-throttling";
        }

        _numPendingOperations -= numOperations;

        // delete the object
        delete (*it2);

        // delete the element from the vector while iterating over the vector
        it2 = operations.erase(it2);

        _logfileManager->decreaseCollectQueueSize(logfile);
      } else {
        // do not delete the object but advance in the operations vector
        ++it2;
      }
    }

    // next collection
  }

  // finally remove all entries from the map with empty vectors
  {
    MUTEX_LOCKER(mutexLocker, _operationsQueueLock);

    for (auto it = _operationsQueue.begin(); it != _operationsQueue.end();
         /* no hoisting */) {
      if ((*it).second.empty()) {
        it = _operationsQueue.erase(it);
      } else {
        ++it;
      }
    }

    // the queue can now be used by others, too
    _operationsQueueInUse = false;
  }

  worked = true;

  return TRI_ERROR_NO_ERROR;
}

/// @brief return the number of queued operations
size_t CollectorThread::numQueuedOperations() {
  MUTEX_LOCKER(mutexLocker, _operationsQueueLock);

  return _operationsQueue.size();
}

/// @brief process a single marker in collector step 2
void CollectorThread::processCollectionMarker(
    arangodb::SingleCollectionTransaction& trx,
    TRI_collection_t* document, CollectorCache* cache,
    CollectorOperation const& operation) {
  auto const* walMarker = reinterpret_cast<TRI_df_marker_t const*>(operation.walPosition);
  TRI_ASSERT(walMarker != nullptr);
  TRI_ASSERT(reinterpret_cast<TRI_df_marker_t const*>(operation.datafilePosition));
  TRI_voc_size_t const datafileMarkerSize = operation.datafileMarkerSize;
  TRI_voc_fid_t const fid = operation.datafileId;


  TRI_df_marker_type_t const type = walMarker->getType();

  if (type == TRI_DF_MARKER_VPACK_DOCUMENT) {
    auto& dfi = createDfi(cache, fid);
    dfi.numberUncollected--;

    VPackSlice slice(reinterpret_cast<char const*>(walMarker) + DatafileHelper::VPackOffset(type));
    TRI_ASSERT(slice.isObject());
    
    VPackSlice keySlice;
    TRI_voc_rid_t revisionId = 0;
    Transaction::extractKeyAndRevFromDocument(slice, keySlice, revisionId);
  
    auto found = document->primaryIndex()->lookupKey(&trx, keySlice);

    if (found == nullptr || found->revisionId() != revisionId ||
        found->getMarkerPtr() != walMarker) {
      // somebody inserted a new revision of the document or the revision
      // was already moved by the compactor
      dfi.numberDead++;
      dfi.sizeDead += DatafileHelper::AlignedSize<int64_t>(datafileMarkerSize);
    } else {
      // we can safely update the master pointer's dataptr value
      found->setVPackFromMarker(reinterpret_cast<TRI_df_marker_t const*>(operation.datafilePosition));
      found->setFid(fid, false); // points to datafile now

      dfi.numberAlive++;
      dfi.sizeAlive += DatafileHelper::AlignedSize<int64_t>(datafileMarkerSize);
    }
  } else if (type == TRI_DF_MARKER_VPACK_REMOVE) {
    auto& dfi = createDfi(cache, fid);
    dfi.numberUncollected--;
    dfi.numberDeletions++;

    VPackSlice slice(reinterpret_cast<char const*>(walMarker) + DatafileHelper::VPackOffset(type));
    TRI_ASSERT(slice.isObject());
    
    VPackSlice keySlice;
    TRI_voc_rid_t revisionId = 0;
    Transaction::extractKeyAndRevFromDocument(slice, keySlice, revisionId);

    auto found = document->primaryIndex()->lookupKey(&trx, keySlice);

    if (found != nullptr && found->revisionId() > revisionId) {
      // somebody re-created the document with a newer revision
      dfi.numberDead++;
      dfi.sizeDead += DatafileHelper::AlignedSize<int64_t>(datafileMarkerSize);
    }
  }
}

/// @brief process all operations for a single collection
int CollectorThread::processCollectionOperations(CollectorCache* cache) {
  arangodb::DatabaseGuard dbGuard(cache->databaseId);
  TRI_vocbase_t* vocbase = dbGuard.database();
  TRI_ASSERT(vocbase != nullptr);

  arangodb::CollectionGuard collectionGuard(vocbase, cache->collectionId, true);
  arangodb::LogicalCollection* collection = collectionGuard.collection();

