path: root/src/mongo/db/storage/wiredtiger/wiredtiger_util.cpp

/**
 *    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.
 */

#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kStorage

#include "mongo/platform/basic.h"

#include "mongo/db/storage/wiredtiger/wiredtiger_util.h"

#include <limits>

#include "mongo/base/simple_string_data_comparator.h"
#include "mongo/bson/bsonobjbuilder.h"
#include "mongo/db/concurrency/write_conflict_exception.h"
#include "mongo/db/server_options.h"
#include "mongo/db/server_parameters.h"
#include "mongo/db/storage/wiredtiger/wiredtiger_recovery_unit.h"
#include "mongo/db/storage/wiredtiger/wiredtiger_session_cache.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/log.h"
#include "mongo/util/mongoutils/str.h"
#include "mongo/util/processinfo.h"
#include "mongo/util/scopeguard.h"

namespace mongo {

using std::string;

Status wtRCToStatus_slow(int retCode, const char* prefix) {
    if (retCode == 0)
        return Status::OK();

    if (retCode == WT_ROLLBACK) {
        throw WriteConflictException();
    }

    // Don't abort on WT_PANIC when repairing, as the error will be handled at a higher layer.
    fassert(28559, retCode != WT_PANIC || storageGlobalParams.repair);

    str::stream s;
    if (prefix)
        s << prefix << " ";
    s << retCode << ": " << wiredtiger_strerror(retCode);

    if (retCode == EINVAL) {
        return Status(ErrorCodes::BadValue, s);
    }

    uassert(ErrorCodes::ExceededMemoryLimit, s, retCode != WT_CACHE_FULL);

    // TODO convert specific codes rather than just using UNKNOWN_ERROR for everything.
    return Status(ErrorCodes::UnknownError, s);
}

void WiredTigerUtil::fetchTypeAndSourceURI(OperationContext* opCtx,
                                           const std::string& tableUri,
                                           std::string* type,
                                           std::string* source) {
    std::string colgroupUri = "colgroup";
    const size_t colon = tableUri.find(':');
    invariant(colon != string::npos);
    colgroupUri += tableUri.substr(colon);
    StatusWith<std::string> colgroupResult = getMetadata(opCtx, colgroupUri);
    invariant(colgroupResult.getStatus());
    WiredTigerConfigParser parser(colgroupResult.getValue());

    WT_CONFIG_ITEM typeItem;
    invariant(parser.get("type", &typeItem) == 0);
    invariant(typeItem.type == WT_CONFIG_ITEM::WT_CONFIG_ITEM_ID);
    *type = std::string(typeItem.str, typeItem.len);

    WT_CONFIG_ITEM sourceItem;
    invariant(parser.get("source", &sourceItem) == 0);
    invariant(sourceItem.type == WT_CONFIG_ITEM::WT_CONFIG_ITEM_STRING);
    *source = std::string(sourceItem.str, sourceItem.len);
}

StatusWith<std::string> WiredTigerUtil::getMetadataRaw(WT_SESSION* session, StringData uri) {
    WT_CURSOR* cursor;
    invariantWTOK(session->open_cursor(session, "metadata:create", nullptr, "", &cursor));
    invariant(cursor);
    ON_BLOCK_EXIT([cursor] { invariantWTOK(cursor->close(cursor)); });

    std::string strUri = uri.toString();
    cursor->set_key(cursor, strUri.c_str());
    int ret = cursor->search(cursor);
    if (ret == WT_NOTFOUND) {
        return StatusWith<std::string>(ErrorCodes::NoSuchKey,
                                       str::stream() << "Unable to find metadata for " << uri);
    } else if (ret != 0) {
        return StatusWith<std::string>(wtRCToStatus(ret));
    }
    const char* metadata = NULL;
    ret = cursor->get_value(cursor, &metadata);
    if (ret != 0) {
        return StatusWith<std::string>(wtRCToStatus(ret));
    }
    invariant(metadata);
    return StatusWith<std::string>(metadata);
}

