Ensure that instances of ActiveModel::Errors can be marshalled

We now use default procs inside of the errors object, which gets
included by default when marshaling anything that includes
`ActiveModel::Validations`. This means that Active Record objects cannot
be marshalled. We strip and apply the default proc ourselves. This will
ensure the objects are YAML serializable as well, since YAML falls back
to marshal implementations now. This is less important, however, as the
errors aren't included when dumping Active Record objects.

This commit does not include a changelog entry, as 5.0 is still in RC
status at the time of writing, and 5.0.0 will not release with the bug
this fixes.

Fixes #25165

activemodel/lib/active_model/errors.rb

@@ -71,8 +71,8 @@ module ActiveModel
     #   end
     def initialize(base)
       @base     = base
-      @messages = Hash.new { |messages, attribute| messages[attribute] = [] }
-      @details  = Hash.new { |details, attribute| details[attribute] = [] }
+      @messages = apply_default_array({})
+      @details = apply_default_array({})
     end
 
     def initialize_dup(other) # :nodoc:
@@ -493,6 +493,16 @@ module ActiveModel
       I18n.translate(key, options)
     end
 
+    def marshal_dump
+      [@base, without_default_proc(@messages), without_default_proc(@details)]
+    end
+
+    def marshal_load(array)
+      @base, @messages, @details = array
+      apply_default_array(@messages)
+      apply_default_array(@details)
+    end
+
   private
     def normalize_message(attribute, message, options)
       case message
@@ -506,6 +516,17 @@ module ActiveModel
     def normalize_detail(message, options)
       { error: message }.merge(options.except(*CALLBACKS_OPTIONS + MESSAGE_OPTIONS))
     end
+
+    def without_default_proc(hash)
+      hash.dup.tap do |new_h|
+        new_h.default_proc = nil
+      end
+    end
+
+    def apply_default_array(hash)
+      hash.default_proc = proc { |h, key| h[key] = [] }
+      hash
+    end
   end
 
   # Raised when a validation cannot be corrected by end users and are considered

activemodel/test/cases/errors_test.rb

@@ -427,4 +427,13 @@ class ErrorsTest < ActiveModel::TestCase
     assert_equal [:name], person.errors.messages.keys
     assert_equal [:name], person.errors.details.keys
   end
+
+  test "errors are marshalable" do
+    errors = ActiveModel::Errors.new(Person.new)
+    errors.add(:name, :invalid)
+    serialized = Marshal.load(Marshal.dump(errors))
+
+    assert_equal errors.messages, serialized.messages
+    assert_equal errors.details, serialized.details
+  end
 end