Introduce `ActionView::Template::Handlers::ERB.escape_whitelist`.

This is a list of mime types where template text is not html escaped
by default. It prevents `Jack & Joe` from rendering as
`Jack & Joe` for the whitelisted mime types. The default whitelist
contains text/plain.

This follows a whitelist approach where plain text templates are
not escaped, and all the others (json, xml) are. The mime type is
assumed to be set by the abstract controller.

actionpack/CHANGELOG.md

@@ -43,6 +43,13 @@
 
     *Josh Peek*
 
+*   Introduce `ActionView::Template::Handlers::ERB.escape_whitelist`. This is a list
+    of mime types where template text is not html escaped by default. It prevents `Jack & Joe`
+    from rendering as `Jack & Joe` for the whitelisted mime types. The default whitelist
+    contains text/plain. Fix #7976
+
+    *Joost Baaij*
+
 *   `assert_template` can be used to assert on the same template with different locals
     Fix #3675
 

actionpack/lib/action_view/template/handlers/erb.rb

@@ -47,6 +47,10 @@ class ERB
         class_attribute :erb_implementation
         self.erb_implementation = Erubis
 
+        # Do not escape templates of these mime types.
+        class_attribute :escape_whitelist
+        self.escape_whitelist = ["text/plain"]
+
         ENCODING_TAG = Regexp.new("\\A(<%#{ENCODING_FLAG}-?%>)[ \\t]*")
 
         def self.call(template)
@@ -78,6 +82,7 @@ def call(template)
 
           self.class.erb_implementation.new(
             erb,
+            :escape => (self.class.escape_whitelist.include? template.type),
             :trim => (self.class.erb_trim_mode == "-")
           ).src
         end

actionpack/test/template/template_test.rb

@@ -26,6 +26,10 @@ def hello
       "Hello"
     end
 
+    def apostrophe
+      "l'apostrophe"
+    end
+
     def partial
       ActionView::Template.new(
         "<%= @virtual_path %>",
@@ -48,7 +52,7 @@ def my_buffer
     end
   end
 
-  def new_template(body = "<%= hello %>", details = {})
+  def new_template(body = "<%= hello %>", details = {format: html})
     ActionView::Template.new(body, "hello template", details.fetch(:handler) { ERBHandler }, {:virtual_path => "hello"}.merge!(details))
   end
 
@@ -72,6 +76,16 @@ def test_basic_template
     assert_equal "Hello", render
   end
 
+  def test_basic_template_does_html_escape
+    @template = new_template("<%= apostrophe %>")
+    assert_equal "l&#39;apostrophe", render
+  end
+
+  def test_text_template_does_not_html_escape
+    @template = new_template("<%= apostrophe %>", format: text)
+    assert_equal "l'apostrophe", render
+  end
+
   def test_raw_template
     @template = new_template("<%= hello %>", :handler => ActionView::Template::Handlers::Raw.new)
     assert_equal "<%= hello %>", render