Separate Borg 1.x legacy code into borg.legacy package

[mr-raj12]

Problem

Borg 2.x code is tangled with 1.x legacy code across ~15 files. manifest.py has 15+ if not self.legacy: branches in the Archives class alone. legacyrepository.py and legacyremote.py already have "legacy" in the name but sit at the package root. src/borg/legacy/ exists but is empty.

What this does

Move the bulk of 1.x code into src/borg/legacy/. The goal is to eliminate almost every last # legacy comment , some one-liner branches are worse to extract than to keep. But the major classes and branch-heavy code (LegacyRepository, LegacyRemoteRepository, RepoObj1, the 4 legacy key types, the entire Archives legacy path) all move out.

After this:

• v2 core modules are mostly free of legacy code
• borg transfer --from-borg1 keeps working via borg.legacy
• Dropping v1 support later = delete borg.legacy + remove a handful of scattered one-liners

Plan (7 phases)

Each phase is a separate PR.

Phase	What happens
1	git mv of legacyrepository.py + legacyremote.py into legacy/
2	Extract RepoObj1, move legacy key types (KeyfileKey, RepoKey, etc.) into legacy/ with imported base classes
3	Extract LegacyArchives from Archives, remove all self.legacy branches in manifest.py
4	Switch remote.py to lazy-import LegacyRepository only inside RepositoryServer.open()
5	git mv of upgrade.py into legacy/
6	Inventory remaining scattered legacy markers, split what can be split, document what stays
7	Audit and expand unit test coverage for the borg1 compatibility path: borg transfer --from-borg1 and the upgrade internals (UpgraderFrom12To20, Pbkdf2FileMixin, hardlink helpers, NSIndex1); end-to-end transfer tests exist but unit coverage of the upgrade internals is thin

done:

• legacy: move LegacyRepository, LegacyRemoteRepository, RepoObj1 into borg.legacy package, refs #9556 #9559 legacy: move LegacyRepository, LegacyRemoteRepository, RepoObj1 into borg.legacy package
• legacy: move legacy AES-CTR key classes into borg.legacy.crypto, refs #9556 #9600 legacy: move legacy AES-CTR key classes into borg.legacy.crypto
• legacy: extract LegacyArchives into legacy/archives.py, add ArchivesInterface, refs #9556 #9644 legacy: extract LegacyArchives into legacy/archives.py, add ArchivesInterface
• testsuite: add Archives and LegacyArchives unit tests, refs #9556 #9648 testsuite: add Archives and LegacyArchives unit tests
• legacy: lazy-import legacy classes inside v1 branches only, refs #9556 #9650 legacy: lazy-import legacy classes inside v1 branches only
• legacy: move upgrade.py into borg.legacy, refs #9556 #9655 legacy: move upgrade.py into borg.legacy
• legacy: move hardlink helpers, NSIndex1, AES, and PBKDF2 key file methods to borg.legacy, refs #9556 #9664 legacy: move AES, hardlink helpers, NSIndex1, Pbkdf2FileMixin into borg.legacy
• legacy: move pbkdf2 static method from FlexiKey to Pbkdf2FileMixin #9668 legacy: move pbkdf2 static method from FlexiKey to Pbkdf2FileMixin
• testsuite: add UpgraderFrom12To20 unit tests, refs #9556 #9675 testsuite: add UpgraderFrom12To20 unit tests

pending:

• testsuite: add borg1_hardlink_master/slave/hardlinkable unit tests, refs #9556 #9685 testsuite: add borg1_hardlink_master/slave/hardlinkable unit tests
• testsuite: add Pbkdf2FileMixin unit tests, refs #9556 #9686 testsuite: add Pbkdf2FileMixin unit tests

Target structure

src/borg/legacy/
├── __init__.py
├── repository.py      <- LegacyRepository
├── remote.py          <- LegacyRemoteRepository
├── repoobj.py         <- RepoObj1
├── archives.py        <- LegacyArchives
├── upgrade.py         <- UpgraderFrom12To20, UpgraderNoOp
├── helpers.py         <- borg1 hardlink helpers
├── hashindex.py       <- NSIndex1
└── crypto/
    ├── __init__.py
    ├── key.py         <- legacy AES-CTR key types (imports bases from crypto/key.py)
    └── low_level.pyx  <- AES-256-CTR Cython extension

Some scattered one-liners can't be moved like version == 1 checks in archive.py, legacy exit codes in helpers/errors.py, repository.version == 1 in cache.py. As extracting those would mean duplicating entire methods for one branch. They go away when v1 support is dropped.

Constraints

• borg transfer --from-borg1 must keep working after every phase
• borg serve must handle both v1 and v2 clients (lazy import in RepositoryServer.open())
• No stubs at old paths, using git mv for whole-file moves (per feedback on legacy: move LegacyRepository, LegacyRemoteRepository, RepoObj1 into borg.legacy package, refs #9556 #9559). Re-exports only where the original file still has v2 code (repoobj.py)

Testing

python -m pytest src/borg/testsuite/ -x -v
python -m pytest src/borg/testsuite/ -x -v -k "transfer"

[ThomasWaldmann]

There are also lots of places marked by "legacy" in a comment or otherwise. But maybe it is not easy to move all pieces into the legacy package.

[mr-raj12]

Phase 2 done with legacy AES-CTR key classes extracted to borg.legacy.crypto.key (PR #9600).