Migration backup

[semohr]

Description

Creates a timestamped .bak copy of the database before any pending schema migration runs, so users can recover if a migration goes wrong. Controlled by the new create_backup_before_migrations config option (default: yes).

Changes:

• Migration.migrate_model: before running migration data, copies the database file to <library.db>.<timestamp>.bak (once per startup, guarded by a ClassVar flag)
• Database.create_copy(dest): copies the SQLite file to the given path

TODOs:

• Documentation.
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[Copilot]

Pull request overview

PR add safety net for db schema migrations: make timestamp .bak copy of library db before first pending migration run. This fit beets startup migration path, so user can roll back when migration go bad.

Changes:

• Add create_backup_before_migrations config (default yes) and document it.
• Add backup creation hook in Migration.migrate_model before migration work runs.
• Add test for config on/off behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
beets/dbcore/db.py	Add backup-before-migration hook and Database.create_copy helper.
beets/config_default.yaml	Set default create_backup_before_migrations: yes.
docs/reference/config.rst	Document new global config option.
docs/changelog.rst	Add changelog entry for migration backup feature.
test/library/test_migrations.py	Add regression test for backup creation controlled by config.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.55%. Comparing base (2337cfb) to head (60d14b8).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6742      +/-   ##
==========================================
+ Coverage   74.52%   74.55%   +0.02%     
==========================================
  Files         162      162              
  Lines       20818    20836      +18     
  Branches     3295     3297       +2     
==========================================
+ Hits        15515    15534      +19     
  Misses       4547     4547              
+ Partials      756      755       -1     

Files with missing lines	Coverage Δ
beets/dbcore/db.py	94.44% <100.00%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

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[snejus]

Would it be possible to move this test to any of existing test classes? Ideally we'd want to reuse the setup

[semohr]

I would wait on #6709 to takle this.