Django 4.2 overwrites user-specified psycopg cursor_factory

[Anders Kaseorg]

Zulip ​configures a custom cursor_factory that ​wraps psycopg2.extensions.cursor to collect timing statistics for logging. But this no longer works in Django 4.2 due to 09ffc5c1212d4ced58b708cbbf3dfbfb77b782ca (#33308) and 0e2649fdf40cedc5be7e2c0e5f7711f315e36b84 (#34255) because DatabaseWrapper.get_new_connection now unconditionally overwrites connection.cursor_factory (even for psycopg2).

The configured cursor_factory is being passed to get_new_connection as conn_params["cursor_factory"]. get_new_connection should leave that alone if it’s set. (And if it’s not, it might also be cleaner for to pass the default cursor_factory via a keyword argument to psycopg[2].connect too, rather than mutating it later.)

[Anders Kaseorg]

Submitted a patch at ​https://github.com/django/django/pull/16736.

[Florian Apolloner]

Sounds like a legit request, we didn't limit this functionality on purpose but rather by accident.

Assuming we fix this we should also make it more pythonic (and I guess I am at fault for it being like it is now) and not check for is True but only truthiness. Either way, a test ensuring that we do not regress here would be good.

[Anders Kaseorg]

Removed is True and added a test.

[Mariusz Felisiak]

Replying to Florian Apolloner:

Assuming we fix this we should also make it more pythonic (and I guess I am at fault for it being like it is now) and not check for is True but only truthiness.

We use is True intentionally, to avoid errors when passing wrong truthy values, e.g."server_side_binding": os.environ.get("USE_SSB") (where os.environ.get("USE_SSB") returns "False").

[Mariusz Felisiak <felisiak.mariusz@…>]

In 73cbb372:

Fixed #34466 -- Reallowed setting cursor_factory in DATABASESoptions on PostgreSQL.

Regression in 09ffc5c1212d4ced58b708cbbf3dfbfb77b782ca.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0bc2bbf:

[4.2.x] Fixed #34466 -- Reallowed setting cursor_factory in DATABASESoptions on PostgreSQL.

Regression in 09ffc5c1212d4ced58b708cbbf3dfbfb77b782ca.

Backport of 73cbb372baa45d1fdafd571e2f430a980831f722 from main