disable controller / view thread spawning in tests

Tests can (and do) access the database from the main thread.  In this
case they were starting a transaction, then making a request.  The
request would create a new thread, which would allocate a new database
connection.  Since the main thread started a transaction that contains
data that the new thread wants to see, the new thread would not see it
due to data visibility from transactions.  Spawning the new thread in
production is fine because middleware should not be doing database
manipulation similar to the test harness.  Before 603fe20c it was
possible to set the database connection id based on a thread local, but
603fe20c changes the connection lookup code to never look at the
"connection id" but only at the thread object itself.  Without that
indirection, we can't force threads to use the same connection pool as
another thread.

Fixes #23483

actionpack/lib/action_controller/metal/live.rb

@@ -237,9 +237,8 @@ module ActionController
       # This processes the action in a child thread. It lets us return the
       # response code and headers back up the rack stack, and still process
       # the body in parallel with sending data to the client
-      Thread.new {
+      new_controller_thread {
         t2 = Thread.current
-        t2.abort_on_exception = true
 
         # Since we're processing the view in a different thread, copy the
         # thread locals from the main thread to the child thread. :'(
@@ -270,6 +269,18 @@ module ActionController
       raise error if error
     end
 
+    # Spawn a new thread to serve up the controller in.  This is to get
+    # around the fact that Rack isn't based around IOs and we need to use
+    # a thread to stream data from the response bodies.  Nobody should call
+    # this method except in Rails internals. Seriously!
+    def new_controller_thread # :nodoc:
+      Thread.new {
+        t2 = Thread.current
+        t2.abort_on_exception = true
+        yield
+      }
+    end
+
     def log_error(exception)
       logger = ActionController::Base.logger
       return unless logger

actionpack/lib/action_controller/test_case.rb

@@ -12,6 +12,16 @@ module ActionController
     include Testing::Functional
   end
 
+  module Live
+    # Disable controller / rendering threads in tests.  User tests can access
+    # the database on the main thread, so they could open a txn, then the
+    # controller thread will open a new connection and try to access data
+    # that's only visible to the main thread's txn.  This is the problem in #23483
+    def new_controller_thread # :nodoc:
+      yield
+    end
+  end
+
   # ActionController::TestCase will be deprecated and moved to a gem in Rails 5.1.
   # Please use ActionDispatch::IntegrationTest going forward.
   class TestRequest < ActionDispatch::TestRequest #:nodoc:

actionpack/test/controller/live_stream_test.rb

@@ -152,7 +152,6 @@ module ActionController
 
       def thread_locals
         tc.assert_equal 'aaron', Thread.current[:setting]
-        tc.assert_not_equal Thread.current.object_id, Thread.current[:originating_thread]
 
         response.headers['Content-Type'] = 'text/event-stream'
         %w{ hello world }.each do |word|
@@ -261,6 +260,14 @@ module ActionController
       end
     end
 
+    def setup
+      super
+
+      def @controller.new_controller_thread
+        Thread.new { yield }
+      end
+    end
+
     def test_set_cookie
       get :set_cookie
       assert_equal({'hello' => 'world'}, @response.cookies)