integrations: Prevent duplicate GitHub pull request review messages.

GitHub sends two almost identical payloads when a pull
request is reviewed, which results in two duplicative
notification messages.

The payloads have different "action" value, with one
having the "submitted" action, whereas the other is
"edited" and has an empty "changes" dict.

We now ignore the payload with the "edited" action type
and an empty "changes" dict.

Fixes #26145.

Co-authored-by: Pieter CK <pieterceka123@gmail.com>

zerver/webhooks/github/view.py

@@ -940,6 +940,12 @@ def api_github_webhook(
     return json_success(request)
 
 
+def is_empty_pull_request_review_event(payload: WildValue) -> bool:
+    action = payload["action"].tame(check_string)
+    changes = payload.get("changes", {})
+    return action == "edited" and len(changes) == 0
+
+
 def get_zulip_event_name(
     header_event: str,
     payload: WildValue,
@@ -970,6 +976,14 @@ def get_zulip_event_name(
             return "pull_request_auto_merge"
         if action in IGNORED_PULL_REQUEST_ACTIONS:
             return None
+    elif header_event == "pull_request_review":
+        if is_empty_pull_request_review_event(payload):
+            # When submitting a review, GitHub has a bug where it'll
+            # send a duplicate empty "edited" event for the main
+            # review body. Ignore those, to avoid triggering
+            # duplicate notifications.
+            return None
+        return "pull_request_review"
     elif header_event == "push":
         if is_merge_queue_push_event(payload):
             return None