path: root/src/mongo/db/repl/bgsync.cpp

/**
 *    Copyright (C) 2012 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.
 */

#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kReplication

#include "mongo/platform/basic.h"

#include "mongo/db/repl/bgsync.h"

#include <memory>

#include "mongo/base/counter.h"
#include "mongo/client/connection_pool.h"
#include "mongo/db/auth/authorization_session.h"
#include "mongo/db/client.h"
#include "mongo/db/commands/fsync.h"
#include "mongo/db/commands/server_status_metric.h"
#include "mongo/db/concurrency/write_conflict_exception.h"
#include "mongo/db/dbhelpers.h"
#include "mongo/db/operation_context_impl.h"
#include "mongo/db/repl/oplog.h"
#include "mongo/db/repl/oplog_interface_local.h"
#include "mongo/db/repl/oplogreader.h"
#include "mongo/db/repl/replication_coordinator_global.h"
#include "mongo/db/repl/replication_coordinator_impl.h"
#include "mongo/db/repl/rollback_source_impl.h"
#include "mongo/db/repl/rs_rollback.h"
#include "mongo/db/repl/rs_sync.h"
#include "mongo/db/stats/timer_stats.h"
#include "mongo/executor/network_interface_factory.h"
#include "mongo/rpc/metadata/repl_set_metadata.h"
#include "mongo/stdx/memory.h"
#include "mongo/util/concurrency/thread_pool.h"
#include "mongo/util/exit.h"
#include "mongo/util/fail_point_service.h"
#include "mongo/util/log.h"
#include "mongo/util/mongoutils/str.h"
#include "mongo/util/scopeguard.h"
#include "mongo/util/time_support.h"

namespace mongo {

using std::string;

namespace repl {

namespace {
const char hashFieldName[] = "h";
int SleepToAllowBatchingMillis = 2;
const int BatchIsSmallish = 40000;  // bytes

/**
 * Returns new thread pool for thead pool task executor.
 */
std::unique_ptr<ThreadPool> makeThreadPool() {
    ThreadPool::Options threadPoolOptions;
    threadPoolOptions.poolName = "rsBackgroundSync";
    return stdx::make_unique<ThreadPool>(threadPoolOptions);
}

/**
 * Checks the criteria for rolling back.
 * 'getNextOperation' returns the first result of the oplog tailing query.
 * 'lastOpTimeFetched' should be consistent with the predicate in the query.
 * Returns RemoteOplogStale if the oplog query has no results.
 * Returns OplogStartMissing if we cannot find the timestamp of the last fetched operation in
 * the remote oplog.
 */
Status checkRemoteOplogStart(stdx::function<StatusWith<BSONObj>()> getNextOperation,
                             OpTime lastOpTimeFetched,
                             long long lastHashFetched) {
    auto result = getNextOperation();
    if (!result.isOK()) {
        // The GTE query from upstream returns nothing, so we're ahead of the upstream.
        return Status(ErrorCodes::RemoteOplogStale,
                      "we are ahead of the sync source, will try to roll back");
    }
    BSONObj o = result.getValue();
    OpTime opTime = fassertStatusOK(28778, OpTime::parseFromOplogEntry(o));
    long long hash = o["h"].numberLong();
    if (opTime != lastOpTimeFetched || hash != lastHashFetched) {
        return Status(ErrorCodes::OplogStartMissing,
                      str::stream() << "our last op time fetched: " << lastOpTimeFetched.toString()
                                    << ". source's GTE: " << opTime.toString() << " hashes: ("
                                    << lastHashFetched << "/" << hash << ")");
    }
    return Status::OK();
}

}  // namespace

MONGO_FP_DECLARE(rsBgSyncProduce);
MONGO_FP_DECLARE(stepDownWhileDrainingFailPoint);

BackgroundSync* BackgroundSync::s_instance = 0;
stdx::mutex BackgroundSync::s_mutex;

// The number and time spent reading batches off the network
static TimerStats getmoreReplStats;
static ServerStatusMetricField<TimerStats> displayBatchesRecieved("repl.network.getmores",
                                                                  &getmoreReplStats);
// The oplog entries read via the oplog reader
static Counter64 opsReadStats;
static ServerStatusMetricField<Counter64> displayOpsRead("repl.network.ops", &opsReadStats);
// The bytes read via the oplog reader
static Counter64 networkByteStats;
static ServerStatusMetricField<Counter64> displayBytesRead("repl.network.bytes", &networkByteStats);