  TRI_ASSERT(collection != nullptr);

  TRI_collection_t* document = collection->_collection;
  TRI_ASSERT(document != nullptr);

  // first try to read-lock the compactor-lock, afterwards try to write-lock the
  // collection
  // if any locking attempt fails, release and try again next time

  TRY_READ_LOCKER(locker, document->_compactionLock);
  
  if (!locker.isLocked()) {
    return TRI_ERROR_LOCK_TIMEOUT;
  }

  arangodb::SingleCollectionTransaction trx(arangodb::StandaloneTransactionContext::Create(document->_vocbase), 
      document->_info.id(), TRI_TRANSACTION_WRITE);
  trx.addHint(TRI_TRANSACTION_HINT_NO_USAGE_LOCK,
              true);  // already locked by guard above
  trx.addHint(TRI_TRANSACTION_HINT_NO_COMPACTION_LOCK,
              true);  // already locked above
  trx.addHint(TRI_TRANSACTION_HINT_NO_BEGIN_MARKER, true);
  trx.addHint(TRI_TRANSACTION_HINT_NO_ABORT_MARKER, true);
  trx.addHint(TRI_TRANSACTION_HINT_TRY_LOCK, true);

  int res = trx.begin();

  if (res != TRI_ERROR_NO_ERROR) {
    // this includes TRI_ERROR_LOCK_TIMEOUT!
    LOG_TOPIC(TRACE, Logger::COLLECTOR) << "wal collector couldn't acquire write lock for collection '"
               << document->_info.name() << "': " << TRI_errno_string(res);

    return res;
  }

  try {
    // now we have the write lock on the collection
    LOG_TOPIC(TRACE, Logger::COLLECTOR) << "wal collector processing operations for collection '"
               << document->_info.name() << "'";

    TRI_ASSERT(!cache->operations->empty());

    for (auto const& it : *(cache->operations)) {
      processCollectionMarker(trx, document, cache, it);
    }

    // finally update all datafile statistics
    LOG_TOPIC(TRACE, Logger::COLLECTOR) << "updating datafile statistics for collection '"
               << document->_info.name() << "'";
    updateDatafileStatistics(document, cache);

    document->_uncollectedLogfileEntries -= cache->totalOperationsCount;
    if (document->_uncollectedLogfileEntries < 0) {
      document->_uncollectedLogfileEntries = 0;
    }

    res = TRI_ERROR_NO_ERROR;
  } catch (arangodb::basics::Exception const& ex) {
    res = ex.code();
  } catch (...) {
    res = TRI_ERROR_INTERNAL;
  }

  // always release the locks
  trx.finish(res);

  LOG_TOPIC(TRACE, Logger::COLLECTOR) << "wal collector processed operations for collection '"
             << document->_info.name() << "' with status: " << TRI_errno_string(res);

  return res;
}

/// @brief collect one logfile
int CollectorThread::collect(Logfile* logfile) {
  TRI_ASSERT(logfile != nullptr);

  LOG_TOPIC(TRACE, Logger::COLLECTOR) << "collecting logfile " << logfile->id();

  TRI_datafile_t* df = logfile->df();

  TRI_ASSERT(df != nullptr);

  TRI_IF_FAILURE("CollectorThreadCollectException") {
    THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
  }

  // We will sequentially scan the logfile for collection:
  TRI_MMFileAdvise(df->_data, df->_maximalSize, TRI_MADVISE_SEQUENTIAL);
  TRI_MMFileAdvise(df->_data, df->_maximalSize, TRI_MADVISE_WILLNEED);
  TRI_DEFER(TRI_MMFileAdvise(df->_data, df->_maximalSize, TRI_MADVISE_RANDOM));

  // create a state for the collector, beginning with the list of failed
  // transactions
  CollectorState state;
  state.failedTransactions = _logfileManager->getFailedTransactions();
  /*
    if (_inRecovery) {
      state.droppedCollections = _logfileManager->getDroppedCollections();
      state.droppedDatabases   = _logfileManager->getDroppedDatabases();
    }
  */

