---
id: review-002-oai11-ref-memoization
name: Memoize $ref expansion to bound resolver work (OAI-11)
status: pending
depends_on: []
scope: moderate
risk: medium
impact: component
level: implementation
tags: [adapters, review-002, from-openapi, openapi-spec]
---

## Description

Review 002 OAI-11 [major, borderline critical]. The OAI-01 fix
(cycle guard + depth budget, `openapi_spec.rs:382-437`) bounds *stack*,
not *work*: every `$ref` hop re-expands its target from a fresh clone
with no memoization. A chain of shared but acyclic refs —

```
S_i = { a: { $ref: "S_{i+1}" }, b: { $ref: "S_{i+1}" } }
```

(the doubly-linked-list `prev`/`next` shape — valid, common) expands as
a full binary tree. Empirically (verbatim harness): 20 levels → 7.3M
node visits / 3.6 s; growth ≈ 2^levels → 30 levels ≈ ~1 hour, 40 ≈ days.
Neither guard fires (`err=false`); the import just wedges — no stack
overflow, no error. The same "valid document kills the process" class
as OAI-01, one mitigation shy.

## Acceptance Criteria

- [ ] `resolve_refs_bounded` memoizes resolved, cycle-free `$ref`
      targets per document (cache keyed on the ref string; clone the
      resolved `Value` on hit instead of re-resolving) — or an
      equivalent total-node-count budget with a clean
      `SchemaParse`-family error (implementer's choice; memoization
      strongly preferred — it preserves full acyclic support)
- [ ] The cycle guard and both depth checks remain (memoization must
      not alter cycle detection semantics — a cycle still errors)
- [ ] Acceptance gate: a 30+ level shared-chain spec imports in
      bounded time (add the test with a wall-clock or node-count
      assert; a 30-level chain must import in < 1 s)
- [ ] Tests: diamond (non-cyclic shared refs) unchanged-correct;
      cycle-through-shared-node still errors; memoized result equals
      the old expansion for a shared-ref schema (golden compare on one
      case)
- [ ] The four missing cycle-shape tests from OAI-16 (array-items,
      allOf, additionalProperties, `$ref`-sibling) land here too —
      same function, same risk envelope
- [ ] `cargo test`, `cargo clippy --all-targets -- -D warnings`,
      `cargo fmt --check` pass

## References

- docs/reviews/002-post-remediation-review.md (Part E', OAI-11 + OAI-16)
- src/adapters/openapi_spec.rs:382-437 (the resolver), :713-805 (existing cycle tests), from_openapi.rs:736 (the requestBody-chain test)
- tasks/adapters/review-001-ref-cycle-guard.md (the OAI-01 fix this completes)
- docs/architecture/decisions/066-from-jsonschema-as-http-adapter.md

## Notes

Clone cost per hit is fine (schemas are small; the win is avoiding
exponential re-resolution, not allocation). If memoizing, note the
`resolving`-set interaction: a memo entry must only be written for
*completed* (cycle-free) expansions — a partial expansion under an
active cycle attempt must never be cached (write-on-success at the
top of the recursion return path, or post-order). This is the one
subtle part; the existing cycle tests + the four new shape tests are
the safety net.