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src/Interpreters/Cache/Metadata.cpp

@@ -355,6 +355,11 @@ public:
                 return;
             inserted = keys.insert(key).second;
         }
+        /// There is an invariant that key cannot be submitted for removal if it is already in removal queue.
+        /// Because
+        /// 1) when submit key to removal it aquires state REMOVING and we submit key for removal only if it has ACITVE state.
+        /// 2) if a key is added to cache and it was found in removal queue - it will be removed from the queue and get state ACTIVE.
+        /// and both these actions are synchronized by the same KeyGuard.
         chassert(inserted);
         if (inserted)
         {