test_linkcheck_allowed_redirects: enable HTTP HEAD request support

[jayaddison]

This is intended to reduce the number of HTTP requests within this particular test to closer-to-the-norm compared to other tests within the test_build_linkcheck.py module; this test has been failing intermittently and it seems possible that timeouts due to HTTP roundtrip traffic are a contributing factor.

Feature or Bugfix

• Bugfix (maybe)

Purpose

• Intended to reduce flakiness in the test_linkcheck_allowed_redirects test -- or to rule out another potential source of that problem
• Also intended to avoid increasing the linkcheck request timeout thresholds for this (and other) tests

Detail

• Should reduce the number of requests made by the unit test to the test HTTP server by half. The server is tiny and basic, and I don't exactly understand why reducing the total request count should have any effect on the duration of the individual requests - but it's an idea to explore, and the logic expressed by the test should remain valid.

Relates

• Relates to Flaky unit tests: linkcheck builder (tests/test_build_linkcheck.py) #11299

[jayaddison]

Thanks for the review @francoisfreitag. After doing more testing, I'm not really convinced that this would fix the test flakiness.

I'd be OK with it being merged.. but if there's no particular reason for it, then I'd probably tend towards closing it unmerged, since otherwise it'll pollute the commit history.

Either way, I'll run a few more tests in jayaddison#1 and perhaps that'll discover more -- or perhaps not :) That has been my Friday, 2023-04-14..

[jayaddison]

(the approach I prefer now, since I couldn't find any particular reason for a performance regression, is simply to increase the timeout thresholds slightly further in #11326)

[francoisfreitag]

My understanding of this change : allowing HEAD saves the linkcheck behavior of retrying with GET after HEAD failed, which makes the test faster. Since the method is not relevant for the test, might as well use HEAD.

[jayaddison]

That is true that this change should provide a very, very small runtime improvement for the tests, yes.

[jayaddison]

Nothing about the experimental evaluation data collected in jayaddison#1 disproved this fix -- that is, no test failures occurred in the linkcheck tests with this change applied -- but I still don't think that the value (reduced test runtime -- extremely marginal) is worth the additional investigation and history overhead.

I'll close this pull request -- if someone later feels that this change is particularly valuable (perhaps it turns out that actually it does fix the problem comprehensively, somehow), then as long as there's supporting evidence for that, I've no problem with it being merged.

Roughly speaking: I don't mind being wrong, but I'd like to understand why. In this case I feel like I'm wrong, but the data hasn't yet confirmed it. So I'll choose not to proceed here.