Before updating the _last_[cp]key (for subsequent .fetch_page())
the pager checks is 'if the pager is not exhausted OR the result
has data'.
The check seems broken: if the pager is not exhausted, but the
result is empty the call for keys will unconditionally try to
reference the last element from empty vector. The not exhausted
condition for empty result can happen if the short_read is set,
which, in turn, unconditionally happens upon meeting partition
end when visiting the partition with result builder.
The correct check should be 'if the pager is not exhausted AND
the result has data': the _last_[pc]key-s should be taken for
continuation (not exhausted), but can be taken if the result is
not empty (has data).
fixes: #7263
tests: unit(dev), but tests don't trigger this corner case
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Message-Id: <20200921124329.21209-1-xemul@scylladb.com>
550fc734d9