Michał Chojnowski 2aac8690c7 cache_flat_mutation_reader: fix a broken iterator validity guarantee in ensure_population_lower_bound() ...

ensure_population_lower_bound() guarantees that _last_row is valid or null.

However, it fails to provide this guarantee in the special rare case when
`_population_range_starts_before_all_rows == true` and _last_row is non-null.

(This can happen in practice if there is a dummy at before_all_clustering_rows
and eviction makes the `(before_all_clustering_rows, ...)` interval
discontinous. When the interval is read in this state, _last_row will point to
the dummy, while _population_range_starts_before_all_rows will still be true.)

In this special case, `ensure_population_lower_bound()` does not refresh
`_last_row`, so it can be non-null but invalid after the call.
If it is accessed in this state, undefined behaviour occurs.
This was observed to happen in a test,
in the `read_from_underlying() -- maybe_drop_last_entry()` codepath.

The proposed fix is to make the meaning of _population_range_starts_before_all_rows
closer to its real intention. Namely: it's supposed to handle the special case of a
left-open interval, not the case of an interval starting at -inf.

2023-11-16 19:01:18 +01:00

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