size-bound 50 MB packs by default, fix stale chunk index after compact pack rewrites

[mr-raj12]

Description

Two changes, one commit each.

Pack sizing defaults (fixes #9843)

Packs are now bound by size: max_size=50_000_000, max_count=None. The old max_count=3 was a testing value. The testsuite sets BORG_PACK_MAX_COUNT=3 via the clean_env fixture, so small test data still produces multiple multi-object packs and the rollover and compaction code paths stay covered.

compact left a stale chunk index after pack rewrites (fixes #9850)

I traced the ObjectNotFound crash from #9850 to this: borg compact computes its keep/drop sets from its own chunk index, but compact_pack() applied the resulting updates to repository.chunks, a different index object. save_chunk_index() then persisted the unchanged index, so after every mixed-pack rewrite the persisted entries still pointed at the deleted old pack. Once a later borg delete made all of that phantom pack's entries unused, the next compact classified it as droppable and crashed trying to delete a pack file that was already gone.

No data is lost in this situation: the kept objects were copied into the new pack, only the persisted pointers were stale. Rebuilding the index from the packs (deleting the index/* objects) repairs an affected repository.

Fixes:

• compact_pack() takes a chunks parameter and compact passes the index it computed keep/drop from, so the updates land in the index that gets persisted. As a side effect this removes a full slow index rebuild that happened mid-compact, when compact_pack() lazily rebuilt repository.chunks after the cached indexes had been invalidated.
• Deleting a pack that is already gone logs a warning instead of crashing. A repository with a stale index then self-heals: compact removes the stale entries and writes a fresh index.
• get_repository_chunks() passes init_flags=F_NONE (found while reading the code: without it, a compact run that has to rebuild the index from the packs marks everything used and frees nothing).

Both bugs have regression tests that fail on master: one drives create/delete/compact through a mixed pack rewrite and checks that the persisted index only references existing packs, that extract still works and that a second delete + compact completes; the other covers garbage collection after the cached index was lost.

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.11%. Comparing base (3969aea) to head (a8c2c7b).
⚠️ Report is 22 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/archiver/compact_cmd.py	71.42%	2 Missing ⚠️
src/borg/repository.py	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9852   +/-   ##
=======================================
  Coverage   85.11%   85.11%           
=======================================
  Files          93       93           
  Lines       15388    15409   +21     
  Branches     2322     2326    +4     
=======================================
+ Hits        13097    13115   +18     
- Misses       1592     1596    +4     
+ Partials      699      698    -1     

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[ThomasWaldmann]

good you found that bug!

some nitpicks.

[ThomasWaldmann]

lgtm!