check: report missing chunks grouped as chunk -> files -> archives, #9218

[mr-raj12]

Fixes #9218.

borg check logged one line per (archive, file, chunk) for each missing file chunk. Now it groups by chunk:

The following chunks are missing in the repository:
- Chunk a8675a3a8a...95dce, 553,688 bytes
    - var/lib/flatpak/repo/objects/2c/6265...52be.file: archive-2022-03-31, archive-2022-04-29
    - var/lib/flatpak/runtime/.../bin/gpgsm: archive-2022-03-31

For each missing chunk you see its size, the files that reference it, and the archives each file is in. The old per-chunk line (with byte offsets) is now logged at debug level.

The report is collected while checking the archives and printed once they have all been analyzed. As a check can run against a badly damaged repository with very many missing chunks, the collection is bounded by ArchiveChecker.MAX_MISSING_CHUNKS (distinct chunks) and MAX_REFS_PER_CHUNK (files per chunk); the report notes when either limit truncated it.

Tests in check_cmd_test.py cover the grouping (one chunk reported once, listing multiple files and archives) and the truncation.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.68%. Comparing base (a50eb6f) to head (083bb31).
⚠️ Report is 20 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archive.py	88.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9965      +/-   ##
==========================================
- Coverage   85.70%   85.68%   -0.03%     
==========================================
  Files          95       95              
  Lines       16815    16902      +87     
  Branches     2577     2591      +14     
==========================================
+ Hits        14411    14482      +71     
- Misses       1668     1684      +16     
  Partials      736      736              

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[ThomasWaldmann]

So, did you ask claude for a review?