## Problem
Two backward-pagination bugs in `ClickHouseRunsRepository.listRunIds`,
both pre-existing (they predate the composite-cursor work in #3852 and
were spotted during/after it):
**1. Wrong slice (straddled pages).** `listRunRows` fetches `page.size +
1` rows to detect `hasMore`. That extra row is the one *farthest from
the cursor* in both directions (forward orders DESC; backward orders
ASC), so it's always the *trailing* element. Forward correctly used
`rows.slice(0, size)`, but backward+`hasMore` used `rows.slice(1, size +
1)` — dropping the row *closest* to the cursor and keeping the has-more
sentinel. The page straddled two logical pages (one run from the correct
previous page + one from the page before it), so paging "newer" across a
boundary **repeated and skipped** runs.
**2. Stranded forward cursor on a partial backward page.** In the
backward `!hasMore` branch, `nextCursor` was `reversedRows.at(page.size
- 1)`. On a partial page (fewer than `page.size` rows — reachable via
`runs.list` by passing a forward page's cursor as `page[before]`), that
index overshoots → `undefined` → `nextCursor` becomes `null`, leaving no
way to page forward again.
## Fix
- **Slice:** both directions now slice `rows.slice(0, size)` (the
sentinel is the trailing element either way).
- **Partial-page cursor:** the backward `!hasMore` branch takes the
oldest row on the page, `rows.at(0)`, for `nextCursor` — equivalent to
the old expression for full pages, correct for partial ones.
Forward pagination, the cursor *values* for full pages, and the `hasMore
=== true` paths were already correct and are unchanged.
## Tests
`runsRepositoryCursor.test.ts` gains two cases (both fail on `main`,
pass here):
- **multi-page backward walk:** forward across all pages, then backward
from the last page — each backward page must *exactly* reproduce the
corresponding forward page (no straddling: `main` returns `{b,c}`
instead of `{c,d}`), and the full traversal covers every run once.
- **partial backward page:** backward onto a partial first page must
still expose a working forward cursor (and paging forward from it
reaches the rest) — `main` returns a `null` nextCursor.
The three existing cursor tests (forward completeness, backward
round-trip, legacy cursor) still pass.
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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Matt Aitken
1b0f2c71dd