Red commit: 67088342ec (CI run: pending)
## Summary
Fixes#1631 — `scanReachRows` swallowed `QueryContext` / `rows.Err()`
failures and returned `nil`. The handler treated that as "genuinely no
reach" and rendered a 200 with empty arrays (or 404 in some flows), so
transient SQLite failures surfaced to operators as "this node has no
reach" — misleading and undiagnosable without log access.
## Fix
`cmd/server/node_reach.go`:
- `scanReachRows` now returns `([]pathRow, error)`; propagates
`QueryContext` + `rows.Err()` failures.
- `computeNodeReach` signature gains an error return: non-nil error
means real backend failure (NOT "unknown node").
- `handleNodeReach` renders **500** on that error path and does **NOT**
cache the failure (next request retries cleanly). Genuinely-empty reach
still renders **200** with empty arrays; unknown/blacklisted nodes still
render 404.
## TDD
- Red commit `67088342`: adds `TestNodeReach_ScanDBErrorReturns500` —
warms the integration DB, drops the `observations` table, asserts
handler returns 500. Pre-fix this got 200 with empty arrays.
- Green commit `5408be3a`: the fix + caller updates. Adds
`TestScanReachRows_ErrorReturn` (unit-level: closed-DB → non-nil err).
- `TestNodeReach_ShapeAndClamp` had to be tightened: the v2 fixture's
`observations` table was missing `observer_idx`; the swallowed error
masked that schema gap. Now rebuilt with the right shape.
## Scope
- `cmd/server/node_reach.go` — fix.
- `cmd/server/node_reach_endpoint_test.go` — new red test +
ShapeAndClamp fixture fix.
- `cmd/server/node_reach_test.go`, `node_reach_bench_test.go` — caller
updates for new signature + one new unit assertion test.
No cache changes (#1629 is separate). No sibling refactors. No frontend.
## Verification
- `go test ./cmd/server/...` — green (48s, all tests).
- pr-preflight — clean (PII, scope, red-commit, CSS vars, LIKE-on-JSON,
async-migration, XSS).
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Co-authored-by: clawbot <bot@kpa-clawbot.local>
Kpa-clawbot
43be1bb76a