// The count of items in the buffer
static Counter64 bufferCountGauge;
static ServerStatusMetricField<Counter64> displayBufferCount("repl.buffer.count",
                                                             &bufferCountGauge);
// The size (bytes) of items in the buffer
static Counter64 bufferSizeGauge;
static ServerStatusMetricField<Counter64> displayBufferSize("repl.buffer.sizeBytes",
                                                            &bufferSizeGauge);
// The max size (bytes) of the buffer
static int bufferMaxSizeGauge = 256 * 1024 * 1024;
static ServerStatusMetricField<int> displayBufferMaxSize("repl.buffer.maxSizeBytes",
                                                         &bufferMaxSizeGauge);


BackgroundSyncInterface::~BackgroundSyncInterface() {}

namespace {
size_t getSize(const BSONObj& o) {
    // SERVER-9808 Avoid Fortify complaint about implicit signed->unsigned conversion
    return static_cast<size_t>(o.objsize());
}
}  // namespace

BackgroundSync::BackgroundSync()
    : _buffer(bufferMaxSizeGauge, &getSize),
      _threadPoolTaskExecutor(makeThreadPool(), executor::makeNetworkInterface()),
      _lastOpTimeFetched(Timestamp(std::numeric_limits<int>::max(), 0),
                         std::numeric_limits<long long>::max()),
      _lastFetchedHash(0),
      _pause(true),
      _appliedBuffer(true),
      _replCoord(getGlobalReplicationCoordinator()),
      _initialSyncRequestedFlag(false),
      _indexPrefetchConfig(PREFETCH_ALL) {}

BackgroundSync* BackgroundSync::get() {
    stdx::unique_lock<stdx::mutex> lock(s_mutex);
    if (s_instance == NULL && !inShutdown()) {
        s_instance = new BackgroundSync();
    }
    return s_instance;
}

void BackgroundSync::shutdown() {
    stdx::lock_guard<stdx::mutex> lock(_mutex);

    // Clear the buffer in case the producerThread is waiting in push() due to a full queue.
    invariant(inShutdown());
    _buffer.clear();
    _pause = true;

    // Wake up producerThread so it notices that we're in shutdown
    _appliedBufferCondition.notify_all();
    _pausedCondition.notify_all();
}

void BackgroundSync::notify(OperationContext* txn) {
    stdx::lock_guard<stdx::mutex> lock(_mutex);

    // If all ops in the buffer have been applied, unblock waitForRepl (if it's waiting)
    if (_buffer.empty()) {
        _appliedBuffer = true;
        _appliedBufferCondition.notify_all();
    }
}

void BackgroundSync::producerThread() {
    Client::initThread("rsBackgroundSync");
    AuthorizationSession::get(cc())->grantInternalAuthorization();

    _threadPoolTaskExecutor.startup();
    ON_BLOCK_EXIT([this]() {
        _threadPoolTaskExecutor.shutdown();
        _threadPoolTaskExecutor.join();
    });

    while (!inShutdown()) {
        try {
            _producerThread();
        } catch (const DBException& e) {
            std::string msg(str::stream() << "sync producer problem: " << e.toString());
            error() << msg;
            _replCoord->setMyHeartbeatMessage(msg);
        } catch (const std::exception& e2) {
            severe() << "sync producer exception: " << e2.what();
            fassertFailed(28546);
        }
    }
    stop();
}

void BackgroundSync::_producerThread() {
    const MemberState state = _replCoord->getMemberState();
    // we want to pause when the state changes to primary
    if (_replCoord->isWaitingForApplierToDrain() || state.primary()) {
        if (!isPaused()) {
            stop();
        }
        if (_replCoord->isWaitingForApplierToDrain() && _buffer.empty()) {
            // This will wake up the applier if it is sitting in blockingPeek().
            _buffer.clear();
        }
        sleepsecs(1);
        return;
    }

    // TODO(spencer): Use a condition variable to await loading a config.
    if (state.startup()) {
        // Wait for a config to be loaded
        sleepsecs(1);
        return;
    }

    // We need to wait until initial sync has started.
    if (_replCoord->getMyLastOptime().isNull()) {
        sleepsecs(1);
        return;
    }
    // we want to unpause when we're no longer primary
    // start() also loads _lastOpTimeFetched, which we know is set from the "if"
    OperationContextImpl txn;
    if (isPaused()) {
        start(&txn);
    }