StatusWith<std::string> WiredTigerUtil::getMetadata(OperationContext* opCtx, StringData uri) {
    invariant(opCtx);

    auto session = WiredTigerRecoveryUnit::get(opCtx)->getSessionNoTxn();
    WT_CURSOR* cursor =
        session->getCursor("metadata:create", WiredTigerSession::kMetadataTableId, false);
    invariant(cursor);
    auto releaser =
        MakeGuard([&] { session->releaseCursor(WiredTigerSession::kMetadataTableId, cursor); });

    std::string strUri = uri.toString();
    cursor->set_key(cursor, strUri.c_str());
    int ret = cursor->search(cursor);
    if (ret == WT_NOTFOUND) {
        return StatusWith<std::string>(ErrorCodes::NoSuchKey,
                                       str::stream() << "Unable to find metadata for " << uri);
    } else if (ret != 0) {
        return StatusWith<std::string>(wtRCToStatus(ret));
    }
    const char* metadata = NULL;
    ret = cursor->get_value(cursor, &metadata);
    if (ret != 0) {
        return StatusWith<std::string>(wtRCToStatus(ret));
    }
    invariant(metadata);
    return StatusWith<std::string>(metadata);
}

Status WiredTigerUtil::getApplicationMetadata(OperationContext* opCtx,
                                              StringData uri,
                                              BSONObjBuilder* bob) {
    StatusWith<std::string> metadataResult = getMetadata(opCtx, uri);
    if (!metadataResult.isOK()) {
        return metadataResult.getStatus();
    }
    WiredTigerConfigParser topParser(metadataResult.getValue());
    WT_CONFIG_ITEM appMetadata;
    if (topParser.get("app_metadata", &appMetadata) != 0) {
        return Status::OK();
    }
    if (appMetadata.len == 0) {
        return Status::OK();
    }
    if (appMetadata.type != WT_CONFIG_ITEM::WT_CONFIG_ITEM_STRUCT) {
        return Status(ErrorCodes::FailedToParse,
                      str::stream() << "app_metadata must be a nested struct. Actual value: "
                                    << StringData(appMetadata.str, appMetadata.len));
    }

    WiredTigerConfigParser parser(appMetadata);
    WT_CONFIG_ITEM keyItem;
    WT_CONFIG_ITEM valueItem;
    int ret;
    auto keysSeen = SimpleStringDataComparator::kInstance.makeStringDataUnorderedSet();
    while ((ret = parser.next(&keyItem, &valueItem)) == 0) {
        const StringData key(keyItem.str, keyItem.len);
        if (keysSeen.count(key)) {
            return Status(ErrorCodes::DuplicateKey,
                          str::stream() << "app_metadata must not contain duplicate keys. "
                                        << "Found multiple instances of key '"
                                        << key
                                        << "'.");
        }
        keysSeen.insert(key);

        switch (valueItem.type) {
            case WT_CONFIG_ITEM::WT_CONFIG_ITEM_BOOL:
                bob->appendBool(key, valueItem.val);
                break;
            case WT_CONFIG_ITEM::WT_CONFIG_ITEM_NUM:
                bob->appendIntOrLL(key, valueItem.val);
                break;
            default:
                bob->append(key, StringData(valueItem.str, valueItem.len));
                break;
        }
    }
    if (ret != WT_NOTFOUND) {
        return wtRCToStatus(ret);
    }

    return Status::OK();
}

StatusWith<BSONObj> WiredTigerUtil::getApplicationMetadata(OperationContext* opCtx,
                                                           StringData uri) {
    BSONObjBuilder bob;
    Status status = getApplicationMetadata(opCtx, uri, &bob);
    if (!status.isOK()) {
        return StatusWith<BSONObj>(status);
    }
    return StatusWith<BSONObj>(bob.obj());
}

