flat_mutation_reader.hh

/*
 * Copyright (C) 2017 ScyllaDB
 */

/*
 * This file is part of Scylla.
 *
 * Scylla is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * Scylla 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
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with Scylla.  If not, see <http://www.gnu.org/licenses/>.
 */

#pragma once

#include <seastar/util/bool_class.hh>
#include <seastar/core/future.hh>

#include "dht/i_partitioner.hh"
#include "position_in_partition.hh"
#include "mutation_fragment.hh"
#include "tracing/trace_state.hh"

#include <seastar/util/gcc6-concepts.hh>
#include <seastar/core/thread.hh>
#include "db/timeout_clock.hh"

#include <deque>

using seastar::future;

class mutation_source;

GCC6_CONCEPT(
    template<typename Consumer>
    concept bool FlatMutationReaderConsumer() {
        return requires(Consumer c, mutation_fragment mf) {
            { c(std::move(mf)) } -> stop_iteration;
        };
    }
)

GCC6_CONCEPT(
    template<typename T>
    concept bool FlattenedConsumer() {
        return StreamedMutationConsumer<T>() && requires(T obj, const dht::decorated_key& dk) {
            obj.consume_new_partition(dk);
            obj.consume_end_of_partition();
        };
    }

    template<typename T>
    concept bool PartitionFilter = requires(T filter, const dht::decorated_key& dk) {
        { filter(dk) } -> bool;
    };
)

/*
 * Allows iteration on mutations using mutation_fragments.
 * It iterates over mutations one by one and for each mutation
 * it returns:
 *      1. partition_start mutation_fragment
 *      2. static_row mutation_fragment if one exists
 *      3. mutation_fragments for all clustering rows and range tombstones
 *         in clustering key order
 *      4. partition_end mutation_fragment
 * The best way to consume those mutation_fragments is to call
 * flat_mutation_reader::consume with a consumer that receives the fragments.
 */
class flat_mutation_reader final {
public:
    // Causes a stream of reversed mutations to be emitted.
    // 1. Static row is still emitted first.
    // 2. Range tombstones are ordered by their end position.
    // 3. Clustered rows and range tombstones are emitted in descending order.
    // Because of 2 and 3 the guarantee that a range tombstone is emitted before
    // any mutation fragment affected by it still holds.
    // Ordering of partitions themselves remains unchanged.
    using consume_reversed_partitions = seastar::bool_class<class consume_reversed_partitions_tag>;

    class impl {
    private:
        circular_buffer<mutation_fragment> _buffer;
        size_t _buffer_size = 0;
        bool _consume_done = false;
    protected:
        size_t max_buffer_size_in_bytes = 8 * 1024;
        bool _end_of_stream = false;
        schema_ptr _schema;
        friend class flat_mutation_reader;
    protected:
        template<typename... Args>
        void push_mutation_fragment(Args&&... args) {
            seastar::memory::on_alloc_point(); // for exception safety tests
            _buffer.emplace_back(std::forward<Args>(args)...);
            _buffer_size += _buffer.back().memory_usage(*_schema);
        }
        void clear_buffer() {
            _buffer.erase(_buffer.begin(), _buffer.end());
            _buffer_size = 0;
        }
        void forward_buffer_to(const position_in_partition& pos);
        void clear_buffer_to_next_partition();
        template<typename Source>
        future<bool> fill_buffer_from(Source&, db::timeout_clock::time_point);
        // When succeeds, makes sure that the next push_mutation_fragment() will not fail.
        void reserve_one() {
            if (_buffer.capacity() == _buffer.size()) {
                _buffer.reserve(_buffer.size() * 2 + 1);
            }
        }
        const circular_buffer<mutation_fragment>& buffer() const {
            return _buffer;
        }
    private:
        static flat_mutation_reader reverse_partitions(flat_mutation_reader::impl&);
    public:
        impl(schema_ptr s) : _schema(std::move(s)) { }
        virtual ~impl() {}
        virtual future<> fill_buffer(db::timeout_clock::time_point) = 0;
        virtual void next_partition() = 0;

        bool is_end_of_stream() const { return _end_of_stream; }
        bool is_buffer_empty() const { return _buffer.empty(); }
        bool is_buffer_full() const { return _buffer_size >= max_buffer_size_in_bytes; }

        mutation_fragment pop_mutation_fragment() {
            auto mf = std::move(_buffer.front());
            _buffer.pop_front();
            _buffer_size -= mf.memory_usage(*_schema);
            return mf;
        }