    _produce(&txn);
}

void BackgroundSync::_produce(OperationContext* txn) {
    // this oplog reader does not do a handshake because we don't want the server it's syncing
    // from to track how far it has synced
    {
        stdx::unique_lock<stdx::mutex> lock(_mutex);
        if (_lastOpTimeFetched.isNull()) {
            // then we're initial syncing and we're still waiting for this to be set
            lock.unlock();
            sleepsecs(1);
            // if there is no one to sync from
            return;
        }

        if (_replCoord->isWaitingForApplierToDrain() || _replCoord->getMemberState().primary() ||
            inShutdownStrict()) {
            return;
        }
    }

    while (MONGO_FAIL_POINT(rsBgSyncProduce)) {
        sleepmillis(0);
    }


    // find a target to sync from the last optime fetched
    OpTime lastOpTimeFetched;
    {
        stdx::unique_lock<stdx::mutex> lock(_mutex);
        lastOpTimeFetched = _lastOpTimeFetched;
        _syncSourceHost = HostAndPort();
    }
    OplogReader syncSourceReader;
    syncSourceReader.connectToSyncSource(txn, lastOpTimeFetched, _replCoord);

    // no server found
    if (syncSourceReader.getHost().empty()) {
        sleepsecs(1);
        // if there is no one to sync from
        return;
    }

    long long lastHashFetched;
    {
        stdx::lock_guard<stdx::mutex> lock(_mutex);
        if (_pause) {
            return;
        }
        lastOpTimeFetched = _lastOpTimeFetched;
        lastHashFetched = _lastFetchedHash;
        _syncSourceHost = syncSourceReader.getHost();
        _replCoord->signalUpstreamUpdater();
    }

    const Milliseconds oplogSocketTimeout(OplogReader::kSocketTimeout);

    const auto isV1ElectionProtocol = _replCoord->isV1ElectionProtocol();
    // Under protocol version 1, make the awaitData timeout (maxTimeMS) dependent on the election
    // timeout. This enables the sync source to communicate liveness of the primary to secondaries.
    // Under protocol version 0, use a default timeout of 2 seconds for awaitData.
    const Milliseconds fetcherMaxTimeMS(
        isV1ElectionProtocol ? _replCoord->getConfig().getElectionTimeoutPeriod() / 2 : Seconds(2));

    // Prefer host in oplog reader to _syncSourceHost because _syncSourceHost may be cleared
    // if sync source feedback fails.
    const HostAndPort source = syncSourceReader.getHost();
    syncSourceReader.resetConnection();
    // no more references to oplog reader from here on.

    // If this status is not OK after the fetcher returns from wait(),
    // proceed to execute rollback
    Status remoteOplogStartStatus = Status::OK();

    auto fetcherCallback = stdx::bind(&BackgroundSync::_fetcherCallback,
                                      this,
                                      stdx::placeholders::_1,
                                      stdx::placeholders::_3,
                                      stdx::cref(source),
                                      lastOpTimeFetched,
                                      lastHashFetched,
                                      fetcherMaxTimeMS,
                                      &remoteOplogStartStatus);


    BSONObjBuilder cmdBob;
    cmdBob.append("find", nsToCollectionSubstring(rsOplogName));
    cmdBob.append("filter", BSON("ts" << BSON("$gte" << lastOpTimeFetched.getTimestamp())));
    cmdBob.append("tailable", true);
    cmdBob.append("oplogReplay", true);
    cmdBob.append("awaitData", true);
    cmdBob.append("maxTimeMS", durationCount<Milliseconds>(fetcherMaxTimeMS));

    BSONObjBuilder metadataBob;
    if (isV1ElectionProtocol) {
        cmdBob.append("term", _replCoord->getTerm());
        metadataBob.append(rpc::kReplSetMetadataFieldName, 1);
    }

    auto dbName = nsToDatabase(rsOplogName);
    auto cmdObj = cmdBob.obj();
    auto metadataObj = metadataBob.obj();
    Fetcher fetcher(&_threadPoolTaskExecutor,
                    source,
                    dbName,
                    cmdObj,
                    fetcherCallback,
                    metadataObj,
                    _replCoord->getConfig().getElectionTimeoutPeriod());

    auto scheduleStatus = fetcher.schedule();
    if (!scheduleStatus.isOK()) {
        warning() << "unable to schedule fetcher to read remote oplog on " << source << ": "
                  << scheduleStatus;
        return;
    }
    fetcher.wait();

