[DB-12019] cursor.close() - fails to clear the result set with error thrown with NuoDB version >=2.1

[VDuda]

The driver does not close the ResultSet and with newer versions of nuodb (>=2.1) an OperationalError is thrown similar to:

...
    raise OperationalError(error_code_string + ': ' + error_string)
pynuodb.exception.OperationalError: 'INVALID_OPERATION: Maximum number of open statements (1000) exceeded'

Below is a code snippet that was used to generate the error.

import pynuodb

connection = pynuodb.connect("local", "localhost", "dba", "dba", options={'schema':'platform'})
for i in range(10000):
    cursor = connection.cursor()
    cursor.execute('select 1 from dual;')
    connection.commit()
    cursor.close()

I believe there was a regression introduced between 2.0.4 and 2.1 that changed the behavior with how we handled ResultSets.

• Added method close_result_set to encodedsession - used to pass messageId 28 over to the server
• Added method close to result_set - result_set objects should call close to officially kill the open resultSet with the server
• Any open result_set objects that are associated with a cursor will be closed when cursor.close() is called. Note that this still allows users to run into OperationError above since it is required to call cursor.close() to clean up after yourself. We could also implicitly cleanup for the user every time a new cursor object is created.

[mjmichaels]

If this is a regression, how did it used to work?

[VDuda]

To be clear I believe the regression occurred in NuoDB core itself, between 2.0.4 and 2.1.

Within the python driver, before this commit, we were never sending a CloseResultSet message (id 28).