        void unpop_mutation_fragment(mutation_fragment mf) {
            const auto memory_usage = mf.memory_usage(*_schema);
            _buffer.emplace_front(std::move(mf));
            _buffer_size += memory_usage;
        }

        future<mutation_fragment_opt> operator()(db::timeout_clock::time_point timeout) {
            if (is_buffer_empty()) {
                if (is_end_of_stream()) {
                    return make_ready_future<mutation_fragment_opt>();
                }
                return fill_buffer(timeout).then([this, timeout] { return operator()(timeout); });
            }
            return make_ready_future<mutation_fragment_opt>(pop_mutation_fragment());
        }

        template<typename Consumer>
        GCC6_CONCEPT(
            requires FlatMutationReaderConsumer<Consumer>()
        )
        // Stops when consumer returns stop_iteration::yes or end of stream is reached.
        // Next call will start from the next mutation_fragment in the stream.
        future<> consume_pausable(Consumer consumer, db::timeout_clock::time_point timeout) {
            _consume_done = false;
            return do_until([this] { return (is_end_of_stream() && is_buffer_empty()) || _consume_done; },
                            [this, consumer = std::move(consumer), timeout] () mutable {
                if (is_buffer_empty()) {
                    return fill_buffer(timeout);
                }

                _consume_done = consumer(pop_mutation_fragment()) == stop_iteration::yes;

                return make_ready_future<>();
            });
        }

        template<typename Consumer, typename Filter>
        GCC6_CONCEPT(
            requires FlatMutationReaderConsumer<Consumer>() && PartitionFilter<Filter>
        )
        // A variant of consume_pausable() that expects to be run in
        // a seastar::thread.
        // Partitions for which filter(decorated_key) returns false are skipped
        // entirely and never reach the consumer.
        void consume_pausable_in_thread(Consumer consumer, Filter filter, db::timeout_clock::time_point timeout) {
            while (true) {
                if (need_preempt()) {
                    seastar::thread::yield();
                }
                if (is_buffer_empty()) {
                    if (is_end_of_stream()) {
                        return;
                    }
                    fill_buffer(timeout).get();
                    continue;
                }
                auto mf = pop_mutation_fragment();
                if (mf.is_partition_start() && !filter(mf.as_partition_start().key())) {
                    next_partition();
                    continue;
                }
                if (consumer(std::move(mf)) == stop_iteration::yes) {
                    return;
                }
            }
        };

    private:
        template<typename Consumer>
        struct consumer_adapter {
            flat_mutation_reader::impl& _reader;
            stdx::optional<dht::decorated_key> _decorated_key;
            Consumer _consumer;
            consumer_adapter(flat_mutation_reader::impl& reader, Consumer c)
                    : _reader(reader)
                      , _consumer(std::move(c))
            { }
            stop_iteration operator()(mutation_fragment&& mf) {
                return std::move(mf).consume(*this);
            }
            stop_iteration consume(static_row&& sr) {
                return handle_result(_consumer.consume(std::move(sr)));
            }
            stop_iteration consume(clustering_row&& cr) {
                return handle_result(_consumer.consume(std::move(cr)));
            }
            stop_iteration consume(range_tombstone&& rt) {
                return handle_result(_consumer.consume(std::move(rt)));
            }
            stop_iteration consume(partition_start&& ps) {
                _decorated_key.emplace(std::move(ps.key()));
                _consumer.consume_new_partition(*_decorated_key);
                if (ps.partition_tombstone()) {
                    _consumer.consume(ps.partition_tombstone());
                }
                return stop_iteration::no;
            }
            stop_iteration consume(partition_end&& pe) {
                return _consumer.consume_end_of_partition();
            }
        private:
            stop_iteration handle_result(stop_iteration si) {
                if (si) {
                    if (_consumer.consume_end_of_partition()) {
                        return stop_iteration::yes;
                    }
                    _reader.next_partition();
                }
                return stop_iteration::no;
            }
        };
    public:
        template<typename Consumer>
        GCC6_CONCEPT(
            requires FlattenedConsumer<Consumer>()
        )
        // Stops when consumer returns stop_iteration::yes from consume_end_of_partition or end of stream is reached.
        // Next call will receive fragments from the next partition.
        // When consumer returns stop_iteration::yes from methods other than consume_end_of_partition then the read
        // of the current partition is ended, consume_end_of_partition is called and if it returns stop_iteration::no
        // then the read moves to the next partition.
        // Reference to the decorated key that is passed to consume_new_partition() remains valid until after
        // the call to consume_end_of_partition().
        //
        // This method is useful because most of current consumers use this semantic.
        //
        //
        // This method returns whatever is returned from Consumer::consume_end_of_stream().S
        auto consume(Consumer consumer, db::timeout_clock::time_point timeout) {
            return do_with(consumer_adapter<Consumer>(*this, std::move(consumer)), [this, timeout] (consumer_adapter<Consumer>& adapter) {
                return consume_pausable(std::ref(adapter), timeout).then([this, &adapter] {
                    return adapter._consumer.consume_end_of_stream();
                });
            });
        }

