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1643549d08
This patch applies the same changes to both kl and mx sstable readers, but because the kl reader is old, we'll focus on the newer one. This patch makes the main sstable reader process a coroutine, allowing to simplify it, by: - using the state saved in the coroutine instead of most of the states saved in the _state variable - removing the switch statement and moving the code of former switch cases, resulting in reduced number of jumps in code - removing repetitive ifs for read statuses, by adding them to the coroutine implementation The coroutine is saved in a new class ```processing_result_generator```, which works like a generator: using its ```generate()``` method, one can order the coroutine to continue until it yields a data_consumer::processing_result value, which was achieved previously by calling the function that is now the coroutine(```do_process_state()```). Before the patch, the main processing method had 558 lines. The patch reduces this number to 345 lines. However, usage of c++ coroutines has a non-negligible effect on the performance of the sstable reader. In the test cases from ```perf_fast_forward``` the new sstable reader performs up to 2% more instructions (per fragment) than the former implementation, and this loss is achieved for cases where we're reading many subsequent rows, without any skips. Thanks to finding an optimization during the development of the patch, the loss is mitigated when we do skip rows, and for some cases, we can even observe an improvement. You can see the full results in attached files: [old_results.txt](https://github.com/scylladb/scylla/files/6793139/old_results.txt), [new_results.txt](https://github.com/scylladb/scylla/files/6793140/new_results.txt) Test: unit(dev) Refs: #7952 Closes #9002 * github.com:scylladb/scylla: mx sstable reader: reduce code blocks mx sstable reader: make ifs consistent sstable readers: make awaiter for read status mx sstable reader: don't yield if the data buffer is not empty mx sstable reader: combine FLAGS and FLAGS_2 states mx sstable reader: reduce placeholder state usage mx sstable reader: replace non_consuming states with a bool mx sstable reader: reduce placeholder state usage mx sstable reader: replace unnecessary states with a placeholder mx sstable reader: remove false if case mx sstable reader: remove row_body_missing_columns_label mx sstable reader: remove row_body_deletion_label mx sstable reader: remove column_end_label mx sstable reader: remove column_cell_path_label mx sstable reader: remove column_ttl_label mx sstable reader: remove column_deletion_time_label mx sstable reader: remove complex_column_2_label mx sstable reader: remove row_body_missing_columns_read_columns_label mx sstable reader: remove row_body_marker_label mx sstable reader: remove row_body_shadowable_deletion_label mx sstable reader: remove row_body_prev_size_label mx sstable reader: remove ck_block_label mx sstable reader: remove ck_block2_label mx sstable reader: remove clustering_row_label and complex_column_label mx sstable reader: remove labels with only one goto mx sstable reader: replace the switch cases with gotos and a new label mx sstable reader: remove states only reached consecutively or from goto mx sstable reader: remove switch breaks for consecutive states mx sstable reader: convert readers main method into a coroutine kl sstable reader: replace states for ending with one state, simplify non_consuming kl sstable reader: remove unnecessary states kl sstable reader: remove unnecessary yield kl sstable reader: remove unnecessary blocks kl sstable reader: fix indentation kl sstable reader: replace switch with standard flow control kl sstable reader: remove state::CELL case kl sstable reader: move states code only reachable from one place kl sstable reader: remove states only reached consecutively kl sstable reader: remove switch breaks for consecutive states kl sstable reader: remove unreachable case kl sstable reader: move testing hack for fragmented buffers outside the coroutine kl sstable reader: convert readers main method into a coroutine sstable readers: create a generator class for coroutines