From aa7136c27649fed0b5505faa382155050b327b7c Mon Sep 17 00:00:00 2001
From: kssenii <sumarokovakseniia@mail.ru>
Date: Fri, 11 Aug 2023 18:05:03 +0200
Subject: [PATCH] Add a comment

---
 src/Interpreters/Cache/Metadata.cpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/Interpreters/Cache/Metadata.cpp b/src/Interpreters/Cache/Metadata.cpp
index 194081e7926..1b0e02e87ce 100644
--- a/src/Interpreters/Cache/Metadata.cpp
+++ b/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)
         {