Iterator is still valid after calling the `Reset` about rocksdb

[git-hulk]

@ajkr Thanks for your answer.

If implementing it in application code, you would probably implement an Iterator subclass that wraps a RocksDB iterator, and you can make the wrapper's Valid() == false as needed

It works well for me.

[ajkr]

Do you mean Refresh()? I don't see a function on Iterator called Reset().

edit: ah sorry you are referring to the Reset() that it inherits from Cleanable. Let me rethink it.

[ajkr]

Probably we should prohibit users from calling Cleanable::Reset() on an Iterator. I can't think of a reason to call it - is there one? The following patch would prevent it, though I don't really want to add four friend classes to Iterator, so would be glad to see any improvement on that -

diff --git a/include/rocksdb/iterator.h b/include/rocksdb/iterator.h
index e90b887b1..11c48e8be 100644
--- a/include/rocksdb/iterator.h
+++ b/include/rocksdb/iterator.h
@@ -27,7 +27,7 @@

 namespace ROCKSDB_NAMESPACE {

-class Iterator : public Cleanable {
+class Iterator : protected Cleanable {
  public:
   Iterator() {}
   // No copying allowed
@@ -152,6 +152,11 @@ class Iterator : public Cleanable {
     assert(false);
     return Slice();
   }
+
+  friend class WriteUnpreparedTxn;
+  friend class WriteUnpreparedTxnDB;
+  friend class WritePreparedTxn;
+  friend class WritePreparedTxnDB;
 };

 // Return an empty iterator (yields nothing).

[git-hulk]

@ajkr Thanks for your reply.

Probably we should prohibit users from calling Cleanable::Reset() on an Iterator.

In our scenario, we want to make the iterator invalid if it mismatches the prefix. But it seems there is no way to do that for now, so I tried Reset() and found it just cleans up some resources. It makes sense to me if the Reset is only used for cleaning up the iterator.

[ajkr]

the iterator invalid if it mismatches the prefix

RocksDB can do it for you if you have a prefix_extractor configured, and you set ReadOptions::prefix_same_as_start. There's some read-amp optimizations too when RocksDB has awareness that scanning never needs to visit another prefix.

If implementing it in application code, you would probably implement an Iterator subclass that wraps a RocksDB iterator, and you can make the wrapper's Valid() == false as needed