Short-circuit get_requires hooks when there are no extra build dependencies

[ichard26]

Summary of changes

The PEP 517 get_requires_for_build_(wheel|sdist) hooks function by running the egg_info command with a patched Distribution that raises a special exception when build dependencies (from setup_requires) are about to be installed.

This works great at minimizing the overhead of the requires hooks... except when there are no dynamic build dependencies. In such a situation, the injected install_build_eggs() shim that raises SetupRequirementsError is never invoked, thus the entire egg_info command will execute. This often results in substantial overhead for modern projects that use [build-system].requires and not setup_requires.

With this simple script at the root of the pip project, this change brings the execution time from ~300ms to ~100ms.

import setuptools.build_meta
setuptools.build_meta.get_requires_for_build_wheel()

This helps a fair bit with reducing the overhead with PEP 517 processing.

[ichard26]

I have no idea why this is causing the prepare metadata hooks to blow up. If anyone else has any ideas, I'm all ears. I'm utterly stumped.

[ichard26]

Oh, it looks like the dist-info command depends on the egg-info command having ran before it...? That's going to kill this idea if true.

[jaraco]

Oh, it looks like the dist-info command depends on the egg-info command having ran before it...? That's going to kill this idea if true.

I do believe that's the case. Soon, however, the biggest dependencies on egg_info (easy_install) should be removed, which gets us much closer to being able to consolidate the metadata handling behavior in dist_info instead of egg_info, so maybe there's hope.