This patch is a backport of the fix for #4733 (merged to master as
0cf4fab). As the shard reader code has been substantially refactored
post the 3.0 branch cut time, that fix cannot be backported at all,
instead this is a separate fix developed specially for 3.0.
To quickly reiterate, the problem at hand is that when recreating a
previously evicted shard reader of a multishard reader, the position of
the last fragment seen by that reader is used as the position after
which the read resumes. For this we just created a clustering range
starting from *after* the key (open bound). This works well in most
cases but when that last key is a non-full prefix this will also ignore
any still unread clustering rows that falls into that prefix.
This patch doesn't attempt to fix the problem in a systematic way like
the fix in master does, making sure reader recreation works properly
with prefixes as well, instead, for the sake of minimizing the impact,
we simply avoid ending the buffer on a prefix key. This fix is more
naive and can cause over-read when the stream contains lots of
successive range tombstones with prefix positions. On the other hand,
this leads to a *much* simpler fix, and anyway, as reader eviction is
much rarer in 3.0 this should have a lesser impact.
A unit test is also added to make sure the problem is fixed.
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20190819120748.28168-1-bdenes@scylladb.com>
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