        template<typename Consumer, typename Filter>
        GCC6_CONCEPT(
            requires FlattenedConsumer<Consumer>() && PartitionFilter<Filter>
        )
        // A variant of consumee() that expects to be run in a seastar::thread.
        // Partitions for which filter(decorated_key) returns false are skipped
        // entirely and never reach the consumer.
        auto consume_in_thread(Consumer consumer, Filter filter, db::timeout_clock::time_point timeout) {
            auto adapter = consumer_adapter<Consumer>(*this, std::move(consumer));
            consume_pausable_in_thread(std::ref(adapter), std::move(filter), timeout);
            return adapter._consumer.consume_end_of_stream();
        };

        /*
         * fast_forward_to is forbidden on flat_mutation_reader created for a single partition.
         */
        virtual future<> fast_forward_to(const dht::partition_range&, db::timeout_clock::time_point timeout) = 0;
        virtual future<> fast_forward_to(position_range, db::timeout_clock::time_point timeout) = 0;

        // Altough for most cases this is a mere getter some readers might have
        // one or more subreaders and will need to account for their buffer-size
        // as well so we need to allow these readers to override the default
        // implementation.
        virtual size_t buffer_size() const {
            return _buffer_size;
        }

        circular_buffer<mutation_fragment> detach_buffer() {
            _buffer_size = 0;
            return std::exchange(_buffer, {});
        }

        void move_buffer_content_to(impl& other) {
            if (other._buffer.empty()) {
                std::swap(_buffer, other._buffer);
                other._buffer_size = std::exchange(_buffer_size, 0);
            } else {
                seastar::memory::on_alloc_point(); // for exception safety tests
                other._buffer.reserve(other._buffer.size() + _buffer.size());
                for (auto&& mf : _buffer) {
                    other._buffer.emplace_back(std::move(mf));
                }
                _buffer.clear();
                other._buffer_size += std::exchange(_buffer_size, 0);
            }
        }
    };
private:
    std::unique_ptr<impl> _impl;

    flat_mutation_reader() = default;
    explicit operator bool() const noexcept { return bool(_impl); }
    friend class optimized_optional<flat_mutation_reader>;
public:
    // Documented in mutation_reader::forwarding in mutation_reader.hh.
    class partition_range_forwarding_tag;
    using partition_range_forwarding = bool_class<partition_range_forwarding_tag>;

    flat_mutation_reader(std::unique_ptr<impl> impl) noexcept : _impl(std::move(impl)) {}

    future<mutation_fragment_opt> operator()(db::timeout_clock::time_point timeout) {
        return _impl->operator()(timeout);
    }

    template <typename Consumer>
    GCC6_CONCEPT(
        requires FlatMutationReaderConsumer<Consumer>()
    )
    auto consume_pausable(Consumer consumer, db::timeout_clock::time_point timeout) {
        return _impl->consume_pausable(std::move(consumer), timeout);
    }

    template <typename Consumer>
    GCC6_CONCEPT(
        requires FlattenedConsumer<Consumer>()
    )
    auto consume(Consumer consumer,
            db::timeout_clock::time_point timeout,
            consume_reversed_partitions reversed = consume_reversed_partitions::no) {
        if (reversed) {
            return do_with(impl::reverse_partitions(*_impl), [&] (auto& reversed_partition_stream) {
                return reversed_partition_stream._impl->consume(std::move(consumer), timeout);
            });
        }
        return _impl->consume(std::move(consumer), timeout);
    }

    template<typename Consumer, typename Filter>
    GCC6_CONCEPT(
        requires FlattenedConsumer<Consumer>() && PartitionFilter<Filter>
    )
    auto consume_in_thread(Consumer consumer, Filter filter, db::timeout_clock::time_point timeout) {
        return _impl->consume_in_thread(std::move(consumer), std::move(filter), timeout);
    }

    template<typename Consumer>
    GCC6_CONCEPT(
        requires FlattenedConsumer<Consumer>()
    )
    auto consume_in_thread(Consumer consumer, db::timeout_clock::time_point timeout) {
        return consume_in_thread(std::move(consumer), [] (const dht::decorated_key&) { return true; }, timeout);
    }

    void next_partition() { _impl->next_partition(); }

    future<> fill_buffer(db::timeout_clock::time_point timeout) { return _impl->fill_buffer(timeout); }

