Memoize live results in RAM to avoid duplicate computation

[vincenzoml]

Tracking issue: #14 — fixes defects §2/§3.1 (no in-RAM image memo → recompute under --no-cache) and §3.2 (repeated disk deserialize under --store-db). Does not address §3.3 (double serialize/write) or §4 (node-level parallelism), which stay open in #14.

---

Summary

First of two stacked PRs addressing the lazy-interpreter caching defects in #14. This one is the minimal correctness fix; the follow-up (#TBD) layers a bounded two-level cache on top.

Scope: avoid duplicate computation only. No parallelization.

The defect

MaterializationStore.put() replaced image-like values (image/bytes/ndarray/overlay) with a node-id placeholder in the in-memory record:

if vox_type in ("bytes", "overlay", "ndarray", "image"):
    stored_value = node_id      # <- real value dropped
else:
    stored_value = value

get() then had to round-trip to the backend to recover the value. With no backend (--no-cache) that round-trip returns None, so _evaluate_node_lazy's top-level cache_lookup misses and the node is recomputed on every demand. A node shared by N consumers (a diamond DAG over images — exactly the BraTS sweeps) recomputes its whole subtree N times. Scalars were unaffected because their real value was kept.

The fix

Keep the real value in the in-memory record for all types; return it directly from get(). Each node is now computed at most once per run regardless of backend. With a backend present, reuse also stops re-reading and re-deserializing the payload from disk on every demand.

Two small edits in storage.py (put, get), plus removal of the now-dead placeholder branch.

Trade-off

Live intermediates are now held in RAM for the duration of the run (previously image-like values were dropped). The stacked follow-up PR bounds this with an LRU memory tier backed by the disk tier — the "two-level cache" of #14.

Tests

• New tests/unit/test_inrun_memo.py: an ndarray result is returned from get() with no backend (the exact case that used to recompute), plus a scalar round-trip.
• test_async_persistence and the storage/execution unit tests still pass.
• End-to-end CLI run under --no-cache verified.

(Pre-existing unrelated failures in test_default_primitives.py — SequenceValue/.compute() API drift — fail identically on main.)

Refs #14

🤖 Generated with Claude Code