    // If the background sync is paused after the fetcher is started, we need to
    // re-evaluate our sync source and oplog common point.
    if (isPaused()) {
        return;
    }

    // Execute rollback if necessary.
    // Rollback is a synchronous operation that uses the task executor and may not be
    // executed inside the fetcher callback.
    if (!remoteOplogStartStatus.isOK()) {
        const int messagingPortTags = 0;
        ConnectionPool connectionPool(messagingPortTags);
        std::unique_ptr<ConnectionPool::ConnectionPtr> connection;
        auto getConnection =
            [&connection, &connectionPool, oplogSocketTimeout, source]() -> DBClientBase* {
                if (!connection.get()) {
                    connection.reset(new ConnectionPool::ConnectionPtr(
                        &connectionPool, source, Date_t::now(), oplogSocketTimeout));
                };
                return connection->get();
            };

        log() << "starting rollback: " << remoteOplogStartStatus;
        _rollback(txn, source, getConnection);
        stop();
    }
}

void BackgroundSync::_fetcherCallback(const StatusWith<Fetcher::QueryResponse>& result,
                                      BSONObjBuilder* bob,
                                      const HostAndPort& source,
                                      OpTime lastOpTimeFetched,
                                      long long lastFetchedHash,
                                      Milliseconds fetcherMaxTimeMS,
                                      Status* remoteOplogStartStatus) {
    // if target cut connections between connecting and querying (for
    // example, because it stepped down) we might not have a cursor
    if (!result.isOK()) {
        return;
    }

    if (inShutdown()) {
        return;
    }

    // Check if we have been paused.
    if (isPaused()) {
        return;
    }

    const auto& queryResponse = result.getValue();
    bool syncSourceHasSyncSource = false;
    OpTime sourcesLastOp;

    // Forward metadata (containing liveness information) to replication coordinator.
    bool receivedMetadata =
        queryResponse.otherFields.metadata.hasElement(rpc::kReplSetMetadataFieldName);
    if (receivedMetadata) {
        auto metadataResult =
            rpc::ReplSetMetadata::readFromMetadata(queryResponse.otherFields.metadata);
        if (!metadataResult.isOK()) {
            error() << "invalid replication metadata from sync source " << source << ": "
                    << metadataResult.getStatus() << ": " << queryResponse.otherFields.metadata;
            return;
        }
        const auto& metadata = metadataResult.getValue();
        _replCoord->processReplSetMetadata(metadata);
        if (metadata.getPrimaryIndex() != rpc::ReplSetMetadata::kNoPrimary) {
            _replCoord->cancelAndRescheduleElectionTimeout();
        }
        syncSourceHasSyncSource = metadata.getSyncSourceIndex() != -1;
        sourcesLastOp = metadata.getLastOpVisible();
    }

    const auto& documents = queryResponse.documents;
    auto documentBegin = documents.cbegin();
    auto documentEnd = documents.cend();

    // Check start of remote oplog and, if necessary, stop fetcher to execute rollback.
    if (queryResponse.first) {
        auto getNextOperation = [&documentBegin, documentEnd]() -> StatusWith<BSONObj> {
            if (documentBegin == documentEnd) {
                return Status(ErrorCodes::OplogStartMissing, "remote oplog start missing");
            }
            return *(documentBegin++);
        };

        *remoteOplogStartStatus =
            checkRemoteOplogStart(getNextOperation, lastOpTimeFetched, lastFetchedHash);
        if (!remoteOplogStartStatus->isOK()) {
            // Stop fetcher and execute rollback.
            return;
        }

        // If this is the first batch and no rollback is needed, we should have advanced
        // the document iterator.
        invariant(documentBegin != documents.cbegin());
    }

    // process documents
    int currentBatchMessageSize = 0;
    for (auto documentIter = documentBegin; documentIter != documentEnd; ++documentIter) {
        if (inShutdown()) {
            return;
        }

        // If we are transitioning to primary state, we need to leave
        // this loop in order to go into bgsync-pause mode.
        if (_replCoord->isWaitingForApplierToDrain() || _replCoord->getMemberState().primary()) {
            LOG(1) << "waiting for draining or we are primary, not adding more ops to buffer";
            return;
        }

