Fix bug in pinned partitioned user key indexes

Backports part of 75d3b6fdf0aa1007c4d26382f65be0adf4519a37.

HISTORY.md

 # Rocksdb Change Log
+## Unreleased
+### Bug Fixes
+* Fixed a bug in the following combination of features: indexes with user keys (`format_version >= 3`), indexes are partitioned (`index_type == kTwoLevelIndexSearch`), and some index partitions are pinned in memory (`BlockBasedTableOptions::pin_l0_filter_and_index_blocks_in_cache`). The bug could cause keys to be truncated when read from the index leading to wrong read results or other unexpected behavior.
+
 ## 6.11.5 (7/23/2020)
 ### Bug Fixes
 * Memtable lookup should report unrecognized value_type as corruption (#7121).

table/iterator_wrapper.h

@@ -142,6 +142,11 @@ class IteratorWrapperBase {
     return result_.value_prepared;
   }
 
+  Slice user_key() const {
+    assert(Valid());
+    return iter_->user_key();
+  }
+
  private:
   void Update() {
     valid_ = iter_->Valid();

table/two_level_iterator.cc

@@ -43,6 +43,10 @@ class TwoLevelIndexIterator : public InternalIteratorBase<IndexValue> {
     assert(Valid());
     return second_level_iter_.key();
   }
+  Slice user_key() const override {
+    assert(Valid());
+    return second_level_iter_.user_key();
+  }
   IndexValue value() const override {
     assert(Valid());
     return second_level_iter_.value();