repo-create: store default compression and chunker params in the repository, fixes #346

[ThomasWaldmann]

Adds per-repository defaults for compression and chunker params, fixes #346 (set at repository creation time).

What it does

• borg repo-create --compression SPEC / --chunker-params PARAMS store repository defaults.
• Compression: the commands that compress data (create, recreate, import-tar, transfer, repo-compress, debug put-obj) use the repository default if no compression was given. borg repo-compress without --compression recompresses to the repository default.
• Chunker params: create and import-tar use the repository default if no chunker params were given. recreate and transfer still only rechunk if --chunker-params is given; --chunker-params default now means the repository default (else the built-in default) instead of always the built-in default.
• Precedence, for both:
  1. the command line, the environment (e.g. BORG_CREATE__COMPRESSION, BORG_CREATE__CHUNKER_PARAMS) or default.yaml
  2. the repository default
  3. the built-in default (lz4, fastcdc,19,23,21,2), as before
• borg repo-info shows Default compression: … and Default chunker params: … (with (built-in) if the repository has no default); --json has a new defaults key.

Storage

The defaults are stored in a new config/defaults store object (msgpacked dict of strings, e.g. {"compression": "zstd,3", "chunker_params": "fastcdc,19,23,21,2"}) in the key's store object envelope, like the index/ and cache/ objects. So they are authenticated: nobody without the key can change them (e.g. remove an obfuscate compression, or turn compression on where none was chosen) without being noticed. config/config can not hold them: it is read before the key is known and is not authenticated.

• missing object / missing entry: no repository default
• failed authentication: IntegrityError
• malformed content or an invalid value: InvalidRepositoryConfig (rc 16), the message names the problem

The resolution happens in one place, the with_repository decorator (default_compression() / default_chunker_params() in archiver/_common.py). --compression options default to None; ChunkerParams("default") returns a DEFAULT_CHUNKER_PARAMS marker, which is also the parser default of create and import-tar. config/defaults is read at most once per Repository object.

Not in this PR

• Changing the defaults of an existing repository (only repo-create sets them).
• borg check does not verify config/defaults; every command using it (incl. repo-info) authenticates it.

Docs / tests

Docs: repo-create epilog ("Repository defaults"), -C / --chunker-params help texts, borg help compression, data-structures.rst (new "Repository defaults" section), frontends.rst, quickstart. Usage docs are not regenerated.

New tests in repo_create_cmd_test.py: defaults used by create/repo-compress/recreate, precedence (command line, env), built-in fallbacks, --chunker-params default, invalid values at creation, tampered and invalid stored defaults. Full test suite passes locally (macOS, Python 3.15).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.70%. Comparing base (9b3b7eb) to head (f694ba3).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/repository.py	86.36%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10438      +/-   ##
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+ Coverage   88.62%   88.70%   +0.07%     
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  Files         103      103              
  Lines       19229    19284      +55     
  Branches     2994     3004      +10     
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+ Hits        17041    17105      +64     
+ Misses       1522     1507      -15     
- Partials      666      672       +6     

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