Prevent double-escaping of rich text blocks on Jinja2

Fixes #2542. Thanks to @aaugustin for the bug report and patch.

wagtail/tests/testapp/jinja2_templates/tests/jinja2/rich_text.html

@@ -0,0 +1 @@
+{{ value }}

wagtail/wagtailcore/rich_text.py

@@ -193,8 +193,11 @@ class RichText(object):
     def __init__(self, source):
         self.source = (source or '')
 
+    def __html__(self):
+        return '<div class="rich-text">' + expand_db_html(self.source) + '</div>'
+
     def __str__(self):
-        return mark_safe('<div class="rich-text">' + expand_db_html(self.source) + '</div>')
+        return mark_safe(self.__html__())
 
     def __bool__(self):
         return bool(self.source)

wagtail/wagtailcore/tests/test_jinja2.py

@@ -52,6 +52,8 @@ class TestCoreJinja(TestCase):
 
 
 class TestJinjaEscaping(TestCase):
+    fixtures = ['test.json']
+
     def test_block_render_result_is_safe(self):
         """
         Ensure that any results of template rendering in block.render are marked safe
@@ -70,3 +72,21 @@ class TestJinjaEscaping(TestCase):
         })
 
         self.assertIn('<p>hello world</p>', result)
+
+    def test_rich_text_is_safe(self):
+        """
+        Ensure that RichText values are marked safe
+        so that they don't get double-escaped when inserted into a parent template (#2542)
+        """
+        stream_block = blocks.StreamBlock([
+            ('paragraph', blocks.RichTextBlock(template='tests/jinja2/rich_text.html'))
+        ])
+        stream_value = stream_block.to_python([
+            {'type': 'paragraph', 'value': '<p>Merry <a linktype="page" id="4">Christmas</a>!</p>'},
+        ])
+
+        result = render_to_string('tests/jinja2/stream.html', {
+            'value': stream_value,
+        })
+
+        self.assertIn('<div class="rich-text"><p>Merry <a href="/events/christmas/">Christmas</a>!</p></div>', result)