[singlehtml]: append docname to section id

Use doc path to make ids unique.

Author: gastmaier

sphinx/transforms/post_transforms/__init__.py

@@ -390,11 +390,30 @@ def run(self, **kwargs: Any) -> None:
                 node['classes'].append(node.parent['domain'])
 
 
+class PrefixIdsWithDocname(SphinxPostTransform):
+    """Prefix all ids with the docname, to ensure unique refid in singlehtml."""
+
+    default_priority = 300
+
+    def run(self, **kwargs: Any) -> None:
+        builder = self.env._app.builder
+        if builder.name != 'singlehtml':
+            return
+        for node in self.document.findall():
+            if 'refid' in node or 'ids' in node:
+                docname = node.document.settings.env.path2doc(node.document['source'])
+            if 'refid' in node:
+                node['refid'] = 'document-' + docname + '#' + node['refid']
+            if 'ids' in node:
+                node['ids'] = ['document-' + docname + '#' + id for id in node['ids']]
+
+
 def setup(app: Sphinx) -> ExtensionMetadata:
     app.add_post_transform(ReferencesResolver)
     app.add_post_transform(OnlyNodeTransform)
     app.add_post_transform(SigElementFallbackTransform)
     app.add_post_transform(PropagateDescDomain)
+    app.add_post_transform(PrefixIdsWithDocname)
 
     return {
         'version': 'builtin',