CurrencyReserves sync() can be overwritten — settlement manipulation vector #1069

hey, noticed that sync() can be called multiple times within a single transaction with no access control. Each call overwrites the previous currency and reserves:

function syncCurrencyAndReserves(Currency currency, uint256 value) internal {
    assembly ("memory-safe") {
        tstore(CURRENCY_SLOT, and(currency, 0xffffffffffffffffffffffffffffffffffffffff))
        tstore(RESERVES_OF_SLOT, value)
    }
}

In _settle(), the paid amount is computed as reservesNow - reservesBefore. If sync() is called again between take() and settle(), the reserves snapshot is overwritten, potentially manipulating the settlement amount.

While NonzeroDeltaCount provides a safety net, the unrestricted sync() combined with transient storage creates a complex attack surface.

Is this a known design trade-off? Or should sync() be restricted?