    // Changes the range of partitions to pr. The range can only be moved
    // forwards. pr.begin() needs to be larger than pr.end() of the previousl
    // used range (i.e. either the initial one passed to the constructor or a
    // previous fast forward target).
    // pr needs to be valid until the reader is destroyed or fast_forward_to()
    // is called again.
    future<> fast_forward_to(const dht::partition_range& pr, db::timeout_clock::time_point timeout) {
        return _impl->fast_forward_to(pr, timeout);
    }
    // Skips to a later range of rows.
    // The new range must not overlap with the current range.
    //
    // In forwarding mode the stream does not return all fragments right away,
    // but only those belonging to the current clustering range. Initially
    // current range only covers the static row. The stream can be forwarded
    // (even before end-of- stream) to a later range with fast_forward_to().
    // Forwarding doesn't change initial restrictions of the stream, it can
    // only be used to skip over data.
    //
    // Monotonicity of positions is preserved by forwarding. That is fragments
    // emitted after forwarding will have greater positions than any fragments
    // emitted before forwarding.
    //
    // For any range, all range tombstones relevant for that range which are
    // present in the original stream will be emitted. Range tombstones
    // emitted before forwarding which overlap with the new range are not
    // necessarily re-emitted.
    //
    // When forwarding mode is not enabled, fast_forward_to()
    // cannot be used.
    future<> fast_forward_to(position_range cr, db::timeout_clock::time_point timeout) {
        return _impl->fast_forward_to(std::move(cr), timeout);
    }
    bool is_end_of_stream() const { return _impl->is_end_of_stream(); }
    bool is_buffer_empty() const { return _impl->is_buffer_empty(); }
    bool is_buffer_full() const { return _impl->is_buffer_full(); }
    mutation_fragment pop_mutation_fragment() { return _impl->pop_mutation_fragment(); }
    void unpop_mutation_fragment(mutation_fragment mf) { _impl->unpop_mutation_fragment(std::move(mf)); }
    const schema_ptr& schema() const { return _impl->_schema; }
    void set_max_buffer_size(size_t size) {
        _impl->max_buffer_size_in_bytes = size;
    }
    // Resolves with a pointer to the next fragment in the stream without consuming it from the stream,
    // or nullptr if there are no more fragments.
    // The returned pointer is invalidated by any other non-const call to this object.
    future<mutation_fragment*> peek(db::timeout_clock::time_point timeout) {
        if (!is_buffer_empty()) {
            return make_ready_future<mutation_fragment*>(&_impl->_buffer.front());
        }
        if (is_end_of_stream()) {
            return make_ready_future<mutation_fragment*>(nullptr);
        }
        return fill_buffer(timeout).then([this, timeout] {
            return peek(timeout);
        });
    }
    // A peek at the next fragment in the buffer.
    // Cannot be called if is_buffer_empty() returns true.
    const mutation_fragment& peek_buffer() const { return _impl->_buffer.front(); }
    // The actual buffer size of the reader.
    // Altough we consistently refer to this as buffer size throught the code
    // we really use "buffer size" as the size of the collective memory
    // used by all the mutation fragments stored in the buffer of the reader.
    size_t buffer_size() const {
        return _impl->buffer_size();
    }
    // Detach the internal buffer of the reader.
    // Roughly equivalent to depleting it by calling pop_mutation_fragment()
    // until is_buffer_empty() returns true.
    // The reader will need to allocate a new buffer on the next fill_buffer()
    // call.
    circular_buffer<mutation_fragment> detach_buffer() {
        return _impl->detach_buffer();
    }
    // Moves the buffer content to `other`.
    //
    // If the buffer of `other` is empty this is very efficient as the buffers
    // are simply swapped. Otherwise the content of the buffer is moved
    // fragmuent-by-fragment.
    // Allows efficient implementation of wrapping readers that do no
    // transformation to the fragment stream.
    void move_buffer_content_to(impl& other) {
        _impl->move_buffer_content_to(other);
    }
};

using flat_mutation_reader_opt = optimized_optional<flat_mutation_reader>;

template<typename Impl, typename... Args>
flat_mutation_reader make_flat_mutation_reader(Args &&... args) {
    return flat_mutation_reader(std::make_unique<Impl>(std::forward<Args>(args)...));
}

