repository: make check and delete work at N=1 with sha256 pack ids

[mr-raj12]

Description

borg check, delete and compact all assumed that a pack file's name is the chunk_id it holds. That is true only at N=1 while pack ids equal chunk ids. With BORG_TESTONLY_SHA256_PACK_ID=1 (and later at N>1) a pack is named sha256(pack_bytes), so the file name is the pack_id, not the chunk_id.

The object header already carries the chunk_id, meta size and data size (repoobj.py), so the chunk index can be rebuilt from the headers (real chunk_ids) instead of the file names, and removal can find the pack through the index.

Changes:

• repoobj.py: add RepoObj.iter_object_headers(), which walks a pack's object headers and yields (chunk_id, obj_offset, obj_size). Works for one object per pack (N=1) and for several (N>1).
• repository.py:
  • delete() becomes a logged no-op. A pack may hold several objects, so we can not remove one object by dropping its pack without losing the others; real removal goes through store_delete at the pack level.
  • list() iterates the chunk index and yields chunk_ids, instead of listing packs/ (which would yield pack_ids).
• cache.py: build_chunkindex_from_repo() rebuilds the index from store_list + iter_object_headers (real chunk_id/offset/size) rather than calling Repository.list(), which would recurse into the very index it is building. Adds an init_flags argument so compact can start the chunks as unused and compute usage itself.
• compact_cmd.py: --stats builds the index from a real header scan instead of a pack_id == chunk_id fake; the delete loop drops each unused chunk's pack via store_delete.
• archive.py / debug_cmd.py: check --repair and debug delete-obj resolve the pack via the chunk index and then store_delete it; debug delete-obj maps a missing pack (stale index entry) back to "not found".
• tests: a delete_chunk helper drops a chunk's pack at the store level (used by the check / extract / mount damage tests), since Repository.delete no longer removes anything; test_empty_repository drops packs through the store; test objects get a unique chunk_id so identical payloads do not collapse into one pack under sha256.

Verified with the local test subset, with and without BORG_TESTONLY_SHA256_PACK_ID=1: the repository- and archive-level missing-chunk checks pass under the switch and nothing regresses without it. Archive --verify-data corruption detection under sha256 pack ids (a few check --verify-data tests) is left for a separate change.

refs #8572

Checklist

• PR is against master
• New code has tests and docs where appropriate
• Tests pass (ran the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.84%. Comparing base (2eef6c8) to head (4121509).
⚠️ Report is 14 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/cache.py	69.23%	3 Missing and 1 partial ⚠️
src/borg/repository.py	83.33%	1 Missing and 2 partials ⚠️
src/borg/archive.py	50.00%	1 Missing ⚠️
src/borg/archiver/debug_cmd.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9783      +/-   ##
==========================================
- Coverage   84.88%   84.84%   -0.05%     
==========================================
  Files          92       92              
  Lines       15172    15166       -6     
  Branches     2275     2281       +6     
==========================================
- Hits        12879    12867      -12     
- Misses       1589     1594       +5     
- Partials      704      705       +1     

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