Cache truststore SSLContext as load_verify_locations() is slow

[ichard26]

SSLContext can be reused across connections as per the Python docs:

SSLContext is designed to be shared and used by multiple connections. Thus, it is thread-safe as long as it is not reconfigured after being used by a connection.

In addition, requests has been using a global SSLContext and the sky hasn't fallen on them so I'm going to say this is pretty safe.

See also python/cpython#95031.

[ichard26]

I didn't include a news entry as the benefit is not significant, but when you combine all of the verify location/path loads in a typical run, they consume ~75 ms (~15ms/load) on my system. This eliminates one load from the self-check session, and sethmlarson/truststore#171 will eliminate one load from truststore's intialization. I may include a news entry for the optimization work in general once it's done.

[ichard26]

The CPython documentation, requests's experience using a global SSLContext¹, and Seth (from the linked truststore PR) all agree this should be fine. I'll merge this now.

Footnotes

1. The fact that requests itself relies on a global SSLContext implies that requests is not reconfiguring the context as warned against in the Python documentation. We don't do anything special with the context within pip, so we are fine. ↩