  // scan all markers in logfile, this will fill the state
  bool result =
      TRI_IterateDatafile(df, &ScanMarker, static_cast<void*>(&state));

  if (!result) {
    return TRI_ERROR_INTERNAL;
  }

  // get an aggregated list of all collection ids
  std::set<TRI_voc_cid_t> collectionIds;
  for (auto it = state.structuralOperations.begin();
       it != state.structuralOperations.end(); ++it) {
    auto cid = (*it).first;

    if (!ShouldIgnoreCollection(&state, cid)) {
      collectionIds.emplace((*it).first);
    }
  }

  for (auto it = state.documentOperations.begin();
       it != state.documentOperations.end(); ++it) {
    auto cid = (*it).first;

    if (state.structuralOperations.find(cid) ==
            state.structuralOperations.end() &&
        !ShouldIgnoreCollection(&state, cid)) {
      collectionIds.emplace(cid);
    }
  }

  // now for each collection, write all surviving markers into collection
  // datafiles
  for (auto it = collectionIds.begin(); it != collectionIds.end(); ++it) {
    auto cid = (*it);

    OperationsType sortedOperations;

    // insert structural operations - those are already sorted by tick
    if (state.structuralOperations.find(cid) !=
        state.structuralOperations.end()) {
      OperationsType const& ops = state.structuralOperations[cid];

      sortedOperations.insert(sortedOperations.begin(), ops.begin(), ops.end());
      TRI_ASSERT(sortedOperations.size() == ops.size());
    }

    // insert document operations - those are sorted by key, not by tick
    if (state.documentOperations.find(cid) != state.documentOperations.end()) {
      DocumentOperationsType const& ops = state.documentOperations[cid];

      for (auto it2 = ops.begin(); it2 != ops.end(); ++it2) {
        sortedOperations.push_back((*it2).second);
      }

      // sort vector by marker tick
      std::sort(sortedOperations.begin(), sortedOperations.end(),
                [](TRI_df_marker_t const* left, TRI_df_marker_t const* right) {
                  return (left->getTick() < right->getTick());
                });
    }

    if (!sortedOperations.empty()) {
      int res = TRI_ERROR_INTERNAL;

      try {
        res = transferMarkers(logfile, cid, state.collections[cid],
                              state.operationsCount[cid], sortedOperations);

        TRI_IF_FAILURE("failDuringCollect") {
          THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG);
        }

      } catch (arangodb::basics::Exception const& ex) {
        res = ex.code();
      } catch (...) {
        res = TRI_ERROR_INTERNAL;
      }

      if (res != TRI_ERROR_NO_ERROR &&
          res != TRI_ERROR_ARANGO_DATABASE_NOT_FOUND &&
          res != TRI_ERROR_ARANGO_COLLECTION_NOT_FOUND) {
        if (res != TRI_ERROR_ARANGO_FILESYSTEM_FULL) {
          // other places already log this error, and making the logging
          // conditional here
          // prevents the log message from being shown over and over again in
          // case the
          // file system is full
          LOG_TOPIC(WARN, Logger::COLLECTOR) << "got unexpected error in CollectorThread::collect: "
                    << TRI_errno_string(res);
        }
        // abort early
        return res;
      }
    }
  }

  // Error conditions TRI_ERROR_ARANGO_DATABASE_NOT_FOUND and
  // TRI_ERROR_ARANGO_COLLECTION_NOT_FOUND are intentionally ignored
  // here since this can actually happen if someone has dropped things
  // in between.

  // remove all handled transactions from failedTransactions list
  if (!state.handledTransactions.empty()) {
    _logfileManager->unregisterFailedTransactions(state.handledTransactions);
  }

  return TRI_ERROR_NO_ERROR;
}

/// @brief transfer markers into a collection
int CollectorThread::transferMarkers(Logfile* logfile,
                                     TRI_voc_cid_t collectionId,
                                     TRI_voc_tick_t databaseId,
                                     int64_t totalOperationsCount,
                                     OperationsType const& operations) {
  TRI_ASSERT(!operations.empty());

  // prepare database and collection
  arangodb::DatabaseGuard dbGuard(databaseId);
  TRI_vocbase_t* vocbase = dbGuard.database();
  TRI_ASSERT(vocbase != nullptr);

  arangodb::CollectionGuard collectionGuard(vocbase, collectionId, true);
  arangodb::LogicalCollection* collection = collectionGuard.collection();
  TRI_ASSERT(collection != nullptr);

  TRI_collection_t* document = collection->_collection;
  TRI_ASSERT(document != nullptr);

