feat(multicall): prevent double initiation

src/BaseBundler.sol

@@ -35,6 +35,8 @@ abstract contract BaseBundler is IMulticall {
     /// @dev Locks the initiator so that the sender can uniquely be identified in callbacks.
     /// @dev All functions delegatecalled must be `payable` if `msg.value` is non-zero.
     function multicall(bytes[] memory data) external payable {
+        require(_initiator == UNSET_INITIATOR, ErrorsLib.ALREADY_INITIATED);
+
         _initiator = msg.sender;
 
         _multicall(data);

src/libraries/ErrorsLib.sol

@@ -8,6 +8,9 @@ pragma solidity ^0.8.0;
 library ErrorsLib {
     /* STANDARD BUNDLERS */
 
+    /// @dev Thrown when a multicall is attempted while the bundler in an initiated execution context.
+    string internal constant ALREADY_INITIATED = "already initiated";
+
     /// @dev Thrown when a call is attempted while the bundler is not in an initiated execution context.
     string internal constant UNINITIATED = "uninitiated";
 

test/forge/BaseBundlerLocalTest.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+pragma solidity ^0.8.0;
+
+import {ErrorsLib} from "src/libraries/ErrorsLib.sol";
+
+import "src/mocks/bundlers/BaseBundlerMock.sol";
+
+import "./helpers/LocalTest.sol";
+
+contract BaseBundlerLocalTest is LocalTest {
+    function setUp() public override {
+        super.setUp();
+
+        bundler = new BaseBundlerMock();
+    }
+
+    function testMulticallEmpty() public {
+        bundler.multicall(bundle);
+    }
+
+    function testNestedMulticall() public {
+        bundle.push(abi.encodeCall(BaseBundler.multicall, (callbackBundle)));
+
+        vm.expectRevert(bytes(ErrorsLib.ALREADY_INITIATED));
+        bundler.multicall(bundle);
+    }
+}