Freeze strings to reduce allocations in integration tests

Moves `X-Request-ID`, `action_dispatch.request_id` and
`HTTP_X_REQUEST_ID` strings to constants and freezes them.

We are freezing these strings to reduce the number of allocations in
Rails integration tests. The tests are spending a lot of time in GC and
this reduces the amount of time spent from 12% to 9% (in combination
with Rack PR that also freezes some strings).

Number of allocations before this change: 1030722
Number of allocations after this change: 967722

1 files changed, 8 insertions, 4 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/request_id.rb b/actionpack/lib/action_dispatch/middleware/request_id.rb
index 25658bac3d..cf31815c23 100644
--- a/actionpack/lib/action_dispatch/middleware/request_id.rb
+++ b/actionpack/lib/action_dispatch/middleware/request_id.rb
@@ -12,19 +12,23 @@ module ActionDispatch
   # The unique request id can be used to trace a request end-to-end and would typically end up being part of log files
   # from multiple pieces of the stack.
   class RequestId
+    X_REQUEST_ID = "X-Request-Id".freeze # :nodoc:
+    ACTION_DISPATCH_REQUEST_ID = "action_dispatch.request_id".freeze # :nodoc:
+    HTTP_X_REQUEST_ID = "HTTP_X_REQUEST_ID".freeze # :nodoc:
+
     def initialize(app)
       @app = app
     end
 
     def call(env)
-      env["action_dispatch.request_id"] = external_request_id(env) || internal_request_id
-      @app.call(env).tap { |_status, headers, _body| headers["X-Request-Id"] = env["action_dispatch.request_id"] }
+      env[ACTION_DISPATCH_REQUEST_ID] = external_request_id(env) || internal_request_id
+      @app.call(env).tap { |_status, headers, _body| headers[X_REQUEST_ID] = env[ACTION_DISPATCH_REQUEST_ID] }
     end
 
     private
       def external_request_id(env)
-        if request_id = env["HTTP_X_REQUEST_ID"].presence
-          request_id.gsub(/[^\w\-]/, "").first(255)
+        if request_id = env[HTTP_X_REQUEST_ID].presence
+          request_id.gsub(/[^\w\-]/, "".freeze).first(255)
         end
       end