StatusWith<int64_t> WiredTigerUtil::checkApplicationMetadataFormatVersion(OperationContext* opCtx,
                                                                          StringData uri,
                                                                          int64_t minimumVersion,
                                                                          int64_t maximumVersion) {
    StatusWith<std::string> result = getMetadata(opCtx, uri);
    if (result.getStatus().code() == ErrorCodes::NoSuchKey) {
        return result.getStatus();
    }
    invariant(result.getStatus());

    WiredTigerConfigParser topParser(result.getValue());
    WT_CONFIG_ITEM metadata;
    if (topParser.get("app_metadata", &metadata) != 0)
        return Status(ErrorCodes::UnsupportedFormat,
                      str::stream() << "application metadata for " << uri << " is missing ");

    if (metadata.type != WT_CONFIG_ITEM::WT_CONFIG_ITEM_STRUCT) {
        return Status(ErrorCodes::FailedToParse,
                      str::stream()
                          << "application metadata must be enclosed in parentheses. Actual value: "
                          << StringData(metadata.str, metadata.len));
    }

    WiredTigerConfigParser parser(metadata);

    int64_t version = 0;
    WT_CONFIG_ITEM versionItem;
    if (parser.get("formatVersion", &versionItem) != 0) {
        // If 'formatVersion' is missing, this metadata was introduced by
        // one of the RC versions (where the format version is 1).
        version = 1;
    } else if (versionItem.type == WT_CONFIG_ITEM::WT_CONFIG_ITEM_NUM) {
        version = versionItem.val;
    } else {
        return Status(ErrorCodes::UnsupportedFormat,
                      str::stream() << "'formatVersion' in application metadata for " << uri
                                    << " must be a number. Current value: "
                                    << StringData(versionItem.str, versionItem.len));
    }

    if (version < minimumVersion || version > maximumVersion) {
        return Status(ErrorCodes::UnsupportedFormat,
                      str::stream() << "Application metadata for " << uri
                                    << " has unsupported format version: "
                                    << version
                                    << ".");
    }

    LOG(2) << "WiredTigerUtil::checkApplicationMetadataFormatVersion "
           << " uri: " << uri << " ok range " << minimumVersion << " -> " << maximumVersion
           << " current: " << version;

    return version;
}

// static
Status WiredTigerUtil::checkTableCreationOptions(const BSONElement& configElem) {
    invariant(configElem.fieldNameStringData() == "configString");

    if (configElem.type() != String) {
        return {ErrorCodes::TypeMismatch, "'configString' must be a string."};
    }

    std::vector<std::string> errors;
    ErrorAccumulator eventHandler(&errors);

    StringData config = configElem.valueStringData();
    // Do NOT allow embedded null characters
    if (config.size() != strlen(config.rawData())) {
        return {ErrorCodes::FailedToParse, "malformed 'configString' value."};
    }

    Status status = wtRCToStatus(
        wiredtiger_config_validate(nullptr, &eventHandler, "WT_SESSION.create", config.rawData()));
    if (!status.isOK()) {
        StringBuilder errorMsg;
        errorMsg << status.reason();
        for (std::string error : errors) {
            errorMsg << ". " << error;
        }
        errorMsg << ".";
        return status.withReason(errorMsg.stringData());
    }
    return Status::OK();
}

// static
StatusWith<uint64_t> WiredTigerUtil::getStatisticsValue(WT_SESSION* session,
                                                        const std::string& uri,
                                                        const std::string& config,
                                                        int statisticsKey) {
    invariant(session);
    WT_CURSOR* cursor = NULL;
    const char* cursorConfig = config.empty() ? NULL : config.c_str();
    int ret = session->open_cursor(session, uri.c_str(), NULL, cursorConfig, &cursor);
    if (ret != 0) {
        return StatusWith<uint64_t>(ErrorCodes::CursorNotFound,
                                    str::stream() << "unable to open cursor at URI " << uri
                                                  << ". reason: "
                                                  << wiredtiger_strerror(ret));
    }
    invariant(cursor);
    ON_BLOCK_EXIT(cursor->close, cursor);

