Exception in pusher on_new_receipts: Cursor needed to be reset

[richvdh]

occasionally /_matrix/client/r0/rooms/{roomId}/read_markers spits out this exception:

2019-11-22 13:44:21,146 - synapse.push.pusherpool - 204 - ERROR - POST-162567 - Exception in pusher on_new_receipts
Traceback (most recent call last):
  File "/home/path/to/server/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
StopIteration: <generator object ReceiptsWorkerStore.get_all_updated_receipts.<locals>.get_all_updated_receipts_txn.<locals>.<genexpr> at 0x7faca013cca8>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/path/to/server/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
StopIteration: <generator object ReceiptsWorkerStore.get_all_updated_receipts.<locals>.get_all_updated_receipts_txn.<locals>.<genexpr> at 0x7faca013cca8>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/path/to/server/lib/python3.6/site-packages/synapse/push/pusherpool.py", line 196, in on_new_receipts
    users_affected = set([r[3] for r in updated_receipts])
  File "/home/path/to/server/lib/python3.6/site-packages/synapse/push/pusherpool.py", line 196, in <listcomp>
    users_affected = set([r[3] for r in updated_receipts])
  File "/home/path/to/server/lib/python3.6/site-packages/synapse/storage/data_stores/main/receipts.py", line 283, in <genexpr>
    return (r[0:5] + (json.loads(r[5]),) for r in txn)
sqlite3.InterfaceError: Cursor needed to be reset because of commit/rollback and can no longer be fetched from.

The symptoms are presumably that people get pushes for messages they have already read.

I think it's because get_all_updated_receipts returns a generator expression which refers to a database cursor, which refers to a database connection, which might get re-used before the results are read.