Consider the following scenario:
1) let's assume tablet 0 has range [1, 5] (pre merge)
2) tablet merge happens, tablet 0 has now range [1, 10]
3) tablet_sstable_set isn't refreshed, so holds a stale state, thinks tablet 0 still has range [1, 5]
4) during a full scan, forward service will intersect the full range with tablet ranges and consume one tablet at a time
5) replica service is asked to consume range [1, 10] of tablet 0 (post merge)
We have two possible outcomes:
With cache bypass:
1) cache reader is bypassed
2) sstable reader is created on range [1, 10]
3) unrefreshed tablet_sstable_set holds stale state, but select correctly all sstables intersecting with range [1, 10]
With cache:
1) cache reader is created
2) finds partition with token 5 is cached
3) sstable reader is created on range [1, 4] (later would fast forward to range [6, 10]; also belongs to tablet 0)
4) incremental selector consumes the pre-merge sstable spanning range [1, 5]
4.1) since the partitioned_sstable_set pre-merge contains only that sstable, EOS is reached
4.2) since EOS is reached, the fast forward to range [6, 10] is not allowed.
So with the set refreshed, sstable set is aligned with tablet ranges, and no premature EOS is signalled, otherwise preventing fast forward to from happening and all data from being properly captured in the read.
This change fixes the bug and triggers a mutation source refresh whenever the number of tablets for the table has changed, not only when we have incoming tablets.
Additionally, includes a fix for range reads that span more than one tablet, which can happen during split execution.
Fixes: https://github.com/scylladb/scylladb/issues/23313
This change needs to be backported to all supported versions which implement tablet merge.
Closesscylladb/scylladb#24287
* github.com:scylladb/scylladb:
replica: Fix range reads spanning sibling tablets
test: add reproducer and test for mutation source refresh after merge
tablets: trigger mutation source refresh on tablet count change
c52aec3d2f