markers are incompatible with pip < 6 and sdists

[Robert Collins]

This originated from https://github.com/testing-cabal/mock/issues/316 and https://github.com/pypa/pip/issues/3111

- Markers in deps *in sdists* (whether extras or install_requires) are not interpreted by pip <6.
- pip < 1.5 chooses sdists over wheels.
- Ubuntu LTS has pip 1.1, and RHEL 7 has 1.5

Thus anything that:
 - uses markers
 - is going to be installed on Ubuntu LTS or RHEL 7 (e.g. another decade or two!)
 - and publishes sdists

will fail to install correctly

Donald stufft suggests the following approach as a workaround:

 - in egg_info generation export unconditional metadata (e.g. use pkg_resources to evaluate the markers immediately)
 - in bdist_wheel export the markers as markers for evaluation by the installing system.

This is used by a number of packages - e.g. https://github.com/pypa/twine/blob/master/setup.cfg#L9-L15 and https://github.com/pypa/twine/blob/master/setup.py#L28-L31 - and apparently works ok with pip.

We'd want careful testing to ensure we don't accidentally export unconditional deps to the wheel, I suggest at least one integration test of a test package that does pip install ., pip install -e ., pip wheel ., parameterised over pip 1.4, 1.5 and latest.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/253289

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/253289
Committed: https://git.openstack.org/cgit/openstack-dev/pbr/commit/?id=7898882fa5316bf85ffd5617340e69df2129e923
Submitter: Jenkins
Branch: master

commit 7898882fa5316bf85ffd5617340e69df2129e923
Author: Sachi King <email address hidden>
Date: Thu Oct 29 12:17:56 2015 +0900

    Handle markers to support sdist on pip < 6

    Old versions of pip do not properly evaluate blank envmarkers such as
    ':python_version>=2.7', so as long as we are not creating a bdist_wheel
    we evaluate these immediately to support these old installations

    Change-Id: I94a5f9bccd658a2529a727d99f5a2b79e6b00aa8
    Closes-Bug: 1502692

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/278210

[OpenStack Infra (hudson-openstack)]

Change abandoned by Sachi King (<email address hidden>) on branch: master
Review: https://review.openstack.org/278210
Reason: Alt implementation https://review.openstack.org/#/c/289628/

Can this one

[Davanum Srinivas (DIMS) (dims-v)]

This issue was fixed in the openstack-dev/pbr 1.9.0 release.