Support safe cleanup of orphaned KV shared SST files

[platinumhamburg]

Search before asking

• I searched in the issues and found nothing similar.

Description

Shared SST files are not owned by a single snapshot directory. They may be reused by multiple retained snapshots and still-in-use snapshots. A client-side cleanup action cannot safely decide that a shared SST is orphaned by only checking currently active snapshot IDs or currently visible shared file handles.

The main correctness challenge is concurrency with snapshot lifecycle:

• a snapshot may have uploaded shared SST files but not committed yet
• a snapshot commit result may be temporarily uncertain
• active refs fetched by the cleanup action are only a point-in-time view
• object-store listing and deletion are not protected by the snapshot lifecycle lock

Deleting a shared SST from the cleanup action can therefore race with an in-flight snapshot and cause a later committed snapshot to reference a deleted SST file.

Shared SST cleanup should be implemented with server-side snapshot lifecycle coordination, or with an equivalent protocol that can protect in-flight snapshots.

Willingness to contribute

• I'm willing to submit a PR!