EIP897 - DelegateProxy

# Simple Summary

Proxy contracts are being increasingly used as both as an upgradeability mechanism and a way to save gas when deploying many instances of a particular contract. This standard proposes a set of interfaces for proxies to signal how they work and what their main implementation is.

# Abstract

Using proxies that delegate their own logic to another contract is becoming an increasingly popular technique for both smart contract upgradeability and creating cheap clone contracts.

We don't believe there is value in standardizing any particular implementation of a DelegateProxy, given its simplicity, but we believe there is a lot of value in agreeing on an interface all proxies use that allows for a standard way to operate with proxies.

# Implementations

aragonOS: AppProxyUpgradeable (opens new window), AppProxyPinned (opens new window) and KernelProxy (opens new window)
zeppelinOS: Proxy (opens new window)

# Standardized interface

interface ERCProxy {
  function proxyType() public pure returns (uint256 proxyTypeId);
  function implementation() public view returns (address codeAddr);
}

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# Code address (`implementation()`)

The returned code address is the address the proxy would delegate calls to at that moment in time, for that message.

# Proxy Type (`proxyType()`)

Checking the proxy type is the way to check whether a contract is a proxy at all. When a contract fails to return to this method or it returns 0, it can be assumed that the contract is not a proxy.

It also allows for communicating a bit more of information about how the proxy operates. It is a pure function, therefore making it effectively constant as it cannot return a different value depending on state changes.

Forwarding proxy (id = 1): The proxy will always forward to the same code address. The following invariant should always be true: once the proxy returns a non-zero code address, that code address should never change.
Upgradeable proxy (id = 2): The proxy code address can be changed depending on some arbitrary logic implemented either at the proxy level or in its forwarded logic.

# Benefits

Source code verification: right now when checking the code of a proxy in explorers like Etherscan, it just shows the code in the proxy itself but not the actual code of the contract. By standardizing this construct, they will be able to show both the actual ABI and code for the contract.