Make slots_per_year configurable instead of a constant

[mikemaccana]

Summary

Move slots_per_year from a hardcoded constant to a configurable parameter in reserve configuration. This allows reserves to adapt to protocol changes in slot time without requiring code updates, preventing borrowers from being charged incorrect wall-clock rates when Solana lowers the slot time.

Key Changes

• Anchor lending program:

  • Remove SLOTS_PER_YEAR constant from constants.rs
  • Add slots_per_year: u64 field to ReserveConfig struct
  • Update ReserveConfig::validate() to reject zero values
  • Modify interest accrual calculations to use config.slots_per_year instead of the constant
  • Add SLOTS_PER_YEAR test constant to common/mod.rs and update default_config()
  • Add comprehensive tests: rejects_zero_slots_per_year and slots_per_year_scales_the_per_slot_rate

• Quasar lending program:

  • Remove SLOTS_PER_YEAR constant from constants.rs
  • Add slots_per_year: u64 field to Reserve state struct
  • Add new update_slots_per_year instruction (discriminator 12) for owner-only retuning
  • Update initialize_reserve to accept slots_per_year parameter
  • Modify borrow_rate_per_slot() and accrue_factor() to accept slots_per_year parameter
  • Add test retuning_slots_per_year_rescales_accrual demonstrating the feature
  • Update README and CHANGELOG with configuration guidance

• Documentation updates:

  • Clarify in multiple constants.rs files that slot-based staleness windows tighten automatically as protocol lowers slot time
  • Add guidance in READMEs about reading current slot time from cluster and keeping reserves in sync
  • Update CHANGELOGs with detailed rationale and testing information

Implementation Details

• The update_slots_per_year instruction accrues interest at the old rate before storing the new divisor, ensuring slots already elapsed are charged at the rate that was in force for them
• Validation rejects zero to prevent division by zero in rate calculations
• Test fixtures use 78,840,000 slots/year (2.5 slots/second × 60 × 60 × 24 × 365), representing a 400ms slot time
• The change is backward-compatible for new reserves; existing reserves would need migration

https://claude.ai/code/session_01TCc6nwbMzXsG3fEom23KXW