range_tombstone_change_generator: fix an edge case in flush()

range_tombstone_change_generator::flush() mishandles the case when two range
tombstones are adjacent and flush(pos, end_of_range=true) is called with pos
equal to the end bound of the lesser-position range tombstone.

In such case, the start change of the greater-position rtc will be accidentally
emitted, and there won't be an end change, which breaks reader assumptions by
ending the stream with an unclosed range tombstone, triggering an assertion.

This is due to a non-strict inequality used in a place where strict inequality
should be used. The modified line was intended to close range tombstones
which end exactly on the flush position, but this is unnecessary because such
range tombstones are handled by the last `if` in the function anyway.
Instead, this line caused range tombstones beginning right after the flush
position to be emitted sometimes.

Fixes #12462

Closes #13906

mutation/range_tombstone_change_generator.hh

@@ -83,7 +83,7 @@ public:
 
         position_in_partition::tri_compare cmp(_schema);
         std::optional<range_tombstone> prev;
-        const bool allow_eq = end_of_range || upper_bound.is_after_all_clustered_rows(_schema);
+        const bool allow_eq = upper_bound.is_after_all_clustered_rows(_schema);
         const auto should_flush = [&] (position_in_partition_view pos) {
             const auto res = cmp(pos, upper_bound);
             if (allow_eq) {