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6e5aaa85b6
Work in this patch is a result of two bugs - spurious MODIFY event, when remove column is used in `update_item` on non-existing item and spurious events, when batch write item mixed noop operations with operations involving actual changes (the former would still emit cdc log entries). The latter issue required rework of Piotr Wieczorek's algorithm, which fixed former issue as well. Piotr Wieczorek previously wrote checks, that should prevent unnecesary cdc events from being written. His implementation missed the fact, that a single `mutation` object passed to cdc code to be analysed for cdc log entries can contain modifications for multiple rows (with the same timestamp - for example as a result to BatchWriteItem call). His code tries to skip whole `mutation`, which in such case is not possible, because BatchWriteItem might have one item that does nothing and second item that does modification (this is the reason for the second bug). His algorithm was extended and moved. Originally it was working as follows - user would sent a `mutation` object with some changes to be "augmented". The cdc would process those changes and built a set of cdc log changes based on them, that would be added to cdc log table. Piotr added a `should_skip` function, which processes user changes and tried to determine if they all should be dropped or not. New version, instead of trying to skip adding rows to cdc log `mutation` object, builds a rows-to-ignore set. After whole cdc log `mutation` object is completed, it processes it and go through it row by row. Any row that was previously added to a `rows_to_ignore` set will now be removed. Remaining rows are written to new cdc log `mutation` with new clustering key (`cdc$batch_seq_no` index value should probably be consecutive - we just want to be safe here) and returns new `mutation` object to be sent to cdc log table. The first bug is fixed as a side effect of new algorithm, which contains more precise checks detecting, if given mutation actually made a difference. Fixes: #28368 Fixes: SCYLLADB-538 Fixes: SCYLLADB-1528 Refs: #28452