In-memory database: add values that expire (TTL)

Context

Welcome to the platform team. The in-memory database in database/ already handles records and scans (levels 1 and 2), and its tests pass. The session service now needs values that disappear on their own. The two new methods are stubbed in database/database.py, and the rules for level 3 are in README.md.

Build level 3 in 35 minutes. The checks are graded one by one, so each part you get working counts even if the rest is not done.

MDB-3: Values that expire (level 3)

Acceptance criteria

• set_with_ttl(timestamp, key, field, value, ttl) stores a value that is visible until just before timestamp + ttl, and gone at timestamp + ttl.
• An expired value behaves like a missing one: get returns None, compare_and_set and compare_and_delete return False, and scan and scan_by_prefix leave it out.
• compare_and_set_with_ttl replaces a matching, unexpired value and gives the new value its own expiry.
• set and compare_and_set still write values that never expire, even over a value that had a TTL.
• Levels 1 and 2 keep working.

Notes

• How you store expiry is up to you. The existing methods will need to know about it.
• Run pytest tests/visible. tests/visible/test_level3.py fails until level 3 works.
• Practice task modelled on candidate reports of Meta's four-level in-memory database online assessment. Original StackYard code, not the actual question; not affiliated with Meta.