/**
 *    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.
 */

#include "mongo/platform/basic.h"

#include "mongo/client/dbclientinterface.h"
#include "mongo/db/commands.h"
#include "mongo/db/query/explain.h"
#include "mongo/s/query/cluster_find.h"

namespace mongo {
namespace {

/**
 * Implements the explain command on mongos.
 *
 * "Old-style" explains (i.e. queries which have the $explain flag set), do not run
 * through this path. Such explains will be supported for backwards compatibility,
 * and must succeed in multiversion clusters.
 *
 * "New-style" explains use the explain command. When the explain command is routed
 * through mongos, it is forwarded to all relevant shards. If *any* shard does not
 * support a new-style explain, then the entire explain will fail (i.e. new-style
 * explains cannot be used in multiversion clusters).
 */
class ClusterExplainCmd : public BasicCommand {
    MONGO_DISALLOW_COPYING(ClusterExplainCmd);

public:
    ClusterExplainCmd() : BasicCommand("explain") {}

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

    /**
     * Running an explain on a secondary requires explicitly setting slaveOk.
     */
    virtual bool slaveOk() const {
        return false;
    }

    virtual bool slaveOverrideOk() const {
        return true;
    }

    virtual bool maintenanceOk() const {
        return false;
    }

    virtual bool adminOnly() const {
        return false;
    }

    virtual void help(std::stringstream& help) const {
        help << "explain database reads and writes";
    }

    /**
     * You are authorized to run an explain if you are authorized to run
     * the command that you are explaining. The auth check is performed recursively
     * on the nested command.
     */
    virtual Status checkAuthForOperation(OperationContext* opCtx,
                                         const std::string& dbname,
                                         const BSONObj& cmdObj) {
        if (Object != cmdObj.firstElement().type()) {
            return Status(ErrorCodes::BadValue, "explain command requires a nested object");
        }

        BSONObj explainObj = cmdObj.firstElement().Obj();

        Command* commToExplain = Command::findCommand(explainObj.firstElementFieldName());
        if (NULL == commToExplain) {
            mongoutils::str::stream ss;
            ss << "unknown command: " << explainObj.firstElementFieldName();
            return Status(ErrorCodes::CommandNotFound, ss);
        }

        return commToExplain->checkAuthForRequest(
            opCtx, OpMsgRequest::fromDBAndBody(dbname, std::move(explainObj)));
    }

    virtual bool run(OperationContext* opCtx,
                     const std::string& dbName,
                     const BSONObj& cmdObj,
                     BSONObjBuilder& result) {
        auto verbosity = ExplainOptions::parseCmdBSON(cmdObj);
        if (!verbosity.isOK()) {
            return appendCommandStatus(result, verbosity.getStatus());
        }

        // This is the nested command which we are explaining. We need to propagate generic
        // arguments into the inner command since it is what is passed to the virtual
        // Command::explain() method.
        const BSONObj explainObj = ([&] {
            const auto innerObj = cmdObj.firstElement().Obj();
            if (auto innerDb = innerObj["$db"]) {
                uassert(ErrorCodes::InvalidNamespace,
                        str::stream() << "Mismatched $db in explain command. Expected " << dbName
                                      << " but got "
                                      << innerDb.checkAndGetStringData(),
                        innerDb.checkAndGetStringData() == dbName);
            }

            BSONObjBuilder bob;
            bob.appendElements(innerObj);
            for (auto outerElem : cmdObj) {
                // If the argument is in both the inner and outer command, we currently let the
                // inner version take precedence.
                const auto name = outerElem.fieldNameStringData();
                if (Command::isGenericArgument(name) && !innerObj.hasField(name)) {
                    bob.append(outerElem);
                }
            }
            return bob.obj();
        }());

        const std::string cmdName = explainObj.firstElementFieldName();
        Command* commToExplain = Command::findCommand(cmdName);
        if (!commToExplain) {
            return appendCommandStatus(
                result,
                Status{ErrorCodes::CommandNotFound,
                       str::stream() << "Explain failed due to unknown command: " << cmdName});
        }

        // Actually call the nested command's explain(...) method.
        Status explainStatus =
            commToExplain->explain(opCtx, dbName, explainObj, verbosity.getValue(), &result);
        if (!explainStatus.isOK()) {
            return appendCommandStatus(result, explainStatus);
        }

        return true;
    }

} cmdExplainCluster;

}  // namespace
}  // namespace mongo