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storage: preserve consistency when applying widening preemptive snaps…
…hots Merges can cause preemptive snapshots that widen existing replicas. For example, consider the following sequence of events: 1. A replica of range A is removed from store S, but is not garbage collected. 2. Range A subsumes its right neighbor B. 3. Range A is re-added to store S. In step 3, S will receive a preemptive snapshot for A that requires widening its existing replica, thanks to the intervening merge. Problematically, the code to check whether this widening was possible, in Store.canApplySnapshotLocked, was incorrectly mutating the range descriptor in the snapshot header! Applying the snapshot would then fail to clear all of the data from the old incarnation of the replica, since the bounds on the range deletion tombstone were wrong. This often resulted in replica inconsistency. Plus, the in-memory copy of the range descriptor would be incorrect until the next descriptor update--though this usually happened quickly, as the replica would apply the change replicas command, which updates the descriptor, soon after applying the preemptive snapshot. To fix the problem, teach Store.canApplySnapshotLocked to make a copy of the range descriptor before it mutates it. To prevent regressions, add an assertion that a range's start key is never changed to the descriptor update path. With this assertion in place, but without the fix itself, TestStoreRangeMergeReadoptedLHSFollower reliably fails. Fixes cockroachdb#29252. Release note: None
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