Michał Chojnowski 25c0510015 row_cache: update _prev_snapshot_pos even if apply_to_incomplete() is preempted ...

Commit e81fc1f095 accidentally broke the control
flow of row_cache::do_update().

Before that commit, the body of the loop was wrapped in a lambda.
Thus, to break out of the loop, `return` was used.

The bad commit removed the lambda, but didn't update the `return` accordingly.
Thus, since the commit, the statement doesn't just break out of the loop as
intended, but also skips the code after the loop, which updates `_prev_snapshot_pos`
to reflect the work done by the loop.

As a result, whenever `apply_to_incomplete()` (the `updater`) is preempted,
`do_update()` fails to update `_prev_snapshot_pos`. It remains in a
stale state, until `do_update()` runs again and either finishes or
is preempted outside of `updater`.

If we read a partition processed by `do_update()` but not covered by
`_prev_snapshot_pos`, we will read stale data (from the previous snapshot),
which will be remembered in the cache as the current data.

This results in outdated data being returned by the replica.
(And perhaps in something worse if range tombstones are involved.
I didn't investigate this possibility in depth).

Note: for queries with CL>1, occurences of this bug are likely to be hidden
by reconciliation, because the reconciled query will only see stale data if
the queried partition is affected by the bug on on *all* queried replicas
at the time of the query.

Fixes #16759

Closes scylladb/scylladb#17138

(cherry picked from commit ed98102c45)

2024-02-04 14:46:26 +02:00

63 KiB

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