aave-protocol-v2/contracts/libraries/openzeppelin-upgradeability/Proxy.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity ^0.6.0;

/**
 * @title Proxy
 * @dev Implements delegation of calls to other contracts, with proper
 * forwarding of return values and bubbling of failures.
 * It defines a fallback function that delegates all calls to the address
 * returned by the abstract _implementation() internal function.
 */
abstract contract Proxy {
    /**
   * @dev Fallback function.
   * Implemented entirely in `_fallback`.
   */
    fallback() external payable {
        _fallback();
    }

    /**
   * @return The Address of the implementation.
   */
    function _implementation() internal virtual view returns (address);

    /**
   * @dev Delegates execution to an implementation contract.
   * This is a low level function that doesn't return to its internal call site.
   * It will return to the external caller whatever the implementation returns.
   * @param implementation Address to delegate.
   */
    function _delegate(address implementation) internal {
        //solium-disable-next-line
        assembly {
            // Copy msg.data. We take full control of memory in this inline assembly
            // block because it will not return to Solidity code. We overwrite the
            // Solidity scratch pad at memory position 0.
            calldatacopy(0, 0, calldatasize())

            // Call the implementation.
            // out and outsize are 0 because we don't know the size yet.
            let result := delegatecall(gas(), implementation, 0, calldatasize(), 0, 0)

            // Copy the returned data.
            returndatacopy(0, 0, returndatasize())

            switch result
                // delegatecall returns 0 on error.
                case 0 {
                    revert(0, returndatasize())
                }
                default {
                    return(0, returndatasize())
                }
        }
    }

    /**
   * @dev Function that is run as the first thing in the fallback function.
   * Can be redefined in derived contracts to add functionality.
   * Redefinitions must call super._willFallback().
   */
    function _willFallback() internal virtual {}

    /**
   * @dev fallback implementation.
   * Extracted to enable manual triggering.
   */
    function _fallback() internal {
        _willFallback();
        _delegate(_implementation());
    }
}