fix: pass comment nodes to the scrubber

Some scrubbers want to allow comments through, but in v1.4.0 didn't
get the chance because only elements were passed through to
`keep_node?`.

This change allows comments and elements through, but still omits
other non-elements like processing instructions (see #115).

CHANGELOG.md

@@ -1,3 +1,15 @@
+## next / unreleased
+
+* Fix regression in v1.4.0 that did not pass comment nodes to the scrubber.
+
+  Some scrubbers will want to override the default behavior and allow comments, but v1.4.0 only
+  passed through elements to the scrubber's `keep_node?` method.
+
+  This change once again allows the scrubber to make the decision on comment nodes, but still skips
+  other non-elements like processing instructions (see #115).
+
+  *Mike Dalessio*
+
 ## 1.4.0 / 2021-08-18
 
 * Processing Instructions are no longer allowed by Rails::Html::PermitScrubber

lib/rails/html/scrubbers.rb

@@ -68,7 +68,7 @@ def scrub(node)
         end
         return CONTINUE if skip_node?(node)
 
-        unless node.element? && keep_node?(node)
+        unless (node.comment? || node.element?) && keep_node?(node)
           return STOP if scrub_node(node) == STOP
         end
 

test/scrubbers_test.rb

@@ -112,6 +112,50 @@ def test_attributes_accessor_validation
   end
 end
 
+class PermitScrubberSubclassTest < ScrubberTest
+  def setup
+    @scrubber = Class.new(::Rails::Html::PermitScrubber) do
+      attr :nodes_seen
+
+      def initialize
+        super()
+        @nodes_seen = []
+      end
+
+      def keep_node?(node)
+        @nodes_seen << node.name
+        super(node)
+      end
+    end.new
+  end
+
+  def test_elements_are_checked
+    html = %Q("<div></div><a></a><tr></tr>")
+    Loofah.scrub_fragment(html, @scrubber)
+    assert_includes(@scrubber.nodes_seen, "div")
+    assert_includes(@scrubber.nodes_seen, "a")
+    assert_includes(@scrubber.nodes_seen, "tr")
+  end
+
+  def test_comments_are_checked
+    # this passes in v1.3.0 but fails in v1.4.0
+    html = %Q("<div></div><!-- ohai --><tr></tr>")
+    Loofah.scrub_fragment(html, @scrubber)
+    assert_includes(@scrubber.nodes_seen, "div")
+    assert_includes(@scrubber.nodes_seen, "comment")
+    assert_includes(@scrubber.nodes_seen, "tr")
+  end
+
+  def test_craftily_named_processing_instructions_are_not_checked
+    # this fails in v1.3.0 but passes in v1.4.0
+    html = %Q("<div></div><?a content><tr></tr>")
+    Loofah.scrub_fragment(html, @scrubber)
+    assert_includes(@scrubber.nodes_seen, "div")
+    refute_includes(@scrubber.nodes_seen, "a")
+    assert_includes(@scrubber.nodes_seen, "tr")
+  end
+end
+
 class TargetScrubberTest < ScrubberTest
   def setup
     @scrubber = Rails::Html::TargetScrubber.new