// Consumes mutation fragments until StopCondition is true.
// The consumer will stop iff StopCondition returns true, in particular
// reaching the end of stream alone won't stop the reader.
template<typename StopCondition, typename ConsumeMutationFragment, typename ConsumeEndOfStream>
GCC6_CONCEPT(requires requires(StopCondition stop, ConsumeMutationFragment consume_mf, ConsumeEndOfStream consume_eos, mutation_fragment mf) {
    { stop() } -> bool;
    { consume_mf(std::move(mf)) } -> void;
    { consume_eos() } -> future<>;
})
future<> consume_mutation_fragments_until(
        flat_mutation_reader& r,
        StopCondition&& stop,
        ConsumeMutationFragment&& consume_mf,
        ConsumeEndOfStream&& consume_eos,
        db::timeout_clock::time_point timeout) {
    return do_until([stop] { return stop(); }, [&r, stop, consume_mf, consume_eos, timeout] {
        while (!r.is_buffer_empty()) {
            consume_mf(r.pop_mutation_fragment());
            if (stop()) {
                return make_ready_future<>();
            }
        }
        if (r.is_end_of_stream()) {
            return consume_eos();
        }
        return r.fill_buffer(timeout);
    });
}

// Creates a stream which is like r but with transformation applied to the elements.
template<typename T>
GCC6_CONCEPT(
    requires StreamedMutationTranformer<T>()
)
flat_mutation_reader transform(flat_mutation_reader r, T t) {
    class transforming_reader : public flat_mutation_reader::impl {
        flat_mutation_reader _reader;
        T _t;
        struct consumer {
            transforming_reader* _owner;
            stop_iteration operator()(mutation_fragment&& mf) {
                _owner->push_mutation_fragment(_owner->_t(std::move(mf)));
                return stop_iteration(_owner->is_buffer_full());
            }
        };
    public:
        transforming_reader(flat_mutation_reader&& r, T&& t)
            : impl(t(r.schema()))
            , _reader(std::move(r))
            , _t(std::move(t))
        {}
        virtual future<> fill_buffer(db::timeout_clock::time_point timeout) override {
            if (_end_of_stream) {
                return make_ready_future<>();
            }
            return _reader.consume_pausable(consumer{this}, timeout).then([this] {
                if (_reader.is_end_of_stream() && _reader.is_buffer_empty()) {
                    _end_of_stream = true;
                }
            });
        }
        virtual void next_partition() override {
            clear_buffer_to_next_partition();
            if (is_buffer_empty()) {
                _reader.next_partition();
            }
        }
        virtual future<> fast_forward_to(const dht::partition_range& pr, db::timeout_clock::time_point timeout) override {
            clear_buffer();
            _end_of_stream = false;
            return _reader.fast_forward_to(pr, timeout);
        }
        virtual future<> fast_forward_to(position_range pr, db::timeout_clock::time_point timeout) override {
            forward_buffer_to(pr.start());
            _end_of_stream = false;
            return _reader.fast_forward_to(std::move(pr), timeout);
        }
        virtual size_t buffer_size() const override {
            return flat_mutation_reader::impl::buffer_size() + _reader.buffer_size();
        }
    };
    return make_flat_mutation_reader<transforming_reader>(std::move(r), std::move(t));
}

inline flat_mutation_reader& to_reference(flat_mutation_reader& r) { return r; }
inline const flat_mutation_reader& to_reference(const flat_mutation_reader& r) { return r; }

