Add dependency injected getTimestamp.

[MicahZoltu]

When writing contracts that integrate with Uniswap, it would be a huge boon to be able to run Uniswap on a testnet with dependency injected getTimestamp function, rather than using the built-in timestamp.

contract TimestampProvider {
    function getTimestamp() external returns (uint256) {
        return block.timestamp;
    }
}
contract UniswapV2Exchange {
    TimestampProvider public timestampProvider;
    constructor(TimestampProvider _timestampProvider) {
        timestampProvider = _timestampProvider;
    }
    ...
}

Also a similar change to the factory so it can pass along the timestamp provider.

The idea here is that in a test environment, you would provide a TimestampProvider that had functions on it for setting the timestamp, rather than pulling it from the block. This makes it so people can test their Uniswap integrated code against real nodes (e.g., Geth, Parity, Nethermind) when their code depends on the passage of time (like oracles do).

Caveat: This will increase the runtime gas costs of Uniswap operations by an SLOAD + CALL + SLOAD (plus some miscellaneous stuff). I can appreciate the argument that it may not be worth it, but as someone who is currently doing a Uniswap oracle integration, it sure would be nice!

[NoahZinsmeister]

this seems excessive, isn't the far more elegant solution to simply mock the timestamp in your test environment?

[MicahZoltu]

That only works if you are testing solely against ganache (and its 880+ transitive dependencies totalling 200+ MB over the wire, including 11 vulnerabilities, and all of the headaches all that brings). If you want to test against Parity/Geth/Nethermind/TurboGeth, you need the ability to do contract-level mocking of timestamps.

[NoahZinsmeister]

this is out of scope for v2, but appreciate the feedback! will keep issues like this in mind for timestamp-related code in the future