Probe and cache HTTP/2 support per-origin

[sethmlarson]

Moved _LockedObject to urllib3._collections.

Adds an "HTTP/2 support" cache which does one of three things if HTTP/2 support is going to be requested from the origin:

• Returns the cached value True or False
• Returns None meaning the current connection is probing and needs to return back a result.
• Blocks waiting for an active probe.

My thinking is that this HTTP/2 support probe cache can evolve into the HTTP/2 socket sharing mechanism. Need to implement probe cache resetting per test case, tests for the probe, and for when it's used by an HTTP/1.1 and HTTP/2 capable function.

Part of #3301

[pquentin]

Thanks! Great step in the right direction.

[pquentin]

Are you defining ALPN_PROTOCOLS outside of the ssl_ module as well in anticipation of h2c?

[sethmlarson]

The ALPN_PROTOCOLS list in ssl_ is reexported to the urllib3.util module even before this change, so updating that list too. We should probably remove the one in utils?

[pquentin]

Yes, although that can be done in a followup PR.

[pquentin]

This looks incredible, thanks! I really like the new direction, that I find much clearer, and that I think will make implement socket sharing easy, at least on the connection side.

LGTM if CI passes!

[sethmlarson]

Addressed your feedback @pquentin (thank you!) and added another test case for blocking timings.

[pquentin]

Thanks! LGTM. The new test may be a bit too ambitious though. Maybe there's a way to make it work by having tracing callbacks in connection.py and make sure that we never have two threads connecting at the same time? Not sure.

[pquentin]

I'm still seeing failures in test_http2_probe_blocked_per_thread, with the times going up to 0.6 in this run: https://github.com/urllib3/urllib3/actions/runs/7984715792/job/21801981185?pr=3324

FAILED test/contrib/test_pyopenssl.py::TestHTTPS_TLSv1_2::test_http2_probe_blocked_per_thread - assert (1708495940.4867585 - 1708495939.9358299) < 0.5
FAILED test/contrib/test_pyopenssl.py::TestHTTPS_TLSv1_3::test_http2_probe_blocked_per_thread - assert (1708495955.7278147 - 1708495955.1609907) < 0.5
FAILED test/with_dummyserver/test_https.py::TestHTTPS_TLSv1_2::test_http2_probe_blocked_per_thread - assert (1708496024.1679983 - 1708496022.7132747) > (0.5 * (5 - 2))
FAILED test/with_dummyserver/test_https.py::TestHTTPS_TLSv1_3::test_http2_probe_blocked_per_thread - assert (1708496034.525668 - 1708496033.9327052) < 0.5

This continues showing that time-based tests are going to be fragile in CI. Is it still useful with a larger time, like say 1s instead of 0.5s?