Fix RLPWriter Memory Allocation (#84)

* add failing test case with this issue

* add _malloc which works

* malloc words not bytes

Author: ben-chain

contracts/optimistic-ethereum/libraries/rlp/Lib_RLPWriter.sol

@@ -89,6 +89,7 @@ library Lib_RLPWriter {
             bytes memory _out
         )
     {
+        _malloc(0x20);
         bytes memory inputBytes;
         assembly {
             let m := mload(0x40)
@@ -308,4 +309,22 @@ library Lib_RLPWriter {
 
         return flattened;
     }
+
+    /**
+     * Clears memory wherever the free memory is pointing.
+     * @param _numBytes Number of bytes to clear out (will be rounded up to nearest word).
+     */
+    function _malloc(
+        uint _numBytes
+    )
+        private
+        pure
+    {
+        assembly {
+            let free_mem := mload(0x40)
+            for { let offset := 0x00 } lt(offset, _numBytes) { offset := add(offset, 0x20) } {
+                mstore(add(free_mem, offset), 0x00)
+            }
+        }
+    }
 }

contracts/test-libraries/rlp/TestLib_RLPWriter.sol

@@ -4,6 +4,7 @@ pragma experimental ABIEncoderV2;
 
 /* Library Imports */
 import { Lib_RLPWriter } from "../../optimistic-ethereum/libraries/rlp/Lib_RLPWriter.sol";
+import { TestERC20 } from "../../test-helpers/TestERC20.sol";
 
 /**
  * @title TestLib_RLPWriter
@@ -93,4 +94,16 @@ contract TestLib_RLPWriter {
     {
         return Lib_RLPWriter.writeBool(_in);
     }
+
+    function writeAddressWithOtherMemory(
+        address _in
+    )
+        public
+        returns (
+            bytes memory _out
+        )
+    {
+        new TestERC20();
+        return Lib_RLPWriter.writeAddress(_in);
+    }
 }

test/contracts/libraries/rlp/Lib_RLPWriter.spec.ts

@@ -41,4 +41,13 @@ describe('Lib_RLPWriter', () => {
       })
     }
   })
+
+  describe.only('Use of library with other memory-modifying operations', () => {
+    it('should allow creation of a contract beforehand and still work', async () => {
+      const randomAddress = '0x1234123412341234123412341234123412341234'
+      const rlpEncodedRandomAddress = '0x941234123412341234123412341234123412341234'
+      const encoded = await Lib_RLPWriter.callStatic.writeAddressWithOtherMemory(randomAddress)
+      expect(encoded).to.eq(rlpEncodedRandomAddress)
+    })
+  })
 })