    cursor->set_key(cursor, statisticsKey);
    ret = cursor->search(cursor);
    if (ret != 0) {
        return StatusWith<uint64_t>(
            ErrorCodes::NoSuchKey,
            str::stream() << "unable to find key " << statisticsKey << " at URI " << uri
                          << ". reason: "
                          << wiredtiger_strerror(ret));
    }

    uint64_t value;
    ret = cursor->get_value(cursor, NULL, NULL, &value);
    if (ret != 0) {
        return StatusWith<uint64_t>(
            ErrorCodes::BadValue,
            str::stream() << "unable to get value for key " << statisticsKey << " at URI " << uri
                          << ". reason: "
                          << wiredtiger_strerror(ret));
    }

    return StatusWith<uint64_t>(value);
}

int64_t WiredTigerUtil::getIdentSize(WT_SESSION* s, const std::string& uri) {
    StatusWith<int64_t> result = WiredTigerUtil::getStatisticsValueAs<int64_t>(
        s, "statistics:" + uri, "statistics=(size)", WT_STAT_DSRC_BLOCK_SIZE);
    const Status& status = result.getStatus();
    if (!status.isOK()) {
        if (status.code() == ErrorCodes::CursorNotFound) {
            // ident gone, so its 0
            return 0;
        }
        uassertStatusOK(status);
    }
    return result.getValue();
}

size_t WiredTigerUtil::getCacheSizeMB(double requestedCacheSizeGB) {
    double cacheSizeMB;
    const double kMaxSizeCacheMB = 10 * 1000 * 1000;
    if (requestedCacheSizeGB == 0) {
        // Choose a reasonable amount of cache when not explicitly specified by user.
        // Set a minimum of 256MB, otherwise use 50% of available memory over 1GB.
        ProcessInfo pi;
        double memSizeMB = pi.getMemSizeMB();
        cacheSizeMB = std::max((memSizeMB - 1024) * 0.5, 256.0);
    } else {
        cacheSizeMB = 1024 * requestedCacheSizeGB;
    }
    if (cacheSizeMB > kMaxSizeCacheMB) {
        log() << "Requested cache size: " << cacheSizeMB << "MB exceeds max; setting to "
              << kMaxSizeCacheMB << "MB";
        cacheSizeMB = kMaxSizeCacheMB;
    }
    return static_cast<size_t>(cacheSizeMB);
}

namespace {
int mdb_handle_error_with_startup_suppression(WT_EVENT_HANDLER* handler,
                                              WT_SESSION* session,
                                              int errorCode,
                                              const char* message) {
    WiredTigerEventHandler* wtHandler = reinterpret_cast<WiredTigerEventHandler*>(handler);

    try {
        StringData sd(message);
        if (!wtHandler->wasStartupSuccessful()) {
            // During startup, storage tries different WiredTiger compatibility modes to determine
            // the state of the data files before FCV can be read. Suppress the error messages
            // regarding expected version compatibility requirements.
            if (sd.find("Version incompatibility detected:") != std::string::npos) {
                return 0;
            }
        }

        error() << "WiredTiger error (" << errorCode << ") " << redact(message)
                << " Raw: " << message;

        // Don't abort on WT_PANIC when repairing, as the error will be handled at a higher layer.
        if (storageGlobalParams.repair) {
            return 0;
        }
        fassert(50853, errorCode != WT_PANIC);
    } catch (...) {
        std::terminate();
    }
    return 0;
}

int mdb_handle_error(WT_EVENT_HANDLER* handler,
                     WT_SESSION* session,
                     int errorCode,
                     const char* message) {
    try {
        error() << "WiredTiger error (" << errorCode << ") " << redact(message);