        // At this point, we are guaranteed to have at least one thing to read out
        // of the fetcher.
        const BSONObj& o = *documentIter;
        currentBatchMessageSize += o.objsize();
        opsReadStats.increment();

        if (MONGO_FAIL_POINT(stepDownWhileDrainingFailPoint)) {
            sleepsecs(20);
        }

        {
            stdx::unique_lock<stdx::mutex> lock(_mutex);
            _appliedBuffer = false;
        }

        OCCASIONALLY {
            LOG(2) << "bgsync buffer has " << _buffer.size() << " bytes";
        }

        bufferCountGauge.increment();
        bufferSizeGauge.increment(getSize(o));
        _buffer.push(o);

        {
            stdx::unique_lock<stdx::mutex> lock(_mutex);
            _lastFetchedHash = o["h"].numberLong();
            _lastOpTimeFetched = fassertStatusOK(28770, OpTime::parseFromOplogEntry(o));
            LOG(3) << "lastOpTimeFetched: " << _lastOpTimeFetched;
        }
    }

    // record time for each batch
    getmoreReplStats.recordMillis(durationCount<Milliseconds>(queryResponse.elapsedMillis));

    networkByteStats.increment(currentBatchMessageSize);

    // Check some things periodically
    // (whenever we run out of items in the
    // current cursor batch)
    if (currentBatchMessageSize > 0 && currentBatchMessageSize < BatchIsSmallish) {
        // on a very low latency network, if we don't wait a little, we'll be
        // getting ops to write almost one at a time.  this will both be expensive
        // for the upstream server as well as potentially defeating our parallel
        // application of batches on the secondary.
        //
        // the inference here is basically if the batch is really small, we are
        // "caught up".
        //
        sleepmillis(SleepToAllowBatchingMillis);
    }

    // If we are transitioning to primary state, we need to leave
    // this loop in order to go into bgsync-pause mode.
    if (_replCoord->isWaitingForApplierToDrain() || _replCoord->getMemberState().primary()) {
        return;
    }

    // re-evaluate quality of sync target
    if (_shouldChangeSyncSource(source, sourcesLastOp, syncSourceHasSyncSource)) {
        return;
    }

    // Check if we have been paused.
    if (isPaused()) {
        return;
    }

    // We fill in 'bob' to signal the fetcher to process with another getMore.
    invariant(bob);
    bob->append("getMore", queryResponse.cursorId);
    bob->append("collection", queryResponse.nss.coll());
    bob->append("maxTimeMS", durationCount<Milliseconds>(fetcherMaxTimeMS));
    if (receivedMetadata) {
        bob->append("term", _replCoord->getTerm());
        _replCoord->getLastCommittedOpTime().append(bob, "lastKnownCommittedOpTime");
    }
}

bool BackgroundSync::_shouldChangeSyncSource(const HostAndPort& syncSource,
                                             const OpTime& syncSourceLastOpTime,
                                             bool syncSourceHasSyncSource) {
    // is it even still around?
    if (getSyncTarget().empty() || syncSource.empty()) {
        return true;
    }

    // check other members: is any member's optime more than MaxSyncSourceLag seconds
    // ahead of the current sync source?
    return _replCoord->shouldChangeSyncSource(
        syncSource, syncSourceLastOpTime, syncSourceHasSyncSource);
}


bool BackgroundSync::peek(BSONObj* op) {
    return _buffer.peek(*op);
}

void BackgroundSync::waitForMore() {
    BSONObj op;
    // Block for one second before timing out.
    // Ignore the value of the op we peeked at.
    _buffer.blockingPeek(op, 1);
}

void BackgroundSync::consume() {
    // this is just to get the op off the queue, it's been peeked at
    // and queued for application already
    BSONObj op = _buffer.blockingPop();
    bufferCountGauge.decrement(1);
    bufferSizeGauge.decrement(getSize(op));
}

void BackgroundSync::_rollback(OperationContext* txn,
                               const HostAndPort& source,
                               stdx::function<DBClientBase*()> getConnection) {
    // Abort only when syncRollback detects we are in a unrecoverable state.
    // In other cases, we log the message contained in the error status and retry later.
    auto status = syncRollback(txn,
                               _replCoord->getMyLastOptime(),
                               OplogInterfaceLocal(txn, rsOplogName),
                               RollbackSourceImpl(getConnection, source, rsOplogName),
                               _replCoord);
    if (status.isOK()) {
        return;
    }
    if (ErrorCodes::UnrecoverableRollbackError == status.code()) {
        fassertNoTrace(28723, status);
    }
    warning() << "rollback cannot proceed at this time (retrying later): " << status;
}

