Chunk 15.0

## Summary In this chunk, the assistant addressed the user's directive to fix the most critical implementation gaps identified by the earlier subagent analysis. The primary focus was implementing the long-stalled `Unlink` method, which had been left as `panic("implement me")` in both `rbstor/rbs.go` and `rbstor/group.go`. The assistant implemented `Group.Unlink` to remove multihash entries from the CQL index via `DropGroup`, update group metadata (dead blocks/bytes counters), and handle the case where the group is offloaded. The `ribBatch.Unlink` was also implemented to wrap this process with the batch session, tracking which groups need flushing. To support these counters, a new `UpdateGroupDeadBlocks` method was added to `RbsDB` in `rbstor/db.go`, and the SQL schema migration (1769890615) was updated to include a `dead_bytes` column alongside the existing `dead_blocks`. 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.

Pragmatic Gap-Filling: Closing the Critical Implementation Gaps in a Distributed S3 Storage System 2904 words

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