path: root/src/mongo/db/commands/distinct.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,
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 *    it in the license file.
 */

#define MONGO_LOGV2_DEFAULT_COMPONENT ::mongo::logv2::LogComponent::kQuery

#include "mongo/platform/basic.h"

#include <memory>
#include <string>
#include <vector>

#include "mongo/db/auth/authorization_session.h"
#include "mongo/db/bson/dotted_path_support.h"
#include "mongo/db/client.h"
#include "mongo/db/clientcursor.h"
#include "mongo/db/commands.h"
#include "mongo/db/commands/run_aggregate.h"
#include "mongo/db/db_raii.h"
#include "mongo/db/exec/working_set_common.h"
#include "mongo/db/jsobj.h"
#include "mongo/db/matcher/extensions_callback_real.h"
#include "mongo/db/namespace_string.h"
#include "mongo/db/query/collection_query_info.h"
#include "mongo/db/query/cursor_response.h"
#include "mongo/db/query/explain.h"
#include "mongo/db/query/find_common.h"
#include "mongo/db/query/get_executor.h"
#include "mongo/db/query/parsed_distinct.h"
#include "mongo/db/query/plan_summary_stats.h"
#include "mongo/db/query/query_planner_common.h"
#include "mongo/db/query/view_response_formatter.h"
#include "mongo/db/s/collection_sharding_state.h"
#include "mongo/db/views/resolved_view.h"
#include "mongo/logv2/log.h"

namespace mongo {
namespace {

namespace dps = dotted_path_support;

class DistinctCommand : public BasicCommand {
public:
    DistinctCommand() : BasicCommand("distinct") {}

    std::string help() const override {
        return "{ distinct : 'collection name' , key : 'a.b' , query : {} }";
    }

    AllowedOnSecondary secondaryAllowed(ServiceContext*) const override {
        return AllowedOnSecondary::kOptIn;
    }

    bool maintenanceOk() const override {
        return false;
    }

    bool supportsWriteConcern(const BSONObj& cmd) const override {
        return false;
    }

    bool canIgnorePrepareConflicts() const override {
        return true;
    }

    ReadConcernSupportResult supportsReadConcern(const BSONObj& cmdObj,
                                                 repl::ReadConcernLevel level) const override {
        return ReadConcernSupportResult::allSupportedAndDefaultPermitted();
    }

    bool shouldAffectReadConcernCounter() const override {
        return true;
    }

    bool supportsReadMirroring(const BSONObj&) const override {
        return true;
    }

    ReadWriteType getReadWriteType() const override {
        return ReadWriteType::kRead;
    }

    std::size_t reserveBytesForReply() const override {
        return FindCommon::kInitReplyBufferSize;
    }

    Status checkAuthForOperation(OperationContext* opCtx,
                                 const std::string& dbname,
                                 const BSONObj& cmdObj) const override {
        AuthorizationSession* authSession = AuthorizationSession::get(opCtx->getClient());

        if (!authSession->isAuthorizedToParseNamespaceElement(cmdObj.firstElement())) {
            return Status(ErrorCodes::Unauthorized, "Unauthorized");
        }

        const auto hasTerm = false;
        return authSession->checkAuthForFind(
            CollectionCatalog::get(opCtx).resolveNamespaceStringOrUUID(
                opCtx, CommandHelpers::parseNsOrUUID(dbname, cmdObj)),
            hasTerm);
    }

    Status explain(OperationContext* opCtx,
                   const OpMsgRequest& request,
                   ExplainOptions::Verbosity verbosity,
                   rpc::ReplyBuilderInterface* result) const override {
        std::string dbname = request.getDatabase().toString();
        const BSONObj& cmdObj = request.body;
        // Acquire locks. The RAII object is optional, because in the case of a view, the locks
        // need to be released.
        boost::optional<AutoGetCollectionForReadCommand> ctx;
        ctx.emplace(opCtx,
                    CommandHelpers::parseNsCollectionRequired(dbname, cmdObj),
                    AutoGetCollectionViewMode::kViewsPermitted);
        const auto nss = ctx->getNss();

