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/**
* Copyright (C) 2022-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/catalog/virtual_collection_options.h"
#include "mongo/db/storage/input_stream.h"
#include "mongo/db/storage/named_pipe.h"
#include "mongo/db/storage/record_store.h"
namespace mongo {
class MultiBsonStreamCursor : public SeekableRecordCursor {
public:
MultiBsonStreamCursor(const VirtualCollectionOptions& vopts)
: _numStreams(vopts.dataSources.size()), _vopts(vopts) {
tassert(6968310, "_numStreams {} <= 0"_format(_numStreams), _numStreams > 0);
_streamReader = getInputStream(_vopts.dataSources[_streamIdx].url);
}
boost::optional<Record> next() override;
// This block of overrides are all essentially no-ops as they are for lock yielding but the
// external data source is read-only.
void save() override {}
void saveUnpositioned() override {}
bool restore(bool tolerateCappedRepositioning) override {
return true;
}
void detachFromOperationContext() override {}
void reattachToOperationContext(OperationContext* opCtx) override {}
void setSaveStorageCursorOnDetachFromOperationContext(bool) override {}
// Seeking is not currently supported.
boost::optional<Record> seekExact(const RecordId& id) {
tasserted(6968300, "MultiBsonStreamCursor::seekExact is not supported");
return boost::none;
}
// Seeking is not currently supported.
boost::optional<Record> seekNear(const RecordId& start) {
tasserted(6968301, "MultiBsonStreamCursor::seekNear is not supported");
return boost::none;
}
private:
void expandBuffer(int32_t bsonSize);
boost::optional<Record> nextFromCurrentStream();
static std::unique_ptr<InputStream<NamedPipeInput>> getInputStream(const std::string& url);
// The size in bytes of a BSON object's "size" prefix.
static constexpr int kSizeSize = static_cast<int>(sizeof(int32_t));
// The buffer starts small and doubles as needed to have room to read the rest of an object that
// spans across the half-way point. Block reads are done at offset 0 and get only half the
// buffer size, so worst case is the last byte of this read is the first byte of the size of the
// following object. (There will always be room to finish reading the size.) The buffer will
// never grow larger than (2 * BSONObjMaxUserSize) as that size is large enough to satisfy any
// stream of objects using the defined algorithm. Except when the buffer is enlarged, none of
// its contents are ever copied.
//
// Buffer starting size should be a power of two since BSONObjMaxUserSize is one. The smallest
// valid starting size in the current implementation is 8 bytes, as a half-buffer block read
// expects to be able to get the 4-byte size of the first BSON object. We start somewhat larger
// to avoid memory fragmentation, as most BSON objects will not be so tiny, however, many may be
// quite small. The starting size is chosen as 8K because at the time of writing the most common
// Linux filesystem block size is 4K, and our block read size is half the buffer size.
int _bufSize = 8 * 1024; // current buffer size
int _blockReadSize = _bufSize / 2; // size of a block read
std::unique_ptr<char[]> _buffer{new char[_bufSize]}; // the buffer itself
int _bufBegin = 0; // index of first unconsumed byte in '_buffer'
int _bufEnd = 0; // index one past the last valid byte in '_buffer'
int64_t _nextRecordId = 0; // for artificially generating the record IDs
int _numStreams; // number of streams in '_vopts'
int _streamIdx = 0; // index in' _vopts' of stream being consumed in '_streamReader'
// Reader for the current stream.
std::unique_ptr<InputStream<NamedPipeInput>> _streamReader = nullptr;
const VirtualCollectionOptions& _vopts; // metadata containing the pipe URLs
};
} // namespace mongo
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