// Licensed to the Apache Software Foundation (ASF) under one

// or more contributor license agreements.  See the NOTICE file

// distributed with this work for additional information

// regarding copyright ownership.  The ASF licenses this file

// to you under the Apache License, Version 2.0 (the

// "License"); you may not use this file except in compliance

// with the License.  You may obtain a copy of the License at

//

//   http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing,

// software distributed under the License is distributed on an

// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY

// KIND, either express or implied.  See the License for the

// specific language governing permissions and limitations

// under the License.

#include "exec/scan/scanner_context.h"

#include <fmt/format.h>

#include <gen_cpp/Metrics_types.h>

#include <glog/logging.h>

#include <zconf.h>

#include <cstdint>

#include <ctime>

#include <memory>

#include <mutex>

#include <ostream>

#include <shared_mutex>

#include <tuple>

#include <utility>

#include "common/config.h"

#include "common/exception.h"

#include "common/logging.h"

#include "common/metrics/doris_metrics.h"

#include "common/status.h"

#include "core/block/block.h"

#include "exec/operator/scan_operator.h"

#include "exec/scan/scan_node.h"

#include "exec/scan/scanner_scheduler.h"

#include "runtime/descriptors.h"

#include "runtime/exec_env.h"

#include "runtime/runtime_profile.h"

#include "runtime/runtime_state.h"

#include "storage/tablet/tablet.h"

#include "util/time.h"

#include "util/uid_util.h"

namespace doris {

using namespace std::chrono_literals;

#include "common/compile_check_begin.h"

ScannerContext::ScannerContext(RuntimeState* state, ScanLocalStateBase* local_state,

                               const TupleDescriptor* output_tuple_desc,

                               const RowDescriptor* output_row_descriptor,

                               const std::list<std::shared_ptr<ScannerDelegate>>& scanners,

                               int64_t limit_, std::shared_ptr<Dependency> dependency,

                               std::atomic<int64_t>* shared_scan_limit

#ifdef BE_TEST

                               ,

                               int num_parallel_instances

#endif

                               )

        : HasTaskExecutionCtx(state),

          _state(state),

          _local_state(local_state),

          _output_tuple_desc(output_row_descriptor

                                     ? output_row_descriptor->tuple_descriptors().front()

                                     : output_tuple_desc),

          _output_row_descriptor(output_row_descriptor),

          _batch_size(state->batch_size()),

          limit(limit_),

          _shared_scan_limit(shared_scan_limit),

          _all_scanners(scanners.begin(), scanners.end()),

#ifndef BE_TEST

          _scanner_scheduler(local_state->scan_scheduler(state)),

          _min_scan_concurrency_of_scan_scheduler(

                  _scanner_scheduler->get_min_active_scan_threads()),

          _max_scan_concurrency(std::min(local_state->max_scanners_concurrency(state),

                                         cast_set<int>(scanners.size()))),

#else

          _scanner_scheduler(state->get_query_ctx()->get_scan_scheduler()),

          _min_scan_concurrency_of_scan_scheduler(0),

          _max_scan_concurrency(num_parallel_instances),

#endif

          _min_scan_concurrency(local_state->min_scanners_concurrency(state)) {

    DCHECK(_state != nullptr);

    DCHECK(_output_row_descriptor == nullptr ||

           _output_row_descriptor->tuple_descriptors().size() == 1);

    _query_id = _state->get_query_ctx()->query_id();

    _resource_ctx = _state->get_query_ctx()->resource_ctx();

    ctx_id = UniqueId::gen_uid().to_string();

    for (auto& scanner : _all_scanners) {

        _pending_scanners.push(std::make_shared<ScanTask>(scanner));

    };

    if (limit < 0) {

        limit = -1;

    }

    _dependency = dependency;

    DorisMetrics::instance()->scanner_ctx_cnt->increment(1);

    if (auto ctx = task_exec_ctx(); ctx) {

        ctx->ref_task_execution_ctx();

    }

}

int64_t ScannerContext::acquire_limit_quota(int64_t desired) {

    DCHECK(desired > 0);

    int64_t remaining = _shared_scan_limit->load(std::memory_order_acquire);

    while (true) {

        if (remaining < 0) {

            // No limit set, grant all desired rows.

            return desired;

        }

        if (remaining == 0) {

            return 0;

        }

        int64_t granted = std::min(desired, remaining);

        if (_shared_scan_limit->compare_exchange_weak(remaining, remaining - granted,

                                                      std::memory_order_acq_rel,

                                                      std::memory_order_acquire)) {

            return granted;

        }

        // CAS failed, `remaining` is updated to current value, retry.

