execute_sql in django/db/models/sql/query.py fails with MySQL due to incorrect sentinel

[tpherndon]

Line 1333: return iter((lambda: cursor.fetchmany(GET_ITERATOR_CHUNK_SIZE)), []) fails under MySQL because the cursor.fetchmany() call returns an empty tuple as sentinel, rather than an empty list. The empty list sentinel is correct for sqlite. I attempted to run runtests.py, which hung on "Running post-sync handlers for application string_lookup". When I changed the sentinel to a tuple, runtests.py ran to completion (four failing tests, but completion) with MySQL, yet hung in the exact same place with sqlite.

[tpherndon]

Sets iterator sentinel to proper sentinel type for sqlite and mysql backends.

[Malcolm Tredinnick]

Nice catch. But I'd prefer to have a function that just returns the "empty sentinel" value in each case, rather than testing for one or the other.

[Malcolm Tredinnick]

(In [7283]) queryset-refactor: Different database backends return different empty sequences
when fetchmany() is exhausted. This change allows for that. Fixed #6807.

Nice debugging from tpherndon.