Why: I tried to solve the issue I found earlier, but found myself stuck in a quagmire because the issues kept coming up while I fix the old one, so I finally decided to rewrite it. Issues with the old one: - A node which should be compressed stays raw This is due to by poor design of quicklist->recompress, the design forgot the situation that a node could stay uncompressed if it can not compress small enough. And if we changed the node, wo should perform compress on it again. See issue redis#12563. - Iterator don't behave like iterator Iterator will be reset and not avaliable for further use after replace or insert, see marcro resetIterator(). The only operation that athe iterator does not get reset is quicklistDelEntry(), and it has a commment about it, but the comment is wrong, the iterator may not behave like the comment says. See issue redis#12614. - Packed node violate size limit Certen call to function quicklistReplaceEntry(), quicklistInsertBefore() and quicklistInsertAfter() will cause a packed node violate size limit. See issue redis#12548. - Merge operation only performed in insert There is no merging in delete nor replace, which can make the quicklist contain adjacent small nodes. See issue redis#12856. - Algorithms that maintain compress depth are not efficient the algorithm to maintain compress depth after add or delete is to check nodes on both sides of the list, time complexity is O(n), where n is the uncompressed depth on both sides of the list. All the changes: - Partition the node Divide the node into three partitions: head, middle and tail. The head and tail partitions hold uncompressed nodes, and the middle partition holds compressed nodes. Therefore,the time complexity of maintaining compress depth after adding or deleting a node will drop to O(1), moving at most one node from one partition to another. - Removed annoying members recompress, attempted_compress and dont_compress from quicklist node structure - Merge structure quicklistIter and quicklistEntry - The historical parameter packed_threshold has been removed This is mentioned by @sundb and @oranagra in pull request redis#12568. - Merge strategy is added That is that any adjacent node in quicklist can not be merged.

Minor comment bug #12548

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