    }

}

// After init function call, should not access _parent

Status ScannerContext::init() {

#ifndef BE_TEST

    _scanner_profile = _local_state->_scanner_profile;

    _newly_create_free_blocks_num = _local_state->_newly_create_free_blocks_num;

    _scanner_memory_used_counter = _local_state->_memory_used_counter;

    // 3. get thread token

    if (!_state->get_query_ctx()) {

        return Status::InternalError("Query context of {} is not set",

                                     print_id(_state->query_id()));

    }

    if (_state->get_query_ctx()->get_scan_scheduler()) {

        _should_reset_thread_name = false;

    }

    auto scanner = _all_scanners.front().lock();

    DCHECK(scanner != nullptr);

    if (auto* task_executor_scheduler =

                dynamic_cast<TaskExecutorSimplifiedScanScheduler*>(_scanner_scheduler)) {

        std::shared_ptr<TaskExecutor> task_executor = task_executor_scheduler->task_executor();

        TaskId task_id(fmt::format("{}-{}", print_id(_state->query_id()), ctx_id));

        _task_handle = DORIS_TRY(task_executor->create_task(

                task_id, []() { return 0.0; },

                config::task_executor_initial_max_concurrency_per_task > 0

                        ? config::task_executor_initial_max_concurrency_per_task

                        : std::max(48, CpuInfo::num_cores() * 2),

                std::chrono::milliseconds(100), std::nullopt));

    }

#endif

    // _max_bytes_in_queue controls the maximum memory that can be used by a single scan operator.

    // scan_queue_mem_limit on FE is 100MB by default, on backend we will make sure its actual value

    // is larger than 10MB.

    _max_bytes_in_queue = std::max(_state->scan_queue_mem_limit(), (int64_t)1024 * 1024 * 10);

    // Provide more memory for wide tables, increase proportionally by multiples of 300

    _max_bytes_in_queue *= _output_tuple_desc->slots().size() / 300 + 1;

    if (_all_scanners.empty()) {

        _is_finished = true;

        _set_scanner_done();

    }

    // when user not specify scan_thread_num, so we can try downgrade _max_thread_num.

    // becaue we found in a table with 5k columns, column reader may ocuppy too much memory.

    // you can refer https://github.com/apache/doris/issues/35340 for details.

    const int32_t max_column_reader_num = _state->max_column_reader_num();

    if (_max_scan_concurrency != 1 && max_column_reader_num > 0) {

        int32_t scan_column_num = cast_set<int32_t>(_output_tuple_desc->slots().size());

        int32_t current_column_num = scan_column_num * _max_scan_concurrency;

        if (current_column_num > max_column_reader_num) {

            int32_t new_max_thread_num = max_column_reader_num / scan_column_num;

            new_max_thread_num = new_max_thread_num <= 0 ? 1 : new_max_thread_num;

            if (new_max_thread_num < _max_scan_concurrency) {

                int32_t origin_max_thread_num = _max_scan_concurrency;

                _max_scan_concurrency = new_max_thread_num;

                LOG(INFO) << "downgrade query:" << print_id(_state->query_id())

                          << " scan's max_thread_num from " << origin_max_thread_num << " to "

                          << _max_scan_concurrency << ",column num: " << scan_column_num

                          << ", max_column_reader_num: " << max_column_reader_num;

            }

        }

    }

    COUNTER_SET(_local_state->_max_scan_concurrency, (int64_t)_max_scan_concurrency);

    COUNTER_SET(_local_state->_min_scan_concurrency, (int64_t)_min_scan_concurrency);

    std::unique_lock<std::mutex> l(_transfer_lock);

    RETURN_IF_ERROR(_scanner_scheduler->schedule_scan_task(shared_from_this(), nullptr, l));

    return Status::OK();

}

ScannerContext::~ScannerContext() {

    SCOPED_SWITCH_THREAD_MEM_TRACKER_LIMITER(_resource_ctx->memory_context()->mem_tracker());

    _tasks_queue.clear();

    BlockUPtr block;

    while (_free_blocks.try_dequeue(block)) {

        // do nothing

    }

    block.reset();

    DorisMetrics::instance()->scanner_ctx_cnt->increment(-1);

    if (_task_handle) {

        if (auto* task_executor_scheduler =

                    dynamic_cast<TaskExecutorSimplifiedScanScheduler*>(_scanner_scheduler)) {

            static_cast<void>(task_executor_scheduler->task_executor()->remove_task(_task_handle));

