net: spurious EADDRINUSE from connect syscall on FreeBSD when connecting from IPv6 wildcard to IPv4 address

[bcmills]

Observed in a freebsd-amd64-12_0 TryBot: https://storage.googleapis.com/go-build-log/2e4c3b9c/freebsd-amd64-12_0_d1b5be43.log

--- FAIL: TestDialerLocalAddr (0.00s)
    dial_test.go:647: tcp [::]:0->127.0.0.1: got dial tcp [::]:0->127.0.0.1:22464: connect: address already in use; want <nil>
FAIL
FAIL	net	54.481s

This test is also flaky on macOS (#22019), but the symptom on FreeBSD is different from the one observed on macOS (connect: address already in use vs. getsockopt: operation timed out).

Note that the freebsd-arm-paulzhol flake reported in #22019 (comment) matches this one.

CC @ianlancetaylor @mikioh

[bcmills]

FAIL: TestDialerLocalAddr.*\n.*address already in use:

2021-06-08T05:02:19-c20bcb6/freebsd-amd64-12_2
2021-04-07T05:19:15-7d5c54e/freebsd-amd64-12_0
2020-09-29T06:10:12-15c8925/freebsd-amd64-race
2019-05-01T16:49:31-fad365b/freebsd-386-12_0

[bcmills]

greplogs --dashboard -md -l -e 'FAIL: TestDialerLocalAddr.*\n.*address already in use' --since=2021-06-09

2021-11-25T00:07:28-f7e34e7/freebsd-amd64-11_4

[bcmills]

These appear to always be for the specific pair [::]:0->127.0.0.1.

The connect: prefix in the error suggests that the error originates at one of these two sites:
https://cs.opensource.google/go/go/+/master:src/net/fd_unix.go;l=83;drc=c4aae23d6426442402b3de0e5f7de1ef8da3842a
https://cs.opensource.google/go/go/+/master:src/net/fd_unix.go;l=165;drc=c4aae23d6426442402b3de0e5f7de1ef8da3842a

This FreeBSD bug seems to match the symptom, but it was believed to be fixed as of FreeBSD 12-STABLE.

[gopherbot]

Change https://golang.org/cl/369157 mentions this issue: net: skip IPv6-to-v4 case in TestDialerLocalAddr on certain FreeBSD builders