Make flash messages cookie compatible with Rails 4

In #18721 we removed the discard key from the session hash used to flash
messages and that broke compatibility with Rails 4 applications because they
try to map in the discarded flash messages and it returns nil.

Fixes #24726.

2 files changed, 3 insertions, 3 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/flash.rb b/actionpack/lib/action_dispatch/middleware/flash.rb
index 06038af571..80703940ed 100644
--- a/actionpack/lib/action_dispatch/middleware/flash.rb
+++ b/actionpack/lib/action_dispatch/middleware/flash.rb
@@ -133,7 +133,7 @@ module ActionDispatch
       def to_session_value #:nodoc:
         flashes_to_keep = @flashes.except(*@discard)
         return nil if flashes_to_keep.empty?
-        {'flashes' => flashes_to_keep}
+        { 'discard' => [], 'flashes' => flashes_to_keep }
       end
 
       def initialize(flashes = {}, discard = []) #:nodoc:
diff --git a/actionpack/test/controller/flash_hash_test.rb b/actionpack/test/controller/flash_hash_test.rb
index 081288ef21..f87077dd86 100644
--- a/actionpack/test/controller/flash_hash_test.rb
+++ b/actionpack/test/controller/flash_hash_test.rb
@@ -57,10 +57,10 @@ module ActionDispatch
 
     def test_to_session_value
       @hash['foo'] = 'bar'
-      assert_equal({'flashes' => {'foo' => 'bar'}}, @hash.to_session_value)
+      assert_equal({ 'discard' => [], 'flashes' => { 'foo' => 'bar' } }, @hash.to_session_value)
 
       @hash.now['qux'] = 1
-      assert_equal({'flashes' => {'foo' => 'bar'}}, @hash.to_session_value)
+      assert_equal({ 'flashes' => { 'foo' => 'bar' }, 'discard' => [] }, @hash.to_session_value)
 
       @hash.discard('foo')
       assert_equal(nil, @hash.to_session_value)