Chunk 15.1

## Summary In this chunk, the assistant addressed the user's directive to implement all remaining critical gaps identified by the earlier comprehensive analysis. The primary focus was fixing the **Prefetcher `Fetch()` implementation** which had been stubbed with a placeholder error. The assistant implemented a proper `Fetch()` method in `retr_provider.go` that leverages the existing retrieval provider's cache hierarchy (L1 → L2 → HTTP), using `FindHashes` via the `Storage()` interface and `getAddrInfoCached` for provider URLs to fetch blocks by CID. The second major implementation was the **L1→L2 cache promotion callback**. The assistant modified `rbcache/arc.go` to add an `evictionCallback` field to the ARCCache struct and created a `SetEvictionCallback()` method. The `evictFromT1()` and `evictFromT2()` methods were updated to call this callback when items are evicted. The callback was then wired up in `retr_provider.go` by calling `SetEvictionCallback` on the L1 cache after initialization, providing a lambda that promotes evicted items to the L2 SSD cache via `l2Cache.Put(key, value)`. A dedicated test file `rbstor/unlink_test.go` was created with two test cases (`TestUnlinkBasic` and `TestUnlinkMultipleBlocks`) that exercise the full put‑unlink‑view cycle: blocks are written to a group, then unlinked, and the test verifies that subsequent `View` calls return `iface.UndefGroupKey` for the unlinked multihashes. The build succeeded across the `rbstor` and `rbdeal` packages, but the test timed out (120 s) due to the overhead of starting a YugabyteDB container. The overall theme is **pragmatic gap‑filling**: the assistant avoided expensive CQL indexes in favor of separate KV‑like tables (aligning with user guidance) and implemented only what was immediately necessary to unblock the data lifecycle. The implementation focused on fixing the critical blocking issues rather than building out the full complexity, ensuring the system can progress without architectural over-engineering.

The Architecture of Pragmatic Gap-Filling: Closing Critical Implementation Gaps in a Distributed Storage System 2017 words

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