Add Rack::Lint to ActionDispatch::ServerTiming tests

To ensure Rails is and remains compliant with [the Rack 3
spec](https://github.com/rack/rack/blob/6d16306192349e665e4ec820a9bfcc0967009b6a/UPGRADE-GUIDE.md)
we can add `Rack::Lint` to the Rails middleware tests.

This adds additional test coverage to `ActionDispatch::ServerTiming` to
validate that its input and output follow the Rack SPEC.

The `Server-Timing` header definition was moved to
`ActionDispatch::Constants` and is now downcased to match the Rack 3
SPEC.

The tests that rely on a `Concurrent::CyclicBarrier` ("events are
tracked by thread") were changed since passing the required proc in the
env is not compatible with the SPEC:

```
Rack::Lint::LintError: env variable proc has non-string value
```

The same can be achieved by invoking the proc as a child Rack app.

actionpack/lib/action_dispatch/constants.rb

@@ -11,12 +11,14 @@ module Constants
       LOCATION = "Location"
       FEATURE_POLICY = "Feature-Policy"
       X_REQUEST_ID = "X-Request-Id"
+      SERVER_TIMING = "Server-Timing"
     else
       VARY = "vary"
       CONTENT_ENCODING = "content-encoding"
       LOCATION = "location"
       FEATURE_POLICY = "feature-policy"
       X_REQUEST_ID = "x-request-id"
+      SERVER_TIMING = "server-timing"
     end
   end
 end

actionpack/lib/action_dispatch/middleware/server_timing.rb

@@ -4,8 +4,6 @@
 
 module ActionDispatch
   class ServerTiming
-    SERVER_TIMING_HEADER = "Server-Timing"
-
     class Subscriber # :nodoc:
       include Singleton
       KEY = :action_dispatch_server_timing_events
@@ -67,8 +65,10 @@ def call(env)
         "%s;dur=%.2f" % [event_name, events_collection.sum(&:duration)]
       end
 
-      header_info.prepend(headers[SERVER_TIMING_HEADER]) if headers[SERVER_TIMING_HEADER].present?
-      headers[SERVER_TIMING_HEADER] = header_info.join(", ")
+      if headers[ActionDispatch::Constants::SERVER_TIMING].present?
+        header_info.prepend(headers[ActionDispatch::Constants::SERVER_TIMING])
+      end
+      headers[ActionDispatch::Constants::SERVER_TIMING] = header_info.join(", ")
 
       response
     end

actionpack/test/dispatch/server_timing_test.rb

@@ -23,7 +23,8 @@ def create
   end
 
   setup do
-    @middlewares = [ActionDispatch::ServerTiming]
+    @middlewares = [Rack::Lint, ActionDispatch::ServerTiming, Rack::Lint]
+    @header_name = ActionDispatch::Constants::SERVER_TIMING
   end
 
   teardown do
@@ -36,49 +37,54 @@ def create
   test "server timing header is included in the response" do
     with_test_route_set do
       get "/"
-      assert_match(/\w+/, @response.headers["Server-Timing"])
+      assert_match(/\w+/, @response.headers[@header_name])
     end
   end
 
   test "includes default action controller events duration" do
     with_test_route_set do
       get "/"
-      assert_match(/start_processing.action_controller;dur=\w+/, @response.headers["Server-Timing"])
-      assert_match(/process_action.action_controller;dur=\w+/, @response.headers["Server-Timing"])
+      assert_match(/start_processing.action_controller;dur=\w+/, @response.headers[@header_name])
+      assert_match(/process_action.action_controller;dur=\w+/, @response.headers[@header_name])
     end
   end
 
   test "includes custom active support events duration" do
     with_test_route_set do
       get "/id"
-      assert_match(/custom.event;dur=\w+/, @response.headers["Server-Timing"])
+      assert_match(/custom.event;dur=\w+/, @response.headers[@header_name])
     end
   end
 
   test "events are tracked by thread" do
     barrier = Concurrent::CyclicBarrier.new(2)
 
     stub_app = -> (env) {
-      env["proc"].call
+      env["action_dispatch.test"].call
       [200, {}, "ok"]
     }
-    app = ActionDispatch::ServerTiming.new(stub_app)
+    app = Rack::Lint.new(
+      ActionDispatch::ServerTiming.new(Rack::Lint.new(stub_app))
+    )
 
     t1 = Thread.new do
-      app.call({ "proc" => -> {
+      proc = -> {
         barrier.wait
         barrier.wait
-      } })
+      }
+      env = Rack::MockRequest.env_for("", { "action_dispatch.test" => proc })
+      app.call(env)
     end
 
     t2 = Thread.new do
       barrier.wait
-
-      response = app.call({ "proc" => -> {
+      proc = -> {
         ActiveSupport::Notifications.instrument("custom.event") do
           true
         end
-      } })
+      }
+      env = Rack::MockRequest.env_for("", { "action_dispatch.test" => proc })
+      response = app.call(env)
 
       barrier.wait
 
@@ -88,8 +94,8 @@ def create
     headers1 = t1.value[1]
     headers2 = t2.value[1]
 
-    assert_match(/custom.event;dur=\w+/, headers2["Server-Timing"])
-    assert_no_match(/custom.event;dur=\w+/, headers1["Server-Timing"])
+    assert_match(/custom.event;dur=\w+/, headers2[@header_name])
+    assert_no_match(/custom.event;dur=\w+/, headers1[@header_name])
   end
 
   test "does not overwrite existing header values" do
@@ -100,15 +106,16 @@ def initialize(app)
 
       def call(env)
         status, headers, body = @app.call(env)
-        headers["Server-Timing"] = [headers["Server-Timing"], %(entry;desc="description")].compact.join(", ")
+        header_name = ActionDispatch::Constants::SERVER_TIMING
+        headers[header_name] = [headers[header_name], %(entry;desc="description")].compact.join(", ")
         [ status, headers, body ]
       end
     end
 
     with_test_route_set do
       get "/"
-      assert_match(/entry;desc="description"/, @response.headers["Server-Timing"])
-      assert_match(/start_processing.action_controller;dur=\w+/, @response.headers["Server-Timing"])
+      assert_match(/entry;desc="description"/, @response.headers[@header_name])
+      assert_match(/start_processing.action_controller;dur=\w+/, @response.headers[@header_name])
     end
   end