Allow consistent permitting classes (#795)

By default, Bridgetown expects to be able to load its special Rb class
for YAML front matter. The original implementation made it hard to have
extra permitted classes in front matter while retaining that
expectation.

This change makes it so the default permitted classes are merged into
any passed permitted classes across both string and file loading.

bridgetown-core/lib/bridgetown-core/yaml_parser.rb

@@ -6,12 +6,23 @@ class YAMLParser
 
     class << self
       def load_file(filename, **kwargs)
-        kwargs = { permitted_classes: PERMITTED_CLASSES }.merge(kwargs)
-        YAML.safe_load_file(filename, **kwargs)
+        YAML.safe_load_file filename, **merge_permitted_classes(kwargs)
       end
 
-      def load(yaml)
-        YAML.safe_load yaml, permitted_classes: PERMITTED_CLASSES
+      def load(yaml, **kwargs)
+        YAML.safe_load yaml, **merge_permitted_classes(kwargs)
+      end
+
+      private
+
+      def merge_permitted_classes(kwargs)
+        if kwargs.key?(:permitted_classes)
+          kwargs[:permitted_classes] |= PERMITTED_CLASSES
+        else
+          kwargs[:permitted_classes] = PERMITTED_CLASSES
+        end
+
+        kwargs
       end
     end
   end

bridgetown-core/test/fixtures/extra_yaml.yml

@@ -0,0 +1,6 @@
+---
+date: 2020-05-14
+time: 2019-07-14 19:22:00 +0100
+rb: !ruby/string:Rb |
+  21 * 2
+symbol: :symbol

bridgetown-core/test/test_yaml_parser.rb

@@ -34,6 +34,21 @@ def initialize
       assert_equal Rb, parsed_yaml["rb"].class
     end
 
+    should "allows passing other permitted classes" do
+      parsed_yaml = Bridgetown::YAMLParser.load(<<~YAML, permitted_classes: [Symbol])
+        date: 2020-05-14
+        time: 2019-07-14 19:22:00 +0100
+        rb: !ruby/string:Rb |
+          21 * 2
+        symbol: :symbol
+      YAML
+
+      assert_equal Date, parsed_yaml["date"].class
+      assert_equal Time, parsed_yaml["time"].class
+      assert_equal Rb, parsed_yaml["rb"].class
+      assert_equal :symbol, parsed_yaml["symbol"]
+    end
+
     should "error when trying to parse types not on the allowlist" do
       assert_raises(Psych::DisallowedClass) do
         Bridgetown::YAMLParser.load(CustomYAMLSerializable.new.to_yaml)
@@ -51,6 +66,16 @@ def initialize
       assert_equal Rb, parsed_yaml["rb"].class
     end
 
+    should "allows passing other permitted classes" do
+      yaml_file = test_dir("fixtures", "extra_yaml.yml")
+      parsed_yaml = Bridgetown::YAMLParser.load_file(yaml_file, permitted_classes: [Symbol])
+
+      assert_equal Date, parsed_yaml["date"].class
+      assert_equal Time, parsed_yaml["time"].class
+      assert_equal Rb, parsed_yaml["rb"].class
+      assert_equal :symbol, parsed_yaml["symbol"]
+    end
+
     should "error when trying to parse types not on the allowlist" do
       yaml_file = test_dir("fixtures", "disallowed_yaml.yml")