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BugZero found this defect 2736 days ago.
In order to work correctly, the merging logic on mongos needs to know that a cursor is tailable. An aggregation with a $changeNotification is tailable, but doesn't communicate that information, so the merging logic mistakenly hangs, repeatedly asking for the next document, since the cursor id is never set to 0.
xgen-internal-githook commented on Wed, 26 Jul 2017 22:05:08 +0000: Author: {'email': 'charlie.swanson@mongodb.com', 'username': 'cswanson310', 'name': 'Charlie Swanson'} Message: SERVER-30246 Error if $changeNotification is sent to mongos. Branch: master https://github.com/mongodb/mongo/commit/7d9038a90fffa22b58323df1a5221aa9b30f853c
Run an aggregate with a $changeNotification stage on a mongos (shards must be replica sets)