        const ExtensionsCallbackReal extensionsCallback(opCtx, &nss);
        auto defaultCollator =
            ctx->getCollection() ? ctx->getCollection()->getDefaultCollator() : nullptr;
        auto parsedDistinct = uassertStatusOK(
            ParsedDistinct::parse(opCtx, nss, cmdObj, extensionsCallback, true, defaultCollator));

        if (ctx->getView()) {
            // Relinquish locks. The aggregation command will re-acquire them.
            ctx.reset();

            auto viewAggregation = parsedDistinct.asAggregationCommand();
            if (!viewAggregation.isOK()) {
                return viewAggregation.getStatus();
            }

            auto viewAggRequest =
                AggregationRequest::parseFromBSON(nss, viewAggregation.getValue(), verbosity);
            if (!viewAggRequest.isOK()) {
                return viewAggRequest.getStatus();
            }

            // An empty PrivilegeVector is acceptable because these privileges are only checked on
            // getMore and explain will not open a cursor.
            return runAggregate(opCtx,
                                nss,
                                viewAggRequest.getValue(),
                                viewAggregation.getValue(),
                                PrivilegeVector(),
                                result);
        }

        const Collection* const collection = ctx->getCollection();

        auto executor = uassertStatusOK(
            getExecutorDistinct(collection, QueryPlannerParams::DEFAULT, &parsedDistinct));

        auto bodyBuilder = result->getBodyBuilder();
        Explain::explainStages(executor.get(), collection, verbosity, BSONObj(), &bodyBuilder);
        return Status::OK();
    }

    bool run(OperationContext* opCtx,
             const std::string& dbname,
             const BSONObj& cmdObj,
             BSONObjBuilder& result) override {
        CommandHelpers::handleMarkKillOnClientDisconnect(opCtx);
        // Acquire locks and resolve possible UUID. The RAII object is optional, because in the case
        // of a view, the locks need to be released.
        boost::optional<AutoGetCollectionForReadCommand> ctx;
        ctx.emplace(opCtx,
                    CommandHelpers::parseNsOrUUID(dbname, cmdObj),
                    AutoGetCollectionViewMode::kViewsPermitted);
        const auto& nss = ctx->getNss();

        if (!ctx->getView()) {
            // Distinct doesn't filter orphan documents so it is not allowed to run on sharded
            // collections in multi-document transactions.
            uassert(
                ErrorCodes::OperationNotSupportedInTransaction,
                "Cannot run 'distinct' on a sharded collection in a multi-document transaction. "
                "Please see http://dochub.mongodb.org/core/transaction-distinct for a recommended "
                "alternative.",
                !opCtx->inMultiDocumentTransaction() ||
                    !CollectionShardingState::get(opCtx, nss)
                         ->getCollectionDescription(opCtx)
                         .isSharded());
        }

        const ExtensionsCallbackReal extensionsCallback(opCtx, &nss);
        auto defaultCollation =
            ctx->getCollection() ? ctx->getCollection()->getDefaultCollator() : nullptr;
        auto parsedDistinct = uassertStatusOK(
            ParsedDistinct::parse(opCtx, nss, cmdObj, extensionsCallback, false, defaultCollation));

        // Check whether we are allowed to read from this node after acquiring our locks.
        auto replCoord = repl::ReplicationCoordinator::get(opCtx);
        uassertStatusOK(replCoord->checkCanServeReadsFor(
            opCtx, nss, ReadPreferenceSetting::get(opCtx).canRunOnSecondary()));

        if (ctx->getView()) {
            // Relinquish locks. The aggregation command will re-acquire them.
            ctx.reset();

            auto viewAggregation = parsedDistinct.asAggregationCommand();
            uassertStatusOK(viewAggregation.getStatus());