  LOG_TOPIC(TRACE, Logger::COLLECTOR) << "collector transferring markers for '"
             << document->_info.name()
             << "', totalOperationsCount: " << totalOperationsCount;

  std::unique_ptr<CollectorCache> cache(
      new CollectorCache(collectionId, databaseId, logfile,
                         totalOperationsCount, operations.size()));

  int res = TRI_ERROR_INTERNAL;

  try {
    res = executeTransferMarkers(collection, cache.get(), operations);
    
    TRI_IF_FAILURE("transferMarkersCrash") {
      // intentionally kill the server
      TRI_SegfaultDebugging("CollectorThreadTransfer");
    }

    if (res == TRI_ERROR_NO_ERROR && !cache->operations->empty()) {
      // now sync the datafile
      res = syncJournalCollection(collection);

      if (res != TRI_ERROR_NO_ERROR) {
        THROW_ARANGO_EXCEPTION(res);
      }

      // note: cache is passed by reference and can be modified by
      // queueOperations
      // (i.e. set to nullptr!)
      queueOperations(logfile, cache);
    }
  } catch (arangodb::basics::Exception const& ex) {
    res = ex.code();
  } catch (...) {
    res = TRI_ERROR_INTERNAL;
  }

  return res;
}

/// @brief transfer markers into a collection, actual work
/// the collection must have been prepared to call this function
int CollectorThread::executeTransferMarkers(LogicalCollection* collection,
                                            CollectorCache* cache,
                                            OperationsType const& operations) {
  // used only for crash / recovery tests
  int numMarkers = 0;

  TRI_voc_tick_t const minTransferTick = collection->_collection->_tickMax;
  TRI_ASSERT(!operations.empty());

  for (auto it2 = operations.begin(); it2 != operations.end(); ++it2) {
    TRI_df_marker_t const* source = (*it2);
    TRI_voc_tick_t const tick = source->getTick();

    if (tick <= minTransferTick) {
      // we have already transferred this marker in a previous run, nothing to
      // do
      continue;
    }

    TRI_IF_FAILURE("CollectorThreadTransfer") {
      if (++numMarkers > 5) {
        // intentionally kill the server
        TRI_SegfaultDebugging("CollectorThreadTransfer");
      }
    }

    TRI_df_marker_type_t const type = source->getType();

    if (type == TRI_DF_MARKER_VPACK_DOCUMENT ||
        type == TRI_DF_MARKER_VPACK_REMOVE) {
      TRI_voc_size_t const size = source->getSize();

      char* dst = nextFreeMarkerPosition(collection, tick, type, size, cache);

      if (dst == nullptr) {
        return TRI_ERROR_OUT_OF_MEMORY;
      }

      auto& dfi = getDfi(cache, cache->lastFid);
      dfi.numberUncollected++;

      memcpy(dst, source, size);

      finishMarker(reinterpret_cast<char const*>(source), dst, collection, tick, cache);
    }
  }

  TRI_IF_FAILURE("CollectorThreadTransferFinal") {
    // intentionally kill the server
    TRI_SegfaultDebugging("CollectorThreadTransferFinal");
  }

  return TRI_ERROR_NO_ERROR;
}

/// @brief insert the collect operations into a per-collection queue
int CollectorThread::queueOperations(arangodb::wal::Logfile* logfile,
                                     std::unique_ptr<CollectorCache>& cache) {
  TRI_ASSERT(cache != nullptr);

  TRI_voc_cid_t cid = cache->collectionId;
  uint64_t numOperations = cache->operations->size();
  uint64_t maxNumPendingOperations = _logfileManager->throttleWhenPending();

  TRI_ASSERT(!cache->operations->empty());

  while (true) {
    {
      MUTEX_LOCKER(mutexLocker, _operationsQueueLock);

      if (!_operationsQueueInUse) {
        // it is only safe to access the queue if this flag is not set
        auto it = _operationsQueue.find(cid);
        if (it == _operationsQueue.end()) {
          _operationsQueue.emplace(cid, std::vector<CollectorCache*>({cache.get()}));
          _logfileManager->increaseCollectQueueSize(logfile);
        } else {
          (*it).second.push_back(cache.get());
          _logfileManager->increaseCollectQueueSize(logfile);
        }
        // now _operationsQueue is responsible for managing the cache entry
        cache.release();

