bugdown: Convert GitHub image-preview urls to image urls.

This makes inline image previews work for links to image files' pages
on GitHub.

Fixes zulip#4658.

zerver/lib/bugdown/__init__.py

@@ -359,6 +359,21 @@ def __init__(self, md, bugdown):
         self.bugdown = bugdown
         markdown.treeprocessors.Treeprocessor.__init__(self, md)
 
+    def get_actual_image_url(self, url):
+        # type: (Text) -> Text
+        # Add specific per-site cases to convert image-preview urls to image urls.
+        # See https://github.com/zulip/zulip/issues/4658 for more information
+        parsed_url = urllib.parse.urlparse(url)
+        if (parsed_url.netloc == 'github.com' or parsed_url.netloc.endswith('.github.com')):
+            # https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png ->
+            # https://raw.githubusercontent.com/zulip/zulip/master/static/images/logo/zulip-icon-128x128.png
+            split_path = parsed_url.path.split('/')
+            if len(split_path) > 3 and split_path[3] == "blob":
+                return urllib.parse.urljoin('https://raw.githubusercontent.com',
+                                            '/'.join(split_path[0:3] + split_path[4:]))
+
+        return url
+
     def is_image(self, url):
         # type: (Text) -> bool
         if not image_preview_enabled_for_realm():
@@ -628,7 +643,7 @@ def run(self, root):
                       class_attr=class_attr)
                 continue
             if self.is_image(url):
-                add_a(root, url, url, title=text)
+                add_a(root, self.get_actual_image_url(url), url, title=text)
                 continue
             if get_tweet_id(url) is not None:
                 if rendered_tweet_count >= self.TWITTER_MAX_TO_PREVIEW:

zerver/tests/test_bugdown.py

@@ -349,6 +349,19 @@ def test_inline_dropbox_bad(self):
             converted = bugdown_convert(msg)
         self.assertEqual(converted, '<p><a href="https://zulip-test.dropbox.com/photos/cl/ROmr9K1XYtmpneM" target="_blank" title="https://zulip-test.dropbox.com/photos/cl/ROmr9K1XYtmpneM">https://zulip-test.dropbox.com/photos/cl/ROmr9K1XYtmpneM</a></p>')
 
+    def test_inline_github_preview(self):
+        # type: () -> None
+        # Test photo album previews
+        msg = 'https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png'
+        converted = bugdown_convert(msg)
+
+        self.assertEqual(converted, '<p><a href="https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png" target="_blank" title="https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png">https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png</a></p>\n<div class="message_inline_image"><a href="https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png" target="_blank" title="https://github.com/zulip/zulip/blob/master/static/images/logo/zulip-icon-128x128.png"><img src="https://raw.githubusercontent.com/zulip/zulip/master/static/images/logo/zulip-icon-128x128.png"></a></div>')
+
+        msg = 'https://developer.github.com/assets/images/hero-circuit-bg.png'
+        converted = bugdown_convert(msg)
+
+        self.assertEqual(converted, '<p><a href="https://developer.github.com/assets/images/hero-circuit-bg.png" target="_blank" title="https://developer.github.com/assets/images/hero-circuit-bg.png">https://developer.github.com/assets/images/hero-circuit-bg.png</a></p>\n<div class="message_inline_image"><a href="https://developer.github.com/assets/images/hero-circuit-bg.png" target="_blank" title="https://developer.github.com/assets/images/hero-circuit-bg.png"><img src="https://developer.github.com/assets/images/hero-circuit-bg.png"></a></div>')
+
     def test_twitter_id_extraction(self):
         # type: () -> None
         self.assertEqual(bugdown.get_tweet_id('http://twitter.com/#!/VizzQuotes/status/409030735191097344'), '409030735191097344')