Merge pull request #42833 from Shopify/fix-memcached-6.1-local-store

MemCacheStore: Properly duplicate local cache values in 6.1 mode

activesupport/lib/active_support/cache/mem_cache_store.rb

@@ -32,6 +32,43 @@ module ActiveSupport
 
       prepend Strategy::LocalCache
 
+      module DupLocalCache
+        class LocalStore < Strategy::LocalCache::LocalStore
+          def write_entry(_key, entry)
+            if entry.is_a?(Entry)
+              entry.dup_value!
+            end
+            super
+          end
+
+          def fetch_entry(key)
+            entry = @data.fetch(key) do
+              new_entry = yield
+              if entry.is_a?(Entry)
+                new_entry.dup_value!
+              end
+              @data[key] = new_entry
+            end
+            entry = entry.dup
+
+            if entry.is_a?(Entry)
+              entry.dup_value!
+            end
+
+            entry
+          end
+        end
+
+        def with_local_cache
+          if ActiveSupport::Cache.format_version == 6.1
+            use_temporary_local_cache(LocalStore.new) { yield }
+          else
+            super
+          end
+        end
+      end
+      prepend DupLocalCache
+
       ESCAPE_KEY_CHARS = /[\x00-\x20%\x7F-\xFF]/n
 
       # Creates a new Dalli::Client instance with specified addresses and options.

activesupport/test/cache/behaviors/local_cache_behavior.rb

@@ -7,15 +7,15 @@ module LocalCacheBehavior
     end
     assert_equal @cache.class.name, events[0].payload[:store]
 
-    events = with_instrumentation "read" do
-      @cache.with_local_cache do
+    @cache.with_local_cache do
+      events = with_instrumentation "read" do
         @cache.read("foo")
         @cache.read("foo")
       end
-    end
 
-    expected = [@cache.class.name, "ActiveSupport::Cache::Strategy::LocalCache::LocalStore"]
-    assert_equal expected, events.map { |p| p.payload[:store] }
+      expected = [@cache.class.name, @cache.send(:local_cache).class.name]
+      assert_equal expected, events.map { |p| p.payload[:store] }
+    end
   end
 
   def test_local_writes_are_persistent_on_the_remote_cache

activesupport/test/cache/stores/mem_cache_store_test.rb

@@ -268,30 +268,6 @@ class MemCacheStoreTest < ActiveSupport::TestCase
     end
   end
 
-  def test_initial_object_mutation_after_fetch
-    if ActiveSupport::Cache.format_version == 6.1
-      skip "Local cache mutation can't be prevented on legacy MemCacheStore"
-    else
-      super
-    end
-  end
-
-  def test_initial_object_mutation_after_write
-    if ActiveSupport::Cache.format_version == 6.1
-      skip "Local cache mutation can't be prevented on legacy MemCacheStore"
-    else
-      super
-    end
-  end
-
-  def test_local_cache_of_read_returns_a_copy_of_the_entry
-    if ActiveSupport::Cache.format_version == 6.1
-      skip "Local cache mutation can't be prevented on legacy MemCacheStore"
-    else
-      super
-    end
-  end
-
   private
     def random_string(length)
       (0...length).map { (65 + rand(26)).chr }.join