Fix `ActionDispatch::IntegrationTest#open_session`

Reset a new session directly after its creation in
`ActionDispatch::IntegrationTest#open_session`. Reset the session to a clean
state before making it available to the client's test code.

Issue #22742 reports unexpected behavior of integration tests that run multiple
sessions. For example an `ActionDispatch::Flash` instance is shared across
multiple sessions, though a client code will rightfully assume that each new
session has its own flash hash.

The following test failed due to this behavior:

    class Issue22742Test < ActionDispatch::IntegrationTest
      test 'issue #22742' do
        integration_session # initialize first session
        a = open_session
        b = open_session

        refute_same(a.integration_session, b.integration_session)
      end
    end

Instead of creating a new `ActionDispatch::Integration::Session` instance,
the same instance is shared across all newly opened test sessions. This is
due to the way how new test sessions are created in
`ActionDispatch::IntegrationTest#open_session`. The already existing
`ActionDispatch::IntegrationTest` instance is duplicated  with `Object#dup`,
This approach was introduced in commit 15c31c7639. `Object#dup` copies the
instance variables, but not the objects they reference. Therefore this issue
only occurred when the current test instance had been tapped in such a way that
the instance variable `@integration_session` was initialized before creating the
new test session.

Close #22742

[Tawan Sierek + Sina Sadeghian]

actionpack/lib/action_dispatch/testing/integration.rb

@@ -368,6 +368,7 @@ module ActionDispatch
       # simultaneously.
       def open_session
         dup.tap do |session|
+          session.reset!
           yield session if block_given?
         end
       end

actionpack/test/controller/integration_test.rb

@@ -356,6 +356,14 @@ class IntegrationProcessTest < ActionDispatch::IntegrationTest
     end
   end
 
+  test "creation of multiple integration sessions" do
+    integration_session # initialize first session
+    a = open_session
+    b = open_session
+
+    refute_same(a.integration_session, b.integration_session)
+  end
+
   def test_get_with_query_string
     with_test_route_set do
       get "/get_with_params?foo=bar"

railties/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Reset a new session directly after its creation in ActionDispatch::IntegrationTest#open_session
+
+    Fixes Issue #22742
+
+    *Tawan Sierek*
+
 *   Add `:skip_sprockets` to `Rails::PluginBuilder::PASSTHROUGH_OPTIONS`
 
     *Tsukuru Tanimichi*