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9ccd4ea416
A schema upgrade appends a MVCC version B after an existing version A. The last dummy in B is added to the front of LRU, so it will be evicted after the entries in A. This alone doesn't quite violate the "older versions are evicted first" rule, because the new last dummy carries no information. But apply_monotonically generally assumes that entries on the same position have the obvious eviction order, even if they carry no information. Thus, after the merge, the rule can become broken. The proposed fix is as follows: - In the case where A is merged into B, the merged last dummy inherits the link of A. - The merging of B into anything is prevented until its merge with A is finished. This is relatively hacky, because it still involves a state that goes against some natural expectations granted by the "older versions..." rule. A less hacky fix would be to ensure that the new dummy is inserted into a proper place in the eviction order to begin with. Or, better yet, we could eliminate the rule altogether. Aside from being very hard to maintain, it also prevents the introduction of any eviction algorithm other than LRU.