Update comments of WriteBatchWithIndex

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/4113

Differential Revision: D8814172

Pulled By: siying

fbshipit-source-id: cabc31db2c74803af9b2f99329155a1086eb1b22

include/rocksdb/utilities/write_batch_with_index.h

@@ -155,6 +155,12 @@ class WriteBatchWithIndex : public WriteBatchBase {
   // The returned iterator should be deleted by the caller.
   // The base_iterator is now 'owned' by the returned iterator. Deleting the
   // returned iterator will also delete the base_iterator.
+  //
+  // Updating write batch with the current key of the iterator is not safe.
+  // We strongly recommand users not to do it. It will invalidate the current
+  // key() and value() of the iterator. This invalidation happens even before
+  // the write batch update finishes. The state may recover after Next() is
+  // called.
   Iterator* NewIteratorWithBase(ColumnFamilyHandle* column_family,
                                 Iterator* base_iterator);
   // default column family

java/src/main/java/org/rocksdb/WriteBatchWithIndex.java

@@ -114,6 +114,12 @@ public class WriteBatchWithIndex extends AbstractWriteBatch {
    * creating a new Iterator that will use {@link org.rocksdb.WBWIRocksIterator}
    * as a delta and baseIterator as a base
    *
+   * Updating write batch with the current key of the iterator is not safe.
+   * We strongly recommand users not to do it. It will invalidate the current
+   * key() and value() of the iterator. This invalidation happens even before
+   * the write batch update finishes. The state may recover after Next() is
+   * called.
+   *
    * @param columnFamilyHandle The column family to iterate over
    * @param baseIterator The base iterator,
    *   e.g. {@link org.rocksdb.RocksDB#newIterator()}