Drop our use of namespace packages

https://blueprints.launchpad.net/oslo-incubator/+spec/drop-namespace-packages

Installing our libraries into a namespace package has caused several types of errors. Rather than continuing to fight with them, I propose that we phase them out.

Problem description

The namespace package support in setuptools is fragile, and some of the ways we use code regularly within OpenStack expose the issues in ways that are hard to debug.

The main issue we have seen is with installing two separate libraries into the same python namespace package using different installation “modes”. If one library installed with pip install -e to enable “editable” mode, and another is installed without the -e option as a regular library, then the import path for the package is broken and some of the installed components are not importable. This happened a lot when devstack installed Oslo libraries editable, and we have changed devstack to stop doing that by default. It also happens when a developer installs something using a system package and then installs another library from source. We have no real way to control that behavior, so it still crops up from time to time.

We also see trouble with virtualenvs configured to allow access to the global site-packages directory, as nova needs for libvirt access. If anything from the oslo namespace package is installed globally, it will shadow the versions visible in the virtualenv. That would normally lead to a version error if some old version is installed globally, but in this case it can also lead to import errors because the namespace package doesn’t not properly span the two site-packages directories.

Proposed change

I propose that we move all current Oslo libraries out of the oslo namespace and create simple packages that can be imported independently. For example, we would change:

from oslo.foo import bar

to:

from oslo_foo import bar

To avoid issues with distros, we should not rename the oslo.foo library to oslo_foo. This may be a little confusing for developers, so we might look at the decision again in the future, but as a first step this is less obtrusive because it means we don’t need to change our requirements lists, repository names, and distro package names.

To support backwards compatibility, we will provide shadow-packages for the oslo namespace so that the old import form still works for the Kilo releases of the libraries. That will give us time to update all of the applications and let them cycle out of the long-term support window before dropping the namespace packages entirely during the M release.

Moving to regular packages will also let us move our unit tests inside the library, so they are delivered and installed under oslo_foo, so we should do that at the same time, retaining just enough of the test suite to ensure that the public API exposed through the namespace package still works.

The distribution names of existing libraries won’t change, and the same pattern will be used to create new libraries to avoid confusion. That means one would pip install oslo.foo and then from oslo_foo import bar.

Alternatives

  1. lifeless has a patch to oslo.db that sets up the namespace package using pkgutils instead of setuptools:

    This retains the ability to use namespace packages, with the extra cost of shipping a real oslo library (https://github.com/rbtcollins/oslo) and maintaining the override code ourselves. We don’t know what other issues we might find with namespace packages, though, so I’m more comfortable with just dropping them entirely.

  2. Python 3 has better native namespace package support, so we could review the decision when we move to Python 3.

  3. Do nothing, and continue to help developers debug problems on their systems when they come up. If we choose this option, we might want to write a script “why_is_oslo_broken.sh” to put in the incubator.

The “oslo_” name prefix was selected to distinguish the libs from the non-production libraries like oslotest and oslosphinx. We could theoretially rename those as well so that all of our branded libraries have the same naming convention. I don’t know if that’s worth doing or not.

Impact on Existing APIs

The imports will change, but we should not need to change other aspects of the libraries’ APIs.

Security impact

None

Performance Impact

None

Configuration Impact

None

Developer Impact

Developers need to be aware of the import changes. We have a few options for communicating that widely:

  1. Add a deprecation warning to the oslo namespace packages so importing them reports a warning.
  2. Add a hacking rule to make importing “from oslo.” illegal.
  3. Rely on reviewers to catch it.

Option 1 is easy for us to do ourselves.

Option 2 has to wait until we have converted all of our own libraries, and updated all the apps, but it would then prevent improper imports being restored.

Option 3 is brittle, so I don’t think we want to rely on it alone.

To ease the transition, we should be able to prepare a bash/sed script to make the required edits to a project. I don’t think we’ll have more than a handful of different types of import statements, but we can update the script as we find new patterns

Testing Impact

The unit tests will be updated in each library so that all of them run against the new module name. Then some of the public API tests will be duplicated to use the namespace package to ensure that we don’t break that.

The existing unit tests in applications should cover the uses of the libraries in the applications. We will need to update any mocks in those tests.

Implementation

Assignee(s)

Primary assignee:
Doug Hellmann
Other contributors:
None

Milestones

Target Milestone for completion: K-2 (I hope early)

Work Items

  1. Rearrange all of our library code, including tests and documentation. See https://review.openstack.org/#/c/127323/ for an example.
  2. Write helper script for liaisons (maybe a liaison can do this?).

Incubation

N/A

Adoption

N/A

Library

N/A

Anticipated API Stabilization

N/A

Documentation Impact

None

Dependencies

None

References

Note

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