Hi,


The latest setuptools (58.0) removed support for "use_2to3" [1]
(deprecated since 46.2). Many OpenStack modules defines
decorator==3.4.0 as lower constraints[2]. However, decorator 3.4.0
cannot be installed anymore with the latest setuptool as it depends on
"use_2to3".

On master, this can be solved easily by bumping the dependency to
decorator 4.0.0. On stable/xena we can still solve it the same way with
a new RC. But on older stable branches such solution might be against
the stable policy as it would require a major bump on our dependencies.

This issue is not limited to lower-constraints testing it just hit us
there first. A similar break could happen in our upper constraints as
well on old stable branches.

The root of the problem is that we always use the latest setuptools in
our CI testing even on old stable branches. Zuul's ensure-tox task[4]
installs tox which installs the virtualenv package which bundles the
latest setuptools package. This happens without applying any
constraints. Then tox is used to install the project's dependencies
under lower or upper constraints with the unconstrained setuptools
available.

During and after yesterday's nova meeting [3] we discussed possible
solutions.

Option 1: Bump the major version of the decorator dependency on stable.
Pros:
* easy to implement
Cons:
* might against the policy / breaks downstream packagers
* needs to be done in each affected project[3] separately
* not future proof. After a future setuptools release we can see a
similar break with another of our dependencies.

@Stable Cores: how do you feel about such bump?


Option 2: Pin the setuptools version during tox installation
Pros:
* single solution for all affected projects
* future proof against breaks introduced by future setuptools releases
Cons:
* complex change as it probably requires to extend the base task in
zuul itself


Option 3: turn off lower-constraints testing
Pros:
* easy to implement
* some of us want to get rid of lower constraints anyhow
Cons:
* needs per project changes
* not future proof against similar break affecting our upper
constraints on old stable branches.


Option 4: utilize pyproject.toml[6] to specify build-time requirements
Unfortunately, I'm not sure if it is possible to restrict the maximum
setuptools version with it


As a side note I tried applying [tox]requires in tox.ini to restrict
setuptools version[7] but It does not seem to take effect in our
case[8].


@Stable Cores: what do you think what could be the way forward?

Cheers,
gibi


[1] https://setuptools.pypa.io/en/latest/history.html#v58-0-2
[2]
https://codesearch.openstack.org/?q=decorator%3D%3D3.4.0&i=nope&literal=nope&files=&excludeFiles=&repos=
[3]
https://meetings.opendev.org/irclogs/%23openstack-nova/%23openstack-nova.2021-09-21.log.html#t2021-09-21T16:31:49
[4]
https://opendev.org/zuul/zuul-jobs/src/branch/master/roles/ensure-tox/tasks/main.yaml#L28
[5] https://tox.readthedocs.io/en/latest/config.html#conf-requires
[6] https://www.python.org/dev/peps/pep-0518/
[7] https://review.opendev.org/c/openstack/placement/+/810293/1/tox.ini
[8]
https://zuul.opendev.org/t/openstack/build/f78a0300c8734e7c8475309af1d2e1a4/log/tox/lower-constraints-0.log#7