        // Don't abort on WT_PANIC when repairing, as the error will be handled at a higher layer.
        if (storageGlobalParams.repair) {
            return 0;
        }
        fassert(28558, errorCode != WT_PANIC);
    } catch (...) {
        std::terminate();
    }
    return 0;
}

int mdb_handle_message(WT_EVENT_HANDLER* handler, WT_SESSION* session, const char* message) {
    try {
        log() << "WiredTiger message " << redact(message);
    } catch (...) {
        std::terminate();
    }
    return 0;
}

int mdb_handle_progress(WT_EVENT_HANDLER* handler,
                        WT_SESSION* session,
                        const char* operation,
                        uint64_t progress) {
    try {
        log() << "WiredTiger progress " << redact(operation) << " " << progress;
    } catch (...) {
        std::terminate();
    }

    return 0;
}

WT_EVENT_HANDLER defaultEventHandlers() {
    WT_EVENT_HANDLER handlers = {};
    handlers.handle_error = mdb_handle_error;
    handlers.handle_message = mdb_handle_message;
    handlers.handle_progress = mdb_handle_progress;
    return handlers;
}
}

WiredTigerEventHandler::WiredTigerEventHandler() {
    WT_EVENT_HANDLER* handler = static_cast<WT_EVENT_HANDLER*>(this);
    invariant((void*)this == (void*)handler);

    handler->handle_error = mdb_handle_error_with_startup_suppression;
    handler->handle_message = mdb_handle_message;
    handler->handle_progress = mdb_handle_progress;
    handler->handle_close = nullptr;
}

WT_EVENT_HANDLER* WiredTigerEventHandler::getWtEventHandler() {
    WT_EVENT_HANDLER* ret = static_cast<WT_EVENT_HANDLER*>(this);
    invariant((void*)this == (void*)ret);

    return ret;
}

WiredTigerUtil::ErrorAccumulator::ErrorAccumulator(std::vector<std::string>* errors)
    : WT_EVENT_HANDLER(defaultEventHandlers()),
      _errors(errors),
      _defaultErrorHandler(handle_error) {
    if (errors) {
        handle_error = onError;
    }
}

// static
int WiredTigerUtil::ErrorAccumulator::onError(WT_EVENT_HANDLER* handler,
                                              WT_SESSION* session,
                                              int error,
                                              const char* message) {
    try {
        ErrorAccumulator* self = static_cast<ErrorAccumulator*>(handler);
        self->_errors->push_back(message);
        return self->_defaultErrorHandler(handler, session, error, message);
    } catch (...) {
        std::terminate();
    }
}

int WiredTigerUtil::verifyTable(OperationContext* opCtx,
                                const std::string& uri,
                                std::vector<std::string>* errors) {
    ErrorAccumulator eventHandler(errors);

    // Try to close as much as possible to avoid EBUSY errors.
    WiredTigerRecoveryUnit::get(opCtx)->getSession()->closeAllCursors(uri);
    WiredTigerSessionCache* sessionCache = WiredTigerRecoveryUnit::get(opCtx)->getSessionCache();
    sessionCache->closeAllCursors(uri);

    // Open a new session with custom error handlers.
    WT_CONNECTION* conn = WiredTigerRecoveryUnit::get(opCtx)->getSessionCache()->conn();
    WT_SESSION* session;
    invariantWTOK(conn->open_session(conn, &eventHandler, NULL, &session));
    ON_BLOCK_EXIT(session->close, session, "");

    // Do the verify. Weird parens prevent treating "verify" as a macro.
    return (session->verify)(session, uri.c_str(), NULL);
}

bool WiredTigerUtil::useTableLogging(NamespaceString ns, bool replEnabled) {
    if (!replEnabled) {
        // All tables on standalones are logged.
        return true;
    }

    if (!serverGlobalParams.enableMajorityReadConcern) {
        return true;
    }

    // Of the replica set configurations:
    if (ns.db() != "local") {
        // All replicated collections are not logged.
        return false;
    }

    if (ns.coll() == "replset.minvalid") {
        // Of local collections, this is derived from the state of the data and therefore
        // not logged.
        return false;
    }

