> But 3.9 and 3.8 are not failing on test_ssl, so do something has clearly changed. Are the failing tests new?

They're flakey in the sense that they're not failing reliably, when they clearly should be :)

If something changed about how any part of the response is written into the buffer, it could change when the OS decides to dispatch it and unblock the receiving thread, which is going to affect the state observed by the test at the time it observes it (and as we already know, there are inherent race conditions that sometimes recognise the close of the connection before it's finished reading). These are difficult tests to write, because there are so many weird interactions like this.

In terms of this actual change, my best guess is that one of the removed checks actually mattered on Windows, probably because of a different compile flag. We just use the defaults as generated by their Perl scripts, and I'd hope that there'd be compile errors if anything too blatant was changed (I don't see anything obvious looking through 39258d3595300bc7b952854c915f63ae2d4b9c3e). But short of that, I don't see anything deterministic changed here.