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5779230d28
In d2a4079bbe, `merger` was modified so
that when we merge a command, `last_group0_state_id` is taken to be the
maximum of the merged command's state_id and the current
`last_group0_state_id`. This is necessary for achieving the same
behavior as if the commands were applied individually instead of being
merged -- where we take the maximum state ID from `group0_history` table
which was applied until now (because the table is sorted using the state
IDs and we take the greatest row).
However, a subtle bug was introduced -- the `std::max` function uses the
`utils::UUID` standard comparison operator which is unfortunately not
the same as timeuuid comparison that Scylla performs when sorting the
`group0_history` table. So in rare cases it could return the *smaller*
of the two timeuuids w.r.t. the correct timeuuid ordering. This would
then lead to commands being applied which should have been turned to
no-ops due to the `prev_state_id` check -- and then, for example,
permanent schema desync or worse.
Fix it by using the correct comparison method.
Fixes: #14600