86 lines
3.3 KiB
Markdown
86 lines
3.3 KiB
Markdown
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# T4.3 — Lazy blob access
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| Field | Value |
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|---|---|
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| Phase | P4 — Verification |
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| Size | S — under 1 day |
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| Status | Not started |
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| Flags | — |
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| Spec | inlined below |
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| Blocks | — |
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## Goal
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A verifier that needs no text performs no blob reads.
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## Facts (inlined — no spec read needed)
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- Context partitions hold **identifiers** — packed, available-but-not-packed,
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dropped — and are small enough to inline. Prompts and outputs are large and go
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**by reference**.
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- Laziness matters concretely: a verifier that shells out and checks an exit code
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needs none of this. Making every verifier carry prompt text penalizes the
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common case and blows broker payload limits.
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- `VerifierCtx.blobs: Arc<dyn BlobStore>` is the mechanism — the verifier calls
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`get` if and only if it wants a body.
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- `get` may return `None` after reduction (T8.4); that is a normal answer, and a
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verifier must not treat it as an error.
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## Steps
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1. Confirm `EpisodeView` carries `BlobRef` only. No `prompt_text` field, no
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eagerly-resolved cache.
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2. Pass the store handle through `VerifierCtx.blobs`.
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3. Instrument the store with a call counter behind a test-only wrapper —
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counting is what turns this from an intention into an assertion.
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4. Write the exit-code verifier used as the acceptance case: it reads the attempt
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state and nothing else.
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5. Write a second, text-reading verifier and assert its call count is exactly the
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number of refs it asked for — no over-fetch, no prefetch-all.
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## Acceptance
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- An exit-code verifier records **zero** `BlobStore::get` calls.
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## Verify
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**Harness:** a counting `BlobStore` decorator wrapping the real store, injected
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through `VerifierCtx.blobs`.
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**Integration test** — `tests/it_lazy_blobs.rs`:
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1. Run a real run with an exit-code verifier that reads only attempt state.
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2. Assert the counter is **exactly 0**.
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3. Run a text-reading verifier that resolves 2 refs. Assert the counter is
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**exactly 2** — not more (no prefetch) and not fewer (no silent cache hit
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masking a missing read).
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4. Assert `EpisodeView` has no field holding blob **bytes** — a compile-level
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check if the type is closed, otherwise a serialization size assertion on a run
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with a 1 MB prompt: the view must stay small.
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5. Reduction interaction: delete a blob body, then run the text verifier; assert
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`get` returns `Ok(None)` and the verifier handles it without erroring.
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6. Assert the counter is per-run, not global, so parallel tests do not mask each
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other.
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**Command:** `cargo test -p verify lazy_blobs`
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**False pass:**
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- Counting at the store level while a caching layer sits above it. The count then
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measures cache misses, not verifier behaviour. Wrap the handle the verifier
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actually receives.
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- Step 4 omitted: a `prompt_text` convenience field makes every verifier pay, and
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steps 1–3 still pass if the resolution happens at view-build time rather than
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through `cx.blobs`. The size assertion catches that.
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## Traps
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- A "convenience" `EpisodeView::prompt_text()` that resolves on access. Every
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verifier then pays, invisibly.
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- Prefetching all refs when the view is built, which is the same regression one
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layer earlier.
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---
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Background (not required to do this task):
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[rust-agentic-sys.md](../../../rust-agentic-sys.md) §10.1 ·
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[rust-agentic-task.md](../../../rust-agentic-task.md)
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