Merge pull request #37587 from Shopify/fix-local-cache-leak

Duplicate the cached value before writing it in the local cache

activesupport/lib/active_support/cache/strategy/local_cache.rb

@@ -64,8 +64,9 @@ def read_multi_entries(keys, **options)
             values
           end
 
-          def write_entry(key, value, **options)
-            @data[key] = value
+          def write_entry(key, entry, **options)
+            entry.dup_value!
+            @data[key] = entry
             true
           end
 

activesupport/test/cache/behaviors/local_cache_behavior.rb

@@ -150,6 +150,25 @@ def test_local_cache_of_read_multi
     end
   end
 
+  def test_initial_object_mutation_after_write
+    @cache.with_local_cache do
+      initial = +"bar"
+      @cache.write("foo", initial)
+      initial << "baz"
+      assert_equal "bar", @cache.read("foo")
+    end
+  end
+
+  def test_initial_object_mutation_after_fetch
+    @cache.with_local_cache do
+      initial = +"bar"
+      @cache.fetch("foo") { initial }
+      initial << "baz"
+      assert_equal "bar", @cache.read("foo")
+      assert_equal "bar", @cache.fetch("foo")
+    end
+  end
+
   def test_middleware
     app = lambda { |env|
       result = @cache.write("foo", "bar")