Clarify the select_for_update logic of cfbot_ingest

Author: JelteF

pgcommitfest/commitfest/views.py

@@ -830,7 +830,12 @@ def cfbot_ingest(message):
         # not it doesn't contain the newest patches that the CFBot knows about.
         return
 
-    old_branch = CfbotBranch.objects.select_for_update().filter(patch_id=patch_id).first()
+    # We don't simply fetch this from the patch.cfbot_branch, because we want
+    # to get it as select_for_update. That way we can use this row as a lock to
+    # ensure only one cfbot HTTP request can change a patch its Branch and
+    # Tasks at the same time. (see also transaction.atomic at the top of this
+    # function)
+    old_branch = CfbotBranch.objects.select_for_update().get(pk=patch_id)
     if old_branch and old_branch.branch_id != branch_id and old_branch.created.replace(tzinfo=timezone.utc) > created:
         # This is a message for an old branch, ignore it.
         return