Repository._delete needs a random read access just for the deleted size

[ThomasWaldmann]

Code: https://github.com/borgbackup/borg/blob/1.2.0/src/borg/repository.py#L1241

Used by:

• repo.put(existing_id, new_data)
• repo.delete(existing_id)

To update the Repository.compact map for the affected segment, the code needs to call LoggedIO.read(segment, offset, existing_id, read_data=False) just to get the size back.

If we had the size information in the repo index (and not just segment and offset), we could save that random read.

[ThomasWaldmann]

This is especially relevant for deleting big archives with many chunks that have refcount 1 (== last reference).

[ThomasWaldmann]

Another similar usage is in _update_index when processing DELETE tags, also just to get the size.

[ThomasWaldmann]

Another place on the client side is LocalCache._fetch_missing_csize, which even fetches all the data, just to determine its csize. This is only needed if --no-cache-sync was used.

[ThomasWaldmann]

The repo index issue will be fixed by #6689.

[ThomasWaldmann]

_fetch_missing_csize is now also gone.