The evictable reader must ensure that each buffer fill makes forward progress, i.e. the last fragment in the buffer has a position larger than the last fragment from the previous buffer-fill. Otherwise, the reader could get stuck in an infinite loop between buffer fills, if the reader is evicted in-between.
The code guranteeing this forward progress had a bug: the comparison between the position after the last buffer-fill and the current last fragment position was done in the wrong direction.
So if the condition that we wanted to achieve was already true, we would continue filling the buffer until partition end which may lead to OOMs such as in #13491.
There was already a fix in this area to handle `partition_start` fragments correctly - #13563 - but it missed that the position comparison was done in the wrong order.
Fix the comparison and adjust one of the tests (added in #13563) to detect this case.
After the fix, the evictable reader starts generating some redundant (but expected) range tombstone change fragments since it's now being paused and resumed. For this we need to adjust mutation source tests which were a bit too specific. We modify `flat_mutation_reader_assertions` to squash the redundant `r_t_c`s.
Fixes#13491Closes#14375
* github.com:scylladb/scylladb:
readers: evictable_reader: don't accidentally consume the entire partition
test: flat_mutation_reader_assertions: squash `r_t_c`s with the same position
(cherry picked from commit 586102b42e)
c89e5f06ba