chore: added a comment

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/server/db_slice.cc

@@ -203,22 +203,14 @@ unsigned PrimeEvictionPolicy::Evict(const PrimeTable::HotspotBuckets& eb, PrimeT
   return 1;
 }
 
-// Helper class to cache and restore fetched_items_ of DbSlice for flows that preempt
-// because some other transaction might conclude and clear the fetched_items_ with OnCbFinish()
+// Deprecated and should be removed.
 class FetchedItemsRestorer {
  public:
-  // using RestoreType = absl::flat_hash_set<CompactObjectView>;
   template <typename U> explicit FetchedItemsRestorer(U&& u) {
-    // cached_ = std::move(*dst_to_restore_);
   }
 
   ~FetchedItemsRestorer() {
-    // *dst_to_restore_ = std::move(cached_);
   }
-
- private:
-  // RestoreType cached_;
-  // RestoreType* dst_to_restore_;
 };
 
 }  // namespace

src/server/db_slice.h

@@ -605,6 +605,13 @@ class DbSlice {
   std::list<std::pair<uint64_t, ChangeCallback>> change_cb_;
 
   // Used in temporary computations in Find item and CbFinish
+  // This set is used to hold fingerprints of key accessed during the run of
+  // a transaction callback (not the whole transaction).
+  // We track them to avoid bumping them again (in any direction) so that the iterators to
+  // the fetched keys will not be invalidated. We must do it for atomic operations,
+  // for operations that preempt in the middle we have another mechanism -
+  // auto laundering iterators, so in case of preemption we do not mind that fetched_items are
+  // cleared or changed.
   mutable absl::flat_hash_set<uint64_t, FpHasher> fetched_items_;
 
   // Registered by shard indices on when first document index is created.