HostAndPort BackgroundSync::getSyncTarget() {
    stdx::unique_lock<stdx::mutex> lock(_mutex);
    return _syncSourceHost;
}

void BackgroundSync::clearSyncTarget() {
    stdx::unique_lock<stdx::mutex> lock(_mutex);
    _syncSourceHost = HostAndPort();
}

void BackgroundSync::cancelFetcher() {
    _threadPoolTaskExecutor.cancelAllCommands();
}

void BackgroundSync::stop() {
    stdx::lock_guard<stdx::mutex> lock(_mutex);

    _pause = true;
    _syncSourceHost = HostAndPort();
    _lastOpTimeFetched = OpTime();
    _lastFetchedHash = 0;
    _appliedBufferCondition.notify_all();
    _pausedCondition.notify_all();
}

void BackgroundSync::start(OperationContext* txn) {
    massert(16235, "going to start syncing, but buffer is not empty", _buffer.empty());

    long long lastFetchedHash = _readLastAppliedHash(txn);
    stdx::lock_guard<stdx::mutex> lk(_mutex);
    _pause = false;

    // reset _last fields with current oplog data
    _lastOpTimeFetched = _replCoord->getMyLastOptime();
    _lastFetchedHash = lastFetchedHash;

    LOG(1) << "bgsync fetch queue set to: " << _lastOpTimeFetched << " " << _lastFetchedHash;
}

bool BackgroundSync::isPaused() const {
    stdx::lock_guard<stdx::mutex> lock(_mutex);
    return _pause;
}

void BackgroundSync::waitUntilPaused() {
    stdx::unique_lock<stdx::mutex> lock(_mutex);
    while (!_pause) {
        _pausedCondition.wait(lock);
    }
}

void BackgroundSync::clearBuffer() {
    _buffer.clear();
}

long long BackgroundSync::_readLastAppliedHash(OperationContext* txn) {
    BSONObj oplogEntry;
    try {
        MONGO_WRITE_CONFLICT_RETRY_LOOP_BEGIN {
            ScopedTransaction transaction(txn, MODE_IX);
            Lock::DBLock lk(txn->lockState(), "local", MODE_X);
            bool success = Helpers::getLast(txn, rsOplogName.c_str(), oplogEntry);
            if (!success) {
                // This can happen when we are to do an initial sync.  lastHash will be set
                // after the initial sync is complete.
                return 0;
            }
        }
        MONGO_WRITE_CONFLICT_RETRY_LOOP_END(txn, "readLastAppliedHash", rsOplogName);
    } catch (const DBException& ex) {
        severe() << "Problem reading " << rsOplogName << ": " << ex.toStatus();
        fassertFailed(18904);
    }
    BSONElement hashElement = oplogEntry[hashFieldName];
    if (hashElement.eoo()) {
        severe() << "Most recent entry in " << rsOplogName << " missing \"" << hashFieldName
                 << "\" field";
        fassertFailed(18902);
    }
    if (hashElement.type() != NumberLong) {
        severe() << "Expected type of \"" << hashFieldName << "\" in most recent " << rsOplogName
                 << " entry to have type NumberLong, but found " << typeName(hashElement.type());
        fassertFailed(18903);
    }
    return hashElement.safeNumberLong();
}

bool BackgroundSync::getInitialSyncRequestedFlag() {
    stdx::lock_guard<stdx::mutex> lock(_initialSyncMutex);
    return _initialSyncRequestedFlag;
}

void BackgroundSync::setInitialSyncRequestedFlag(bool value) {
    stdx::lock_guard<stdx::mutex> lock(_initialSyncMutex);
    _initialSyncRequestedFlag = value;
}

void BackgroundSync::pushTestOpToBuffer(const BSONObj& op) {
    stdx::lock_guard<stdx::mutex> lock(_mutex);
    _buffer.push(op);
}


}  // namespace repl
}  // namespace mongo