Add a warning about getMerkleRoot (#336)

packages/contracts/contracts/optimistic-ethereum/OVM/chain/OVM_CanonicalTransactionChain.sol

@@ -545,6 +545,9 @@ contract OVM_CanonicalTransactionChain is iOVM_CanonicalTransactionChain, Lib_Ad
             blockNumber = lastElement.blockNumber;
         }
 
+        // For efficiency reasons getMerkleRoot modifies the `leaves` argument in place
+        // while calculating the root hash therefore any arguments passed to it must not
+        // be used again afterwards
         _appendBatch(
             Lib_MerkleTree.getMerkleRoot(leaves),
             totalElementsToAppend,
@@ -937,18 +940,18 @@ contract OVM_CanonicalTransactionChain is iOVM_CanonicalTransactionChain, Lib_Ad
         // If there are existing elements, this batch must come later.
         if (getTotalElements() > 0) {
             (,, uint40 lastTimestamp, uint40 lastBlockNumber) = _getBatchExtraData();
-    
+
             require(
                 _firstContext.blockNumber >= lastBlockNumber,
                 "Context block number is lower than last submitted."
             );
-    
+
             require(
                 _firstContext.timestamp >= lastTimestamp,
                 "Context timestamp is lower than last submitted."
             );
         }
-    
+
         // Sequencer cannot submit contexts which are more than the force inclusion period old.
         require(
             _firstContext.timestamp + forceInclusionPeriodSeconds >= block.timestamp,

packages/contracts/contracts/optimistic-ethereum/OVM/chain/OVM_StateCommitmentChain.sol

@@ -20,7 +20,7 @@ import '@openzeppelin/contracts/math/SafeMath.sol';
 /**
  * @title OVM_StateCommitmentChain
  * @dev The State Commitment Chain (SCC) contract contains a list of proposed state roots which
- * Proposers assert to be a result of each transaction in the Canonical Transaction Chain (CTC). 
+ * Proposers assert to be a result of each transaction in the Canonical Transaction Chain (CTC).
  * Elements here have a 1:1 correspondence with transactions in the CTC, and should be the unique
  * state root calculated off-chain by applying the canonical transactions one by one.
  *
@@ -334,6 +334,9 @@ contract OVM_StateCommitmentChain is iOVM_StateCommitmentChain, Lib_AddressResol
             );
         }
 
+        // For efficiency reasons getMerkleRoot modifies the `_batch` argument in place
+        // while calculating the root hash therefore any arguments passed to it must not
+        // be used again afterwards
         Lib_OVMCodec.ChainBatchHeader memory batchHeader = Lib_OVMCodec.ChainBatchHeader({
             batchIndex: getTotalBatches(),
             batchRoot: Lib_MerkleTree.getMerkleRoot(_batch),

packages/contracts/contracts/optimistic-ethereum/libraries/utils/Lib_MerkleTree.sol

@@ -16,6 +16,7 @@ library Lib_MerkleTree {
      * of leaves passed in is not a power of two, it pads out the tree with zero hashes.
      * If you do not know the original length of elements for the tree you are verifying,
      * then this may allow empty leaves past _elements.length to pass a verification check down the line.
+     * Note that the _elements argument is modified, therefore it must not be used again afterwards
      * @param _elements Array of hashes from which to generate a merkle root.
      * @return Merkle root of the leaves, with zero hashes for non-powers-of-two (see above).
      */
@@ -108,12 +109,12 @@ library Lib_MerkleTree {
 
     /**
      * Verifies a merkle branch for the given leaf hash.  Assumes the original length
-     * of leaves generated is a known, correct input, and does not return true for indices 
+     * of leaves generated is a known, correct input, and does not return true for indices
      * extending past that index (even if _siblings would be otherwise valid.)
      * @param _root The Merkle root to verify against.
      * @param _leaf The leaf hash to verify inclusion of.
      * @param _index The index in the tree of this leaf.
-     * @param _siblings Array of sibline nodes in the inclusion proof, starting from depth 0 (bottom of the tree). 
+     * @param _siblings Array of sibline nodes in the inclusion proof, starting from depth 0 (bottom of the tree).
      * @param _totalLeaves The total number of leaves originally passed into.
      * @return Whether or not the merkle branch and leaf passes verification.
      */
@@ -188,7 +189,7 @@ library Lib_MerkleTree {
         returns (
             uint256
         )
-    {   
+    {
         require(
             _in > 0,
             "Lib_MerkleTree: Cannot compute ceil(log_2) of 0."