        // exit the loop
        break;
      }
    }

    // wait outside the mutex for the flag to be cleared
    usleep(10000);
  }

  if (maxNumPendingOperations > 0 &&
      _numPendingOperations < maxNumPendingOperations &&
      (_numPendingOperations + numOperations) >= maxNumPendingOperations) {
    // activate write-throttling!
    _logfileManager->activateWriteThrottling();
    LOG_TOPIC(WARN, Logger::COLLECTOR)
        << "queued more than " << maxNumPendingOperations
        << " pending WAL collector operations. now activating write-throttling";
  }

  _numPendingOperations += numOperations;

  return TRI_ERROR_NO_ERROR;
}

/// @brief update a collection's datafile information
int CollectorThread::updateDatafileStatistics(
    TRI_collection_t* document, CollectorCache* cache) {
  // iterate over all datafile infos and update the collection's datafile stats
  for (auto it = cache->dfi.begin(); it != cache->dfi.end();
       /* no hoisting */) {
    document->_datafileStatistics.update((*it).first, (*it).second);

    // flush the local datafile info so we don't update the statistics twice
    // with the same values
    (*it).second.reset();
    it = cache->dfi.erase(it);
  }

  return TRI_ERROR_NO_ERROR;
}

/// @brief sync all journals of a collection
int CollectorThread::syncJournalCollection(
    LogicalCollection* collection) {
  TRI_IF_FAILURE("CollectorThread::syncDatafileCollection") {
    return TRI_ERROR_DEBUG;
  }

  return collection->syncActiveJournal();
}

/// @brief get the next position for a marker of the specified size
char* CollectorThread::nextFreeMarkerPosition(
    LogicalCollection* collection, TRI_voc_tick_t tick,
    TRI_df_marker_type_t type, TRI_voc_size_t size, CollectorCache* cache) {
  
  // align the specified size
  size = DatafileHelper::AlignedSize<TRI_voc_size_t>(size);

  char* dst = nullptr; // will be modified by reserveJournalSpace()
  TRI_datafile_t* datafile = nullptr; // will be modified by reserveJournalSpace()
  int res = collection->reserveJournalSpace(tick, size, dst, datafile);

  if (res != TRI_ERROR_NO_ERROR) {
    // could not reserve space, for whatever reason
    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_NO_JOURNAL);
  }

  // if we get here, we successfully reserved space in the datafile

  TRI_ASSERT(datafile != nullptr);

  if (cache->lastFid != datafile->_fid) {
    if (cache->lastFid > 0) {
      // rotated the existing journal... now update the old journal's stats
      auto& dfi = createDfi(cache, cache->lastFid);
      collection->_collection->_datafileStatistics.increaseUncollected(cache->lastFid,
                                                        dfi.numberUncollected);
      // and reset them afterwards
      dfi.numberUncollected = 0;
    }
 
    // reset datafile in cache   
    cache->lastDatafile = datafile;
    cache->lastFid = datafile->_fid;
    
    // create a local datafile info struct
    createDfi(cache, datafile->_fid);

    // we only need the ditches when we are outside the recovery
    // the compactor will not run during recovery
    auto ditch =
        collection->_collection->ditches()->createDocumentDitch(false, __FILE__, __LINE__);

    if (ditch == nullptr) {
      THROW_ARANGO_EXCEPTION(TRI_ERROR_OUT_OF_MEMORY);
    }

    cache->addDitch(ditch);
  }
  
  TRI_ASSERT(dst != nullptr);
  
  DatafileHelper::InitMarker(reinterpret_cast<TRI_df_marker_t*>(dst), type, size);

  return dst;
}

/// @brief set the tick of a marker and calculate its CRC value
void CollectorThread::finishMarker(char const* walPosition,
                                   char* datafilePosition,
                                   LogicalCollection* collection,
                                   TRI_voc_tick_t tick, CollectorCache* cache) {
  TRI_df_marker_t* marker =
      reinterpret_cast<TRI_df_marker_t*>(datafilePosition);

  TRI_datafile_t* datafile = cache->lastDatafile;
  TRI_ASSERT(datafile != nullptr);

  // update ticks
  TRI_UpdateTicksDatafile(datafile, marker);

  TRI_ASSERT(collection->_collection->_tickMax < tick);
  collection->_collection->_tickMax = tick;

  cache->operations->emplace_back(CollectorOperation(
      datafilePosition, marker->getSize(), walPosition, cache->lastFid));
}