Closes safe-global#246: Send along msg.sender to fallback handler (sa…

…fe-global#270)

base/FallbackManager.sol

@@ -47,7 +47,11 @@ contract FallbackManager is SelfAuthorized {
             // solium-disable-next-line security/no-inline-assembly
             assembly {
                 calldatacopy(0, 0, calldatasize())
-                let success := call(gas, handler, 0, 0, calldatasize(), 0, 0)
+                // The msg.sender address is shifted to the left by 12 bytes to remove the padding
+                // Then the address without padding is stored right after the calldata
+                mstore(calldatasize(), shl(96, caller()))
+                // Add 20 bytes for the address appended add the end
+                let success := call(gas, handler, 0, 0, add(calldatasize(), 20), 0, 0)
                 returndatacopy(0, 0, returndatasize())
                 if eq(success, 0) { revert(0, returndatasize()) }
                 return(0, returndatasize())

handler/HandlerContext.sol

@@ -0,0 +1,19 @@
+pragma solidity >=0.5.0 <0.6.0;
+
+/// @title Handler Context - allows to extract calling context
+/// @author Richard Meissner - <richard@gnosis.pm>
+/// @notice based on https://github.com/OpenZeppelin/openzeppelin-contracts/blob/f8cc8b844a9f92f63dc55aa581f7d643a1bc5ac1/contracts/metatx/ERC2771Context.sol
+contract HandlerContext {
+
+    // This function does not rely on a trusted forwarder. Use the returned value only to check information against the calling manager.
+    function _msgSender() internal pure returns (address sender) {
+        // The assembly code is more direct than the Solidity version using `abi.decode`.
+        assembly { sender := shr(96, calldataload(sub(calldatasize(), 20))) }
+    }
+
+    // Function do differentiate more clearly between msg.sender and the calling manager
+    function _manager() internal view returns (address) {
+        return msg.sender;
+    }
+
+}

test/TestHandler.sol

@@ -0,0 +1,7 @@
+pragma solidity >=0.5.0 <0.6.0;
+import "../handler/HandlerContext.sol";
+contract TestHandler is HandlerContext {
+	function dudududu() external returns (address sender, address manager) {
+		return (_msgSender(), _manager());
+	}
+}