[comandanto]

Description

UsdcLendingPoolContract::withdraw could only close a depositor's whole position, and set_yield_rate let the admin change annual_yield_bps by any amount in one call. withdraw now takes an amount of principal, so a depositor can withdraw part of the position and keep the rest earning. set_yield_rate now limits a single update to 500 bps.

Related Issue

Closes #421

Motivation and Context

Liquidity providers had to exit completely to access part of their funds, and the admin could shock the yield rate with no notice.

Changes:

• contracts/usdc_lending_pool/src/lib.rs: withdraw(env, depositor, amount). It pays amount principal plus all accrued yield. Remaining principal is stored with the current yield index, so it keeps accruing and no yield is paid twice. Full withdrawal still removes the record. Panics: "amount must be > 0", "amount exceeds deposited principal". set_yield_rate panics with "yield rate change exceeds max step" when the change is above MAX_YIELD_RATE_CHANGE_BPS (500).
• contracts/usdc_lending_pool/src/test.rs: existing withdraw tests pass the full principal; added tests for partial withdrawal (balances, pool totals, continued accrual, backing), partial then full close, zero and over-principal amounts, and the rate step limit (max step up/down, over the step in both directions).
• lib/contracts/usdc-lending-pool.ts: withdraw wrapper takes amountStroops.

Note: the withdraw signature change is breaking for callers of the contract ABI.

How Has This Been Tested?

Run on Windows (stable Rust, wasm32-unknown-unknown installed), from contracts/:

cargo test -p usdc_lending_pool   -> test result: ok. 39 passed; 0 failed
cargo test                        -> all crates ok, 0 failed (incl. lending 135 passed, usdc_lending_pool 39 passed)
cargo clippy -p usdc_lending_pool -- -D warnings                 -> Finished, no warnings
cargo clippy -p usdc_lending_pool --all-targets -- -D warnings   -> Finished, no warnings
cargo build --target wasm32-unknown-unknown --release            -> Finished `release` profile

cargo fmt -p usdc_lending_pool --check reports 9 diffs, the same count as on unmodified main, so they are pre-existing and I did not reformat unrelated code. I did not run the TypeScript build; the TS change is a one-line signature update of the wrapper, which has no callers in the repo.

Screenshots (if appropriate):

n/a

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change) (withdraw now requires an amount argument)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation. (n/a, behaviour is documented in the contract doc comments, which are updated)
• I have updated the documentation accordingly. (n/a, see above)
• I have read the CONTRIBUTING document.

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