        }

        _task_handle = nullptr;

    }

    if (auto ctx = task_exec_ctx(); ctx) {

        ctx->unref_task_execution_ctx();

    }

}

BlockUPtr ScannerContext::get_free_block(bool force) {

    BlockUPtr block = nullptr;

    if (_free_blocks.try_dequeue(block)) {

        DCHECK(block->mem_reuse());

        _block_memory_usage -= block->allocated_bytes();

        _scanner_memory_used_counter->set(_block_memory_usage);

        // A free block is reused, so the memory usage should be decreased

        // The caller of get_free_block will increase the memory usage

    } else if (_block_memory_usage < _max_bytes_in_queue || force) {

        _newly_create_free_blocks_num->update(1);

        block = Block::create_unique(_output_tuple_desc->slots(), 0);

    }

    return block;

}

void ScannerContext::return_free_block(BlockUPtr block) {

    // If under low memory mode, should not return the freeblock, it will occupy too much memory.

    if (!_local_state->low_memory_mode() && block->mem_reuse() &&

        _block_memory_usage < _max_bytes_in_queue) {

        size_t block_size_to_reuse = block->allocated_bytes();

        _block_memory_usage += block_size_to_reuse;

        _scanner_memory_used_counter->set(_block_memory_usage);

        block->clear_column_data();

        // Free blocks is used to improve memory efficiency. Failure during pushing back

        // free block will not incur any bad result so just ignore the return value.

        _free_blocks.enqueue(std::move(block));

    }

}

Status ScannerContext::submit_scan_task(std::shared_ptr<ScanTask> scan_task,

                                        std::unique_lock<std::mutex>& /*transfer_lock*/) {

    // increase _num_finished_scanners no matter the scan_task is submitted successfully or not.

    // since if submit failed, it will be added back by ScannerContext::push_back_scan_task

    // and _num_finished_scanners will be reduced.

    // if submit succeed, it will be also added back by ScannerContext::push_back_scan_task

    // see ScannerScheduler::_scanner_scan.

    _num_scheduled_scanners++;

    return _scanner_scheduler->submit(shared_from_this(), scan_task);

}

void ScannerContext::clear_free_blocks() {

    clear_blocks(_free_blocks);

}

void ScannerContext::push_back_scan_task(std::shared_ptr<ScanTask> scan_task) {

    if (scan_task->status_ok()) {

        for (const auto& [block, _] : scan_task->cached_blocks) {

            if (block->rows() > 0) {

                Status st = validate_block_schema(block.get());

                if (!st.ok()) {

                    scan_task->set_status(st);

                    break;

                }

            }

        }

    }

    std::lock_guard<std::mutex> l(_transfer_lock);

    if (!scan_task->status_ok()) {

        _process_status = scan_task->get_status();

    }

    _tasks_queue.push_back(scan_task);

    _num_scheduled_scanners--;

    _dependency->set_ready();

}

Status ScannerContext::get_block_from_queue(RuntimeState* state, Block* block, bool* eos, int id) {

    if (state->is_cancelled()) {

        _set_scanner_done();

        return state->cancel_reason();

    }

    std::unique_lock l(_transfer_lock);

    if (!_process_status.ok()) {

        _set_scanner_done();

        return _process_status;

    }

    std::shared_ptr<ScanTask> scan_task = nullptr;

    if (!_tasks_queue.empty() && !done()) {

        // https://en.cppreference.com/w/cpp/container/list/front

        // The behavior is undefined if the list is empty.

        scan_task = _tasks_queue.front();

    }

    if (scan_task != nullptr) {

        // The abnormal status of scanner may come from the execution of the scanner itself,

        // or come from the scanner scheduler, such as TooManyTasks.

        if (!scan_task->status_ok()) {

            // TODO: If the scanner status is TooManyTasks, maybe we can retry the scanner after a while.