    // The remainder of local gets logged. In particular, the oplog and user created collections.
    return true;
}

Status WiredTigerUtil::setTableLogging(OperationContext* opCtx, const std::string& uri, bool on) {
    // Try to close as much as possible to avoid EBUSY errors.
    WiredTigerRecoveryUnit::get(opCtx)->getSession()->closeAllCursors(uri);
    WiredTigerSessionCache* sessionCache = WiredTigerRecoveryUnit::get(opCtx)->getSessionCache();
    sessionCache->closeAllCursors(uri);

    // Use a dedicated session for alter operations to avoid transaction issues.
    WiredTigerSession session(sessionCache->conn());
    return setTableLogging(session.getSession(), uri, on);
}

Status WiredTigerUtil::setTableLogging(WT_SESSION* session, const std::string& uri, bool on) {
    std::string setting;
    if (on) {
        setting = "log=(enabled=true)";
    } else {
        setting = "log=(enabled=false)";
    }

    // This method does some "weak" parsing to see if the table is in the expected logging
    // state. Only attempt to alter the table when a change is needed. This avoids grabbing heavy
    // locks in WT when creating new tables for collections and indexes. Those tables are created
    // with the proper settings and consequently should not be getting changed here.
    //
    // If the settings need to be changed (only expected at startup), the alter table call must
    // succeed.
    std::string existingMetadata = getMetadataRaw(session, uri).getValue();
    if (existingMetadata.find("log=(enabled=true)") != std::string::npos &&
        existingMetadata.find("log=(enabled=false)") != std::string::npos) {
        // Sanity check against a table having multiple logging specifications.
        invariant(false,
                  str::stream() << "Table has contradictory logging settings. Uri: " << uri
                                << " Conf: "
                                << existingMetadata);
    }

    if (existingMetadata.find(setting) != std::string::npos) {
        // The table is running with the expected logging settings.
        return Status::OK();
    }

    LOG(1) << "Changing table logging settings. Uri: " << uri << " Enable? " << on;
    int ret = session->alter(session, uri.c_str(), setting.c_str());
    if (ret) {
        severe() << "Failed to update log setting. Uri: " << uri << " Enable? " << on
                 << " Ret: " << ret << " MD: " << redact(existingMetadata)
                 << " Msg: " << session->strerror(session, ret);
        fassertFailed(50756);
    }

    return Status::OK();
}

Status WiredTigerUtil::exportTableToBSON(WT_SESSION* session,
                                         const std::string& uri,
                                         const std::string& config,
                                         BSONObjBuilder* bob) {
    invariant(session);
    invariant(bob);
    WT_CURSOR* c = NULL;
    const char* cursorConfig = config.empty() ? NULL : config.c_str();
    int ret = session->open_cursor(session, uri.c_str(), NULL, cursorConfig, &c);
    if (ret != 0) {
        return Status(ErrorCodes::CursorNotFound,
                      str::stream() << "unable to open cursor at URI " << uri << ". reason: "
                                    << wiredtiger_strerror(ret));
    }
    bob->append("uri", uri);
    invariant(c);
    ON_BLOCK_EXIT(c->close, c);

    std::map<string, BSONObjBuilder*> subs;
    const char* desc;
    uint64_t value;
    while (c->next(c) == 0 && c->get_value(c, &desc, NULL, &value) == 0) {
        StringData key(desc);

