Test that query cache works correctly on multiple handlers when forking processes

In a forking process web server like Unicorn, connections are
reconnected after fork. This test ensures that when connects are
reconnected after fork when using multiple databases, the query cache
will be on and work correctly.

This test fails on Rails 6.0 for now.

activerecord/test/cases/query_cache_test.rb

@@ -76,6 +76,67 @@ def test_query_cache_is_applied_to_connections_in_all_handlers
     ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
   end
 
+  def test_query_cache_with_multiple_handlers_and_forked_processes
+    ActiveRecord::Base.connection_handlers = {
+      writing: ActiveRecord::Base.default_connection_handler,
+      reading: ActiveRecord::ConnectionAdapters::ConnectionHandler.new
+    }
+
+    ActiveRecord::Base.connected_to(role: :reading) do
+      ActiveRecord::Base.establish_connection(ActiveRecord::Base.configurations["arunit"])
+    end
+
+    rd, wr = IO.pipe
+    rd.binmode
+    wr.binmode
+
+    pid = fork {
+      rd.close
+      status = 0
+
+      middleware { |env|
+        begin
+          assert_cache :clean
+
+          # first request dirties cache
+          ActiveRecord::Base.connected_to(role: :reading) do
+            Post.first
+            assert_cache :dirty
+          end
+
+          # should clear the cache
+          Post.create!(title: "a new post", body: "and a body")
+
+          # fails because cache is still dirty
+          ActiveRecord::Base.connected_to(role: :reading) do
+            assert_cache :clean
+            Post.first
+          end
+
+        rescue Minitest::Assertion => e
+          wr.write Marshal.dump e
+          status = 1
+        end
+      }.call({})
+
+      wr.close
+      exit!(status)
+    }
+
+    wr.close
+
+    Process.waitpid pid
+    if !$?.success?
+      raise Marshal.load(rd.read)
+    else
+      assert_predicate $?, :success?
+    end
+
+    rd.close
+  ensure
+    ActiveRecord::Base.connection_handlers = { writing: ActiveRecord::Base.default_connection_handler }
+  end
+
   def test_query_cache_across_threads
     with_temporary_connection_pool do
       if in_memory_db?