From bf14d3a401e4ffa245e3daf316d85100157d6c9f Mon Sep 17 00:00:00 2001
From: Takeshi KOMIYA <i.tkomiya@gmail.com>
Date: Sat, 22 May 2021 14:13:19 +0900
Subject: [PATCH] Concept: Close #9016: linkcheck builder failed to check the
 anchors of github.com

---
 sphinx/builders/linkcheck.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/sphinx/builders/linkcheck.py b/sphinx/builders/linkcheck.py
index a46b80c081b..db389123228 100644
--- a/sphinx/builders/linkcheck.py
+++ b/sphinx/builders/linkcheck.py
@@ -627,6 +627,10 @@ def run(self, **kwargs: Any) -> None:
             if 'refuri' not in refnode:
                 continue
             uri = refnode['refuri']
+            newuri = self.app.emit_firstresult('linkcheck-process-uri', uri)
+            if newuri:
+                uri = newuri
+
             lineno = get_node_line(refnode)
             uri_info = Hyperlink(uri, self.env.docname, lineno)
             if uri not in hyperlinks:
@@ -636,12 +640,29 @@ def run(self, **kwargs: Any) -> None:
         for imgnode in self.document.traverse(nodes.image):
             uri = imgnode['candidates'].get('?')
             if uri and '://' in uri:
+                newuri = self.app.emit_firstresult('linkcheck-process-uri', uri)
+                if newuri:
+                    uri = newuri
+
                 lineno = get_node_line(imgnode)
                 uri_info = Hyperlink(uri, self.env.docname, lineno)
                 if uri not in hyperlinks:
                     hyperlinks[uri] = uri_info
 
 
+def rewrite_github_anchor(app: Sphinx, uri: str) -> Optional[str]:
+    """Rewrite anchor name of the hyperlink to github.com
+
+    The hyperlink anchors in github.com are dynamically generated.  This rewrites
+    them before checking and makes them comparable.
+    """
+    if re.search('://github.com/', uri) and '#' in uri:
+        baseuri, anchor = uri.split('#', 1)
+        return '#'.join([baseuri, 'user-content-' + anchor])
+    else:
+        return None
+
+
 def setup(app: Sphinx) -> Dict[str, Any]:
     app.add_builder(CheckExternalLinksBuilder)
     app.add_post_transform(HyperlinkCollector)
@@ -658,6 +679,9 @@ def setup(app: Sphinx) -> Dict[str, Any]:
     app.add_config_value('linkcheck_anchors_ignore', ["^!"], None)
     app.add_config_value('linkcheck_rate_limit_timeout', 300.0, None)
 
+    app.add_event('linkcheck-process-uri')
+    app.connect('linkcheck-process-uri', rewrite_github_anchor)
+
     return {
         'version': 'builtin',
         'parallel_read_safe': True,