Michael Livshin 9f656b96ac short-circuit flat mutation reader upgrades and downgrades ...

When asked to upgrade a reader that itself is a downgrade, try to
return the original v2 reader instead, and likewise when downgrading
upgraded v1 readers.

This is desirable because version transformations can result from,
say, entering/leaving a reader concurrency semaphore, and the amount
of such transformations is practically unbounded.

Such short-circuiting is only done if it is safe, that is: the
transforming reader's buffer is empty and its internal range tombstone
tracking state is discardable.

Signed-off-by: Michael Livshin <michael.livshin@scylladb.com>

2021-12-21 11:26:17 +02:00

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