            _process_status = scan_task->get_status();

            _set_scanner_done();

            return _process_status;

        }

        // No need to worry about small block, block is merged together when they are appended to cached_blocks.

        if (!scan_task->cached_blocks.empty()) {

            auto [current_block, block_size] = std::move(scan_task->cached_blocks.front());

            scan_task->cached_blocks.pop_front();

            _block_memory_usage -= block_size;

            // consume current block

            block->swap(*current_block);

            return_free_block(std::move(current_block));

        }

        VLOG_DEBUG << fmt::format(

                "ScannerContext {} get block from queue, task_queue size {}, current scan "

                "task remaing cached_block size {}, eos {}, scheduled tasks {}",

                ctx_id, _tasks_queue.size(), scan_task->cached_blocks.size(), scan_task->is_eos(),

                _num_scheduled_scanners);

        if (scan_task->cached_blocks.empty()) {

            // All Cached blocks are consumed, pop this task from task_queue.

            if (!_tasks_queue.empty()) {

                _tasks_queue.pop_front();

            }

            if (scan_task->is_eos()) {

                // 1. if eos, record a finished scanner.

                _num_finished_scanners++;

                RETURN_IF_ERROR(

                        _scanner_scheduler->schedule_scan_task(shared_from_this(), nullptr, l));

            } else {

                RETURN_IF_ERROR(

                        _scanner_scheduler->schedule_scan_task(shared_from_this(), scan_task, l));

            }

        }

    }

    // Mark finished when either:

    // (1) all scanners completed normally, or

    // (2) shared limit exhausted and no scanners are still running.

    if (_tasks_queue.empty() && (_num_finished_scanners == _all_scanners.size() ||

                                 (_shared_scan_limit->load(std::memory_order_acquire) == 0 &&

                                  _num_scheduled_scanners == 0))) {

        _set_scanner_done();

        _is_finished = true;

    }

    *eos = done();

    if (_tasks_queue.empty()) {

        _dependency->block();

    }

    return Status::OK();

}

Status ScannerContext::validate_block_schema(Block* block) {

    size_t index = 0;

    for (auto& slot : _output_tuple_desc->slots()) {

        auto& data = block->get_by_position(index++);

        if (data.column->is_nullable() != data.type->is_nullable()) {

            return Status::Error<ErrorCode::INVALID_SCHEMA>(

                    "column(name: {}) nullable({}) does not match type nullable({}), slot(id: "

                    "{}, "

                    "name:{})",

                    data.name, data.column->is_nullable(), data.type->is_nullable(), slot->id(),

                    slot->col_name());

        }

        if (data.column->is_nullable() != slot->is_nullable()) {

            return Status::Error<ErrorCode::INVALID_SCHEMA>(

                    "column(name: {}) nullable({}) does not match slot(id: {}, name: {}) "

                    "nullable({})",

                    data.name, data.column->is_nullable(), slot->id(), slot->col_name(),

                    slot->is_nullable());

        }

    }

    return Status::OK();

}

void ScannerContext::stop_scanners(RuntimeState* state) {

    std::lock_guard<std::mutex> l(_transfer_lock);

    if (_should_stop) {

        return;

    }

    _should_stop = true;

    _set_scanner_done();

    for (const std::weak_ptr<ScannerDelegate>& scanner : _all_scanners) {

        if (std::shared_ptr<ScannerDelegate> sc = scanner.lock()) {

            sc->_scanner->try_stop();

        }

    }

    _tasks_queue.clear();

    if (_task_handle) {

        if (auto* task_executor_scheduler =

                    dynamic_cast<TaskExecutorSimplifiedScanScheduler*>(_scanner_scheduler)) {

            static_cast<void>(task_executor_scheduler->task_executor()->remove_task(_task_handle));

        }

        _task_handle = nullptr;

    }

    // TODO yiguolei, call mark close to scanners

    if (state->enable_profile()) {

        std::stringstream scanner_statistics;

        std::stringstream scanner_rows_read;

        std::stringstream scanner_wait_worker_time;

        std::stringstream scanner_projection;

        scanner_statistics << "[";

        scanner_rows_read << "[";

        scanner_wait_worker_time << "[";

        scanner_projection << "[";

        // Scanners can in 3 state

        //  state 1: in scanner context, not scheduled

        //  state 2: in scanner worker pool's queue, scheduled but not running

        //  state 3: scanner is running.

        for (auto& scanner_ref : _all_scanners) {

            auto scanner = scanner_ref.lock();

            if (scanner == nullptr) {

                continue;

            }

            // Add per scanner running time before close them

            scanner_statistics << PrettyPrinter::print(scanner->_scanner->get_time_cost_ns(),

                                                       TUnit::TIME_NS)

                               << ", ";

            scanner_projection << PrettyPrinter::print(scanner->_scanner->projection_time(),

                                                       TUnit::TIME_NS)

                               << ", ";

            scanner_rows_read << PrettyPrinter::print(scanner->_scanner->get_rows_read(),

                                                      TUnit::UNIT)

                              << ", ";

            scanner_wait_worker_time

                    << PrettyPrinter::print(scanner->_scanner->get_scanner_wait_worker_timer(),

                                            TUnit::TIME_NS)

                    << ", ";

            // since there are all scanners, some scanners is running, so that could not call scanner

            // close here.

