Week 4 - M1: Object-store hardening + packfile groundwork

[aman-a-shah]

Owner: @mahis1067
Files: minigit/objects.py, tests/test_objects.py, docs/packfiles.md
Depends on: #16
Branch: week4/m1-object-store-hardening

Make corrupt objects fail clearly. Prepare the storage code for Week 5 packfiles.

Steps

1. Start from updated main after Week 3 - M1: Tree object serialization #16 merges. Keep the existing object API and byte formats.
2. Tighten read_object validation. Reject broken zlib data, malformed headers, unknown types, bad lengths, and hash mismatches with ObjectCorruptError.
   • Length means bytes, including for UTF-8 content.
   • Missing objects still raise ObjectNotFoundError.
3. Validate hashes before building disk paths. Accept full 40-character lowercase hex hashes. Reject other input with ObjectNotFoundError.
4. Make loose-object writes atomic: write a temporary file beside the destination, then rename it into place.
   • Clean up the temporary file on failure.
   • A duplicate write must validate the existing object before returning its hash.
   • An existing corrupt object raises ObjectCorruptError.
5. Add has_object(hash: str) -> bool for M4.
   • Use read_object. Return False only for ObjectNotFoundError.
   • Let corruption errors reach the caller.
6. Keep loose-file lookup in a private helper. Write a short docs/packfiles.md proposal covering record format, hash-to-offset lookup, and loose-file fallback. Packfile implementation starts in Week 5.
7. Run scripts/quality-check.sh. Open a PR linked to this issue and request a teammate review.

Tests

• Round-trip empty and binary blobs, trees, and commits through a fresh store.
• Corrupt the header, length, payload, and compressed stream separately.
• Check Unicode byte lengths and invalid hashes.
• Simulate a failed write: no partial object at the final path; retry succeeds.
• Check valid duplicates, corrupt duplicates, and all three has_object outcomes.
• Use tmp_path for every test, including CLI tests.

Done when

• Bad objects raise the shared errors.
• Failed writes leave no partial object at its final path.
• M4 can check object presence through has_object.
• Packfile proposal is ready for review.
• Quality check passes; PR open and review requested.