        StringData prefix;
        StringData suffix;

        size_t idx = key.find(':');
        if (idx != string::npos) {
            prefix = key.substr(0, idx);
            suffix = key.substr(idx + 1);
        } else {
            idx = key.find(' ');
        }

        if (idx != string::npos) {
            prefix = key.substr(0, idx);
            suffix = key.substr(idx + 1);
        } else {
            prefix = key;
            suffix = "num";
        }

        long long v = _castStatisticsValue<long long>(value);

        if (prefix.size() == 0) {
            bob->appendNumber(desc, v);
        } else {
            BSONObjBuilder*& sub = subs[prefix.toString()];
            if (!sub)
                sub = new BSONObjBuilder();
            sub->appendNumber(mongoutils::str::ltrim(suffix.toString()), v);
        }
    }

    for (std::map<string, BSONObjBuilder*>::const_iterator it = subs.begin(); it != subs.end();
         ++it) {
        const std::string& s = it->first;
        bob->append(s, it->second->obj());
        delete it->second;
    }
    return Status::OK();
}

Status WiredTigerUtil::exportOperationStatsInfoToBSON(WT_SESSION* session,
                                                      const std::string& uri,
                                                      const std::string& config,
                                                      BSONObjBuilder* bob) {
    invariant(session);
    invariant(bob);

    // Map the statistics to a name and type desired for user consumption. There are two types of
    // operation statistics - data and wait.
    enum class Section { DATA, WAIT };
    static std::map<int, std::pair<StringData, Section>> statNameMap = {
        {WT_STAT_SESSION_BYTES_READ, std::make_pair("bytesRead", Section::DATA)},
        {WT_STAT_SESSION_BYTES_WRITE, std::make_pair("bytesWritten", Section::DATA)},
        {WT_STAT_SESSION_LOCK_DHANDLE_WAIT, std::make_pair("handleLock", Section::WAIT)},
        {WT_STAT_SESSION_READ_TIME, std::make_pair("timeReadingMicros", Section::DATA)},
        {WT_STAT_SESSION_WRITE_TIME, std::make_pair("timeWritingMicros", Section::DATA)},
        {WT_STAT_SESSION_LOCK_SCHEMA_WAIT, std::make_pair("schemaLock", Section::WAIT)},
        {WT_STAT_SESSION_CACHE_TIME, std::make_pair("cache", Section::WAIT)}};

    WT_CURSOR* c = nullptr;
    const char* cursorConfig = config.empty() ? nullptr : config.c_str();
    int ret = session->open_cursor(session, uri.c_str(), nullptr, cursorConfig, &c);
    if (ret != 0) {
        return Status(ErrorCodes::CursorNotFound,
                      str::stream() << "unable to open cursor at URI " << uri << ". reason: "
                                    << wiredtiger_strerror(ret));
    }
    invariant(c);
    ON_BLOCK_EXIT(c->close, c);

    BSONObjBuilder* dataSection = nullptr;
    BSONObjBuilder* waitSection = nullptr;
    const char* desc;
    uint64_t value;
    uint64_t key;
    while (c->next(c) == 0 && c->get_key(c, &key) == 0) {
        fassert(51035, c->get_value(c, &desc, nullptr, &value) == 0);

        StringData statName;
        // Find the user consumable name for this statistic.
        auto statIt = statNameMap.find(key);
        invariant(statIt != statNameMap.end());
        statName = statIt->second.first;

        Section subs = statIt->second.second;
        long long casted_val = _castStatisticsValue<long long>(value);

        // Add this statistic only if higher than zero.
        if (casted_val > 0) {
            // Gather the statistic into its own subsection in the BSONObj.
            switch (subs) {
                case Section::DATA:
                    if (!dataSection)
                        dataSection = new BSONObjBuilder();

                    dataSection->append(statName, casted_val);
                    break;
                case Section::WAIT:
                    if (!waitSection)
                        waitSection = new BSONObjBuilder();

                    waitSection->append(statName, casted_val);
                    break;
                default:
                    return Status(ErrorCodes::BadValue,
                                  str::stream() << "Unexpected storage statistics type.");
            }
        }
    }

    if (dataSection)
        bob->append("data", dataSection->obj());
    if (waitSection)
        bob->append("timeWaitingMicros", waitSection->obj());

    // Reset the statistics so that the next fetch gives the recent values.
    c->reset(c);
    return Status::OK();
}

}  // namespace mongo