            BSONObj aggResult = CommandHelpers::runCommandDirectly(
                opCtx, OpMsgRequest::fromDBAndBody(dbname, std::move(viewAggregation.getValue())));
            uassertStatusOK(ViewResponseFormatter(aggResult).appendAsDistinctResponse(&result));
            return true;
        }

        const Collection* const collection = ctx->getCollection();

        auto executor =
            getExecutorDistinct(collection, QueryPlannerParams::DEFAULT, &parsedDistinct);
        uassertStatusOK(executor.getStatus());

        {
            stdx::lock_guard<Client> lk(*opCtx->getClient());
            CurOp::get(opCtx)->setPlanSummary_inlock(executor.getValue()->getPlanSummary());
        }

        const auto key = cmdObj[ParsedDistinct::kKeyField].valuestrsafe();

        std::vector<BSONObj> distinctValueHolder;
        BSONElementSet values(executor.getValue()->getCanonicalQuery()->getCollator());

        const int kMaxResponseSize = BSONObjMaxUserSize - 4096;

        try {
            size_t listApproxBytes = 0;
            BSONObj obj;
            while (PlanExecutor::ADVANCED == executor.getValue()->getNext(&obj, nullptr)) {
                // Distinct expands arrays.
                //
                // If our query is covered, each value of the key should be in the index key and
                // available to us without this.  If a collection scan is providing the data, we may
                // have to expand an array.
                BSONElementSet elts;
                dps::extractAllElementsAlongPath(obj, key, elts);

                for (BSONElementSet::iterator it = elts.begin(); it != elts.end(); ++it) {
                    BSONElement elt = *it;
                    if (values.count(elt)) {
                        continue;
                    }

                    // This is an approximate size check which safeguards against use of unbounded
                    // memory by the distinct command. We perform a more precise check at the end of
                    // this method to confirm that the response size is less than 16MB.
                    listApproxBytes += elt.size();
                    uassert(
                        17217, "distinct too big, 16mb cap", listApproxBytes < kMaxResponseSize);

                    auto distinctObj = elt.wrap();
                    values.insert(distinctObj.firstElement());
                    distinctValueHolder.push_back(std::move(distinctObj));
                }
            }
        } catch (DBException& exception) {
            LOGV2_WARNING(23797,
                          "Plan executor error during distinct command: {error}, "
                          "stats: {stats}",
                          "Plan executor error during distinct command",
                          "error"_attr = exception.toStatus(),
                          "stats"_attr = redact(executor.getValue()->getStats()));

            exception.addContext("Executor error during distinct command");
            throw;
        }

        auto curOp = CurOp::get(opCtx);

        // Get summary information about the plan.
        PlanSummaryStats stats;
        executor.getValue()->getSummaryStats(&stats);
        if (collection) {
            CollectionQueryInfo::get(collection).notifyOfQuery(opCtx, collection, stats);
        }
        curOp->debug().setPlanSummaryMetrics(stats);

        if (curOp->shouldDBProfile()) {
            curOp->debug().execStats = executor.getValue()->getStats();
        }

        BSONArrayBuilder valueListBuilder(result.subarrayStart("values"));
        for (const auto& value : values) {
            valueListBuilder.append(value);
        }
        valueListBuilder.doneFast();

        if (!opCtx->inMultiDocumentTransaction() &&
            repl::ReadConcernArgs::get(opCtx).getArgsAtClusterTime()) {
            result.append("atClusterTime"_sd,
                          repl::ReadConcernArgs::get(opCtx).getArgsAtClusterTime()->asTimestamp());
        }

        uassert(31299, "distinct too big, 16mb cap", result.len() < kMaxResponseSize);
        return true;
    }

    void appendMirrorableRequest(BSONObjBuilder* bob, const BSONObj& cmdObj) const override {
        static const auto kMirrorableKeys = [] {
            BSONObjBuilder keyBob;
            keyBob.append("distinct", 1);
            keyBob.append("key", 1);
            keyBob.append("query", 1);
            keyBob.append("collation", 1);
            return keyBob.obj();
        }();

        // Filter the keys that can be mirrored
        cmdObj.filterFieldsUndotted(bob, kMirrorableKeys, true);
    }


} distinctCmd;

}  // namespace
}  // namespace mongo