        }

        scanner_statistics << "]";

        scanner_rows_read << "]";

        scanner_wait_worker_time << "]";

        scanner_projection << "]";

        _scanner_profile->add_info_string("PerScannerRunningTime", scanner_statistics.str());

        _scanner_profile->add_info_string("PerScannerRowsRead", scanner_rows_read.str());

        _scanner_profile->add_info_string("PerScannerWaitTime", scanner_wait_worker_time.str());

        _scanner_profile->add_info_string("PerScannerProjectionTime", scanner_projection.str());

    }

}

std::string ScannerContext::debug_string() {

    return fmt::format(

            "id: {}, total scanners: {}, pending tasks: {},"

            " _should_stop: {}, _is_finished: {}, free blocks: {},"

            " limit: {}, remaining_limit: {}, _num_running_scanners: {}, _max_thread_num: {},"

            " _max_bytes_in_queue: {}, query_id: {}",

            ctx_id, _all_scanners.size(), _tasks_queue.size(), _should_stop, _is_finished,

            _free_blocks.size_approx(), limit, _shared_scan_limit->load(std::memory_order_relaxed),

            _num_scheduled_scanners, _max_scan_concurrency, _max_bytes_in_queue,

            print_id(_query_id));

}

void ScannerContext::_set_scanner_done() {

    _dependency->set_always_ready();

}

void ScannerContext::update_peak_running_scanner(int num) {

    _local_state->_peak_running_scanner->add(num);

}

int32_t ScannerContext::_get_margin(std::unique_lock<std::mutex>& transfer_lock,

                                    std::unique_lock<std::shared_mutex>& scheduler_lock) {

    // margin_1 is used to ensure each scan operator could have at least _min_scan_concurrency scan tasks.

    int32_t margin_1 = _min_scan_concurrency -

                       (cast_set<int32_t>(_tasks_queue.size()) + _num_scheduled_scanners);

    // margin_2 is used to ensure the scan scheduler could have at least _min_scan_concurrency_of_scan_scheduler scan tasks.

    int32_t margin_2 =

            _min_scan_concurrency_of_scan_scheduler -

            (_scanner_scheduler->get_active_threads() + _scanner_scheduler->get_queue_size());

    if (margin_1 <= 0 && margin_2 <= 0) {

        return 0;

    }

    int32_t margin = std::max(margin_1, margin_2);

    if (low_memory_mode()) {

        // In low memory mode, we will limit the number of running scanners to `low_memory_mode_scanners()`.

        // So that we will not submit too many scan tasks to scheduler.

        margin = std::min(low_memory_mode_scanners() - _num_scheduled_scanners, margin);

    }

    VLOG_DEBUG << fmt::format(

            "[{}|{}] schedule scan task, margin_1: {} = {} - ({} + {}), margin_2: {} = {} - "

            "({} + {}), margin: {}",

            print_id(_query_id), ctx_id, margin_1, _min_scan_concurrency, _tasks_queue.size(),

            _num_scheduled_scanners, margin_2, _min_scan_concurrency_of_scan_scheduler,

            _scanner_scheduler->get_active_threads(), _scanner_scheduler->get_queue_size(), margin);

    return margin;

}

// This function must be called with:

// 1. _transfer_lock held.

// 2. ScannerScheduler::_lock held.

Status ScannerContext::schedule_scan_task(std::shared_ptr<ScanTask> current_scan_task,

                                          std::unique_lock<std::mutex>& transfer_lock,

                                          std::unique_lock<std::shared_mutex>& scheduler_lock) {

    if (current_scan_task &&

        (!current_scan_task->cached_blocks.empty() || current_scan_task->is_eos())) {

        throw doris::Exception(ErrorCode::INTERNAL_ERROR, "Scanner scheduler logical error.");

    }

    std::list<std::shared_ptr<ScanTask>> tasks_to_submit;

    int32_t margin = _get_margin(transfer_lock, scheduler_lock);

    // margin is less than zero. Means this scan operator could not submit any scan task for now.

    if (margin <= 0) {

        // Be careful with current scan task.

        // We need to add it back to task queue to make sure it could be resubmitted.

        if (current_scan_task) {

            // This usually happens when we should downgrade the concurrency.

