Prevent edit conflicts for topic tags

tildes/tildes/templates/intercooler/topic_tags_edit.jinja2

@@ -12,6 +12,7 @@
     data-ic-target="#sidebar .topic-tags"
     data-js-remove-on-success
 >
+  <input type="hidden" name="conflict_check" id="conflict_check" value="{{ topic.tags|join(",") }}">
   {{ topic_tagging(value=topic.tags|join(', '), auto_focus=True, autocomplete_options=topic.group.common_topic_tags) }}
   <div class="form-buttons">
     <button class="btn btn-primary" type="submit">Save tags</button>

tildes/tildes/views/api/web/topic.py

@@ -161,11 +161,23 @@ def get_topic_tags(request: Request) -> dict:
     renderer="topic_tags.jinja2",
     permission="tag",
 )
-@use_kwargs({"tags": String()})
-def put_tag_topic(request: Request, tags: str) -> dict:
+@use_kwargs({"tags": String(), "conflict_check": String()})
+def put_tag_topic(request: Request, tags: str, conflict_check: str) -> dict:
     """Apply tags to a topic with Intercooler."""
+    # pylint: disable=too-many-branches
     topic = request.context
 
+    # check for edit conflict by verifying tags didn't change after they loaded the form
+    if conflict_check:
+        conflict_check_tags = conflict_check.split(",")
+    else:
+        conflict_check_tags = []
+
+    if conflict_check_tags != topic.tags:
+        raise ValidationError(
+            {"tags": ["Someone else edited simultaneously, please cancel and retry"]}
+        )
+
     if tags:
         # split the tag string on commas
         new_tags = tags.split(",")