net/http: apparent deadlocks in TestDisableKeepAliveUpgrade on longtest builders

[bcmills]

2020-12-07T21:01:46-7ad6596/windows-amd64-longtest
2020-12-04T08:49:16-b67b7dd/windows-amd64-longtest
2020-12-02T16:43:58-10240b9/windows-amd64-longtest

Marking as release-blocker for Go 1.16: this is a new test, merged Dec. 1 in CL 213277 for #36381.

CC @bradfitz @nhooyr @neild @fraenkel @golang/release

[bcmills]

This regression was partially masked in the builders by #42942.

[nhooyr]

Hmm, interesting that it only occurs on Windows.

Looks like the io.ReadFull on line 6495 is stuck as the peer isn't echoing back the written bytes for some reason.

[nhooyr]

Nothing stands out, the test may be surfacing an unrelated regression.

[nhooyr]

Actually I'm going to push up a change that may fix it.

[nhooyr]

Opened #43086

[gopherbot]

Change https://golang.org/cl/276375 mentions this issue: net/http: attempt deadlock fix in TestDisableKeepAliveUpgrade