runtime: TestNetpollDeadlock timeout on freebsd-amd64-race

[bcmills]

greplogs -l -e 'FAIL: TestNetpollDeadlock .*(?:\n\s.*)*SIGQUIT' --since=2022-06-01
2022-08-18T17:26:17-07cf24b/freebsd-amd64-race

--- FAIL: TestNetpollDeadlock (60.04s)
    testenv.go:468: [/tmp/workdir/tmp/go-build2829729280/testprognet.exe NetpollDeadlock] exit status: exit status 2
    crash_test.go:463: output does not start with "done\n":
        dialing
        SIGQUIT: quit
        PC=0x46403e m=0 sigcode=65537
…

CC @golang/runtime @golang/freebsd

See previously #42500, #35353, #22403, #22280, #22019.

[prattmic]

The expectation is that the connection will be refused immediately. It seems that it is instead hanging (probably to time out with a longer period than our test timeout?).

[bcmills]

It's probably hanging because some other test on the machine has actually opened that hard-coded port. 😅

(It's a bad test — you can't just go around opening arbitrary ports on a heavily-loaded machine and assuming their behavior!)

[bcmills]

In particular: if something actually opened port 14356 and started listening on it, and filled up the kernel accept buffer without closing the listener (if freebsd even buffers accept operations?), then it seems possible for a Dial of that port to hang indefinitely.

(And if the port was opened by another test process, the spurious connection may well mess with the behavior of that test too.)