3 files changed, 20 insertions, 3 deletions
diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index 646d84f304..db9dc09e35 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Inject `Rack::Lock` if `config.eager_load` is false.
+
+    Fixes #15089.
+
+    *Xavier Noria*
+
 *   Change the path of dummy app location in plugin's test_helper.rb for cases
     you specify dummy_path option.
 
diff --git a/railties/lib/rails/application/default_middleware_stack.rb b/railties/lib/rails/application/default_middleware_stack.rb
index a00afe008c..d1789192ef 100644
--- a/railties/lib/rails/application/default_middleware_stack.rb
+++ b/railties/lib/rails/application/default_middleware_stack.rb
@@ -66,7 +66,11 @@ module Rails
         end
 
         def allow_concurrency?
-          config.allow_concurrency.nil? ? config.cache_classes : config.allow_concurrency
+          if config.allow_concurrency.nil?
+            config.cache_classes && config.eager_load
+          else
+            config.allow_concurrency
+          end
         end
 
         def load_rack_cache
diff --git a/railties/test/application/middleware_test.rb b/railties/test/application/middleware_test.rb
index a905598d80..caef39d16f 100644
--- a/railties/test/application/middleware_test.rb
+++ b/railties/test/application/middleware_test.rb
@@ -94,13 +94,20 @@ module ApplicationTests
       assert !middleware.include?("ActiveRecord::Migration::CheckPending")
     end
 
-    test "removes lock if cache classes is set" do
+    test "includes lock if cache_classes is set but eager_load is not" do
       add_to_config "config.cache_classes = true"
       boot!
+      assert middleware.include?("Rack::Lock")
+    end
+
+    test "does not include lock if cache_classes is set and so is eager_load" do
+      add_to_config "config.cache_classes = true"
+      add_to_config "config.eager_load = true"
+      boot!
       assert !middleware.include?("Rack::Lock")
     end
 
-    test "removes lock if allow concurrency is set" do
+    test "does not include lock if allow_concurrency is set" do
       add_to_config "config.allow_concurrency = true"
       boot!
       assert !middleware.include?("Rack::Lock")