Don't expect defined protect_against_forgery? in {token,csrf_meta}_tag

The `#csrf_meta_tags` and `#token_tag` Action View helper methods are
expecting the class in which are included to explicitly define the
method `#protect_against_forgery?` or else they will fail with
`NoMethodError`.

This is a problem if you want to use Action View outside of Rails
applications. For example, in #34788 I used the `#button_to` helper
inside of the error pages templates that have a custom
`ActionView::Base` subclass, which did not defined
`#protect_against_forgery?` and trying to call the button failed.

I had to dig inside of Action View to find-out what's was going on. I
think we should either set a default method implementation in the
helpers or check for the method definition, but don't explicitly require
the presence of `#protect_against_forgery?` in every `ActionViews::Base`
subclass as the errors are hard to figure out.

2 files changed, 56 insertions, 0 deletions

diff --git a/actionview/test/template/csrf_helper_test.rb b/actionview/test/template/csrf_helper_test.rb
new file mode 100644
index 0000000000..dd9821eb6c
--- /dev/null
+++ b/actionview/test/template/csrf_helper_test.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+class CsrfHelperTest < ActiveSupport::TestCase
+  cattr_accessor :request_forgery, default: false
+
+  include ActionView::Helpers::CsrfHelper
+  include ActionView::Helpers::TagHelper
+  include Rails::Dom::Testing::Assertions::DomAssertions
+
+  def test_csrf_meta_tags_without_request_forgery_protection
+    assert_dom_equal "", csrf_meta_tags
+  end
+
+  def test_csrf_meta_tags_with_request_forgery_protection
+    self.request_forgery = true
+
+    assert_dom_equal <<~DOM.chomp, csrf_meta_tags
+      <meta name="csrf-param" content="form_token" />
+      <meta name="csrf-token" content="secret" />
+    DOM
+  ensure
+    self.request_forgery = false
+  end
+
+  def test_csrf_meta_tags_without_protect_against_forgery_method
+    self.class.undef_method(:protect_against_forgery?)
+
+    assert_dom_equal "", csrf_meta_tags
+  ensure
+    self.class.define_method(:protect_against_forgery?) { request_forgery }
+  end
+
+  def protect_against_forgery?
+    request_forgery
+  end
+
+  def form_authenticity_token(*args)
+    "secret"
+  end
+
+  def request_forgery_protection_token
+    "form_token"
+  end
+end
diff --git a/actionview/test/template/url_helper_test.rb b/actionview/test/template/url_helper_test.rb
index 1ab28e4749..632b32f09f 100644
--- a/actionview/test/template/url_helper_test.rb
+++ b/actionview/test/template/url_helper_test.rb
@@ -119,6 +119,16 @@ class UrlHelperTest < ActiveSupport::TestCase
     )
   end
 
+  def test_button_to_without_protect_against_forgery_method
+    self.class.undef_method(:protect_against_forgery?)
+    assert_dom_equal(
+      %{<form method="post" action="http://www.example.com" class="button_to"><input type="submit" value="Hello" /></form>},
+      button_to("Hello", "http://www.example.com")
+    )
+  ensure
+    self.class.define_method(:protect_against_forgery?) { request_forgery }
+  end
+
   def test_button_to_with_straight_url
     assert_dom_equal %{<form method="post" action="http://www.example.com" class="button_to"><input type="submit" value="Hello" /></form>}, button_to("Hello", "http://www.example.com")
   end