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/**
* 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.
*/
#pragma once
#include "mongo/db/pipeline/document_source.h"
#include "mongo/executor/task_executor.h"
#include "mongo/s/query/blocking_results_merger.h"
#include "mongo/s/query/router_stage_merge.h"
namespace mongo {
/**
* A stage used only internally to merge results that are being gathered from remote hosts, possibly
* including this host.
*
* Does not assume ownership of cursors until the first call to getNext(). This is to allow this
* stage to be used on mongos without actually iterating the cursors. For example, when this stage
* is parsed on mongos it may later be decided that the merging should happen on one of the shards.
* Then this stage is forwarded to the merging shard, and it should not kill the cursors when it
* goes out of scope on mongos.
*/
class DocumentSourceMergeCursors : public DocumentSource {
public:
static constexpr StringData kStageName = "$mergeCursors"_sd;
/**
* Parses a serialized version of this stage.
*/
static boost::intrusive_ptr<DocumentSource> createFromBson(
BSONElement, const boost::intrusive_ptr<ExpressionContext>&);
/**
* Creates a new DocumentSourceMergeCursors from the given parameters.
*/
static boost::intrusive_ptr<DocumentSourceMergeCursors> create(
executor::TaskExecutor*,
AsyncResultsMergerParams,
const boost::intrusive_ptr<ExpressionContext>&);
/**
* Extracts the remote cursors and converts the execution machinery from a DocumentSource to a
* RouterStage interface. Can only be called at planning time before any call to getNext().
*/
std::unique_ptr<RouterStageMerge> convertToRouterStage();
const char* getSourceName() const final {
return kStageName.rawData();
}
void detachFromOperationContext() final;
void reattachToOperationContext(OperationContext*) final;
/**
* Serializes this stage to be sent to perform the merging on a different host.
*/
Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
StageConstraints constraints(Pipeline::SplitState pipeState) const final {
StageConstraints constraints(StreamType::kStreaming,
PositionRequirement::kFirst,
HostTypeRequirement::kNone,
DiskUseRequirement::kNoDiskUse,
FacetRequirement::kNotAllowed,
TransactionRequirement::kAllowed);
constraints.requiresInputDocSource = false;
return constraints;
}
boost::optional<MergingLogic> mergingLogic() final {
return boost::none;
}
GetNextResult getNext() final;
std::size_t getNumRemotes() const;
/**
* Returns the high water mark sort key for the given cursor, if it exists; otherwise, returns
* an empty BSONObj. Calling this method causes the underlying BlockingResultsMerger to be
* populated and assumes ownership of the remote cursors.
*/
BSONObj getHighWaterMark();
bool remotesExhausted() const;
void setExecContext(RouterExecStage::ExecContext execContext) {
_execContext = execContext;
}
Status setAwaitDataTimeout(Milliseconds awaitDataTimeout) {
if (!_blockingResultsMerger) {
// In cases where a cursor was established with a batchSize of 0, the first getMore
// might specify a custom maxTimeMS (AKA await data timeout). In these cases we will not
// have iterated the cursor yet so will not have populated the merger, but need to
// remember/track the custom await data timeout. We will soon iterate the cursor, so we
// just populate the merger now and let it track the await data timeout itself.
populateMerger();
}
return _blockingResultsMerger->setAwaitDataTimeout(awaitDataTimeout);
}
/**
* Adds the specified shard cursors to the set of cursors to be merged. The results from the
* new cursors will be returned as normal through getNext().
*/
void addNewShardCursors(std::vector<RemoteCursor>&& newCursors) {
invariant(_blockingResultsMerger);
_blockingResultsMerger->addNewShardCursors(std::move(newCursors));
}
/**
* Marks the remote cursors as unowned, meaning that they won't be killed upon disposing of this
* DocumentSource.
*/
void dismissCursorOwnership() {
_ownCursors = false;
}
protected:
void doDispose() final;
private:
DocumentSourceMergeCursors(executor::TaskExecutor*,
AsyncResultsMergerParams,
const boost::intrusive_ptr<ExpressionContext>&,
boost::optional<BSONObj> ownedParamsSpec = boost::none);
/**
* Converts '_armParams' into the execution machinery to merge the cursors. See below for why
* this is done lazily. Clears '_armParams' and populates '_blockingResultsMerger'.
*/
void populateMerger();
// When we have parsed the params out of a BSONObj, the object needs to stay around while the
// params are in use. We store them here.
boost::optional<BSONObj> _armParamsObj;
executor::TaskExecutor* _executor;
// '_blockingResultsMerger' is lazily populated. Until we need to use it, '_armParams' will be
// populated with the parameters. Once we start using '_blockingResultsMerger', '_armParams'
// will become boost::none. We do this to prevent populating '_blockingResultsMerger' on mongos
// before serializing this stage and sending it to a shard to perform the merge. If we always
// populated '_blockingResultsMerger', then the destruction of this stage would cause the
// cursors within '_blockingResultsMerger' to be killed prematurely. For example, if this stage
// is parsed on mongos then forwarded to the shards, it should not kill the cursors when it goes
// out of scope on mongos.
boost::optional<AsyncResultsMergerParams> _armParams;
boost::optional<BlockingResultsMerger> _blockingResultsMerger;
// The ExecContext is needed because if we're a tailable, awaitData cursor, we only want to
// 'await data' if we 1) are in a getMore and 2) don't already have data to return. This context
// allows us to determine which situation we're in.
RouterExecStage::ExecContext _execContext = RouterExecStage::ExecContext::kInitialFind;
// Indicates whether the cursors stored in _armParams are "owned", meaning the cursors should be
// killed upon disposal of this DocumentSource.
bool _ownCursors = true;
};
} // namespace mongo
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