template <typename Underlying>
class delegating_reader : public flat_mutation_reader::impl {
    Underlying _underlying;
public:
    delegating_reader(Underlying&& r) : impl(to_reference(r).schema()), _underlying(std::forward<Underlying>(r)) { }
    virtual future<> fill_buffer(db::timeout_clock::time_point timeout) override {
        return fill_buffer_from(to_reference(_underlying), timeout).then([this] (bool underlying_finished) {
            _end_of_stream = underlying_finished;
        });
    }
    virtual future<> fast_forward_to(position_range pr, db::timeout_clock::time_point timeout) override {
        _end_of_stream = false;
        forward_buffer_to(pr.start());
        return to_reference(_underlying).fast_forward_to(std::move(pr), timeout);
    }
    virtual void next_partition() override {
        clear_buffer_to_next_partition();
        if (is_buffer_empty()) {
            to_reference(_underlying).next_partition();
        }
        _end_of_stream = to_reference(_underlying).is_end_of_stream() && to_reference(_underlying).is_buffer_empty();
    }
    virtual future<> fast_forward_to(const dht::partition_range& pr, db::timeout_clock::time_point timeout) override {
        _end_of_stream = false;
        clear_buffer();
        return to_reference(_underlying).fast_forward_to(pr, timeout);
    }
    virtual size_t buffer_size() const override {
        return flat_mutation_reader::impl::buffer_size() + to_reference(_underlying).buffer_size();
    }
};
flat_mutation_reader make_delegating_reader(flat_mutation_reader&);

flat_mutation_reader make_forwardable(flat_mutation_reader m);

flat_mutation_reader make_nonforwardable(flat_mutation_reader, bool);

flat_mutation_reader make_empty_flat_reader(schema_ptr s);

flat_mutation_reader flat_mutation_reader_from_mutations(std::vector<mutation>, const dht::partition_range& pr = query::full_partition_range, streamed_mutation::forwarding fwd = streamed_mutation::forwarding::no);
inline flat_mutation_reader flat_mutation_reader_from_mutations(std::vector<mutation> ms, streamed_mutation::forwarding fwd) {
    return flat_mutation_reader_from_mutations(std::move(ms), query::full_partition_range, fwd);
}
flat_mutation_reader
flat_mutation_reader_from_mutations(std::vector<mutation> ms,
                                    const query::partition_slice& slice,
                                    streamed_mutation::forwarding fwd = streamed_mutation::forwarding::no);

/// Make a reader that enables the wrapped reader to work with multiple ranges.
///
/// \param ranges An range vector that has to contain strictly monotonic
///     partition ranges, such that successively calling
///     `flat_mutation_reader::fast_forward_to()` with each one is valid.
///     An range vector range with 0 or 1 elements is also valid.
/// \param fwd_mr It is only respected when `ranges` contains 0 or 1 partition
///     ranges. Otherwise the reader is created with
///     mutation_reader::forwarding::yes.
flat_mutation_reader
make_flat_multi_range_reader(schema_ptr s, mutation_source source, const dht::partition_range_vector& ranges,
                             const query::partition_slice& slice, const io_priority_class& pc = default_priority_class(),
                             tracing::trace_state_ptr trace_state = nullptr,
                             flat_mutation_reader::partition_range_forwarding fwd_mr = flat_mutation_reader::partition_range_forwarding::yes);

/// Make a reader that enables the wrapped reader to work with multiple ranges.
///
/// Generator overload. The ranges returned by the generator have to satisfy the
/// same requirements as the `ranges` param of the vector overload.
flat_mutation_reader
make_flat_multi_range_reader(
        schema_ptr s,
        mutation_source source,
        std::function<std::optional<dht::partition_range>()> generator,
        const query::partition_slice& slice,
        const io_priority_class& pc = default_priority_class(),
        tracing::trace_state_ptr trace_state = nullptr,
        flat_mutation_reader::partition_range_forwarding fwd_mr = flat_mutation_reader::partition_range_forwarding::yes);

flat_mutation_reader
make_flat_mutation_reader_from_fragments(schema_ptr, std::deque<mutation_fragment>);

// Calls the consumer for each element of the reader's stream until end of stream
// is reached or the consumer requests iteration to stop by returning stop_iteration::yes.
// The consumer should accept mutation as the argument and return stop_iteration.
// The returned future<> resolves when consumption ends.
template <typename Consumer>
inline
future<> consume_partitions(flat_mutation_reader& reader, Consumer consumer, db::timeout_clock::time_point timeout) {
    static_assert(std::is_same<future<stop_iteration>, futurize_t<std::result_of_t<Consumer(mutation&&)>>>::value, "bad Consumer signature");
    using futurator = futurize<std::result_of_t<Consumer(mutation&&)>>;

    return do_with(std::move(consumer), [&reader, timeout] (Consumer& c) -> future<> {
        return repeat([&reader, &c, timeout] () {
            return read_mutation_from_flat_mutation_reader(reader, timeout).then([&c] (mutation_opt&& mo) -> future<stop_iteration> {
                if (!mo) {
                    return make_ready_future<stop_iteration>(stop_iteration::yes);
                }
                return futurator::apply(c, std::move(*mo));
            });
        });
    });
}