            _pending_scanners.push(current_scan_task);

            VLOG_DEBUG << fmt::format(

                    "{} push back scanner to task queue, because diff <= 0, task_queue size "

                    "{}, _num_scheduled_scanners {}",

                    ctx_id, _tasks_queue.size(), _num_scheduled_scanners);

        }

#ifndef NDEBUG

        // This DCHECK is necessary.

        // We need to make sure each scan operator could have at least 1 scan tasks.

        // Or this scan operator will not be re-scheduled.

        if (!_pending_scanners.empty() && _num_scheduled_scanners == 0 && _tasks_queue.empty()) {

            throw doris::Exception(ErrorCode::INTERNAL_ERROR, "Scanner scheduler logical error.");

        }

#endif

        return Status::OK();

    }

    bool first_pull = true;

    while (margin-- > 0) {

        std::shared_ptr<ScanTask> task_to_run;

        const int32_t current_concurrency = cast_set<int32_t>(

                _tasks_queue.size() + _num_scheduled_scanners + tasks_to_submit.size());

        VLOG_DEBUG << fmt::format("{} currenct concurrency: {} = {} + {} + {}", ctx_id,

                                  current_concurrency, _tasks_queue.size(), _num_scheduled_scanners,

                                  tasks_to_submit.size());

        if (first_pull) {

            task_to_run = _pull_next_scan_task(current_scan_task, current_concurrency);

            if (task_to_run == nullptr) {

                // In two situations we will get nullptr.

                // 1. current_concurrency already reached _max_scan_concurrency.

                // 2. all scanners are finished.

                if (current_scan_task) {

                    DCHECK(current_scan_task->cached_blocks.empty());

                    DCHECK(!current_scan_task->is_eos());

                    if (!current_scan_task->cached_blocks.empty() || current_scan_task->is_eos()) {

                        // This should not happen.

                        throw doris::Exception(ErrorCode::INTERNAL_ERROR,

                                               "Scanner scheduler logical error.");

                    }

                    // Current scan task is not eos, but we can not resubmit it.

                    // Add current_scan_task back to task queue, so that we have chance to resubmit it in the future.

                    _pending_scanners.push(current_scan_task);

                }

            }

            first_pull = false;

        } else {

            task_to_run = _pull_next_scan_task(nullptr, current_concurrency);

        }

        if (task_to_run) {

            tasks_to_submit.push_back(task_to_run);

        } else {

            break;

        }

    }

    if (tasks_to_submit.empty()) {

        return Status::OK();

    }

    VLOG_DEBUG << fmt::format("[{}:{}] submit {} scan tasks to scheduler, remaining scanner: {}",

                              print_id(_query_id), ctx_id, tasks_to_submit.size(),

                              _pending_scanners.size());

    for (auto& scan_task_iter : tasks_to_submit) {

        Status submit_status = submit_scan_task(scan_task_iter, transfer_lock);

        if (!submit_status.ok()) {

            _process_status = submit_status;

            _set_scanner_done();

            return _process_status;

        }

    }

    return Status::OK();

}

std::shared_ptr<ScanTask> ScannerContext::_pull_next_scan_task(

        std::shared_ptr<ScanTask> current_scan_task, int32_t current_concurrency) {

    if (current_concurrency >= _max_scan_concurrency) {

        VLOG_DEBUG << fmt::format(

                "ScannerContext {} current concurrency {} >= _max_scan_concurrency {}, skip "

                "pull",

                ctx_id, current_concurrency, _max_scan_concurrency);

        return nullptr;

    }

    if (current_scan_task != nullptr) {

        if (!current_scan_task->cached_blocks.empty() || current_scan_task->is_eos()) {

            // This should not happen.

            throw doris::Exception(ErrorCode::INTERNAL_ERROR, "Scanner scheduler logical error.");

        }

        return current_scan_task;

    }

    if (!_pending_scanners.empty()) {

        // If shared limit quota is exhausted, do not submit new scanners from pending queue.

        int64_t remaining = _shared_scan_limit->load(std::memory_order_acquire);

        if (remaining == 0) {

            return nullptr;

        }

        std::shared_ptr<ScanTask> next_scan_task;

        next_scan_task = _pending_scanners.top();

        _pending_scanners.pop();

        return next_scan_task;

    } else {

        return nullptr;

    }

}

bool ScannerContext::low_memory_mode() const {

    return _local_state->low_memory_mode();

}

#include "common/compile_check_end.h"

} // namespace doris