runtime: TestFutexsleep failing on freebsd and linux

[bradfitz]

FreeBSD failure I've seen a few times now...

https://storage.googleapis.com/go-build-log/deaf5ba3/freebsd-amd64-12_0_6ba9394d.log

##### GOMAXPROCS=2 runtime -cpu=1,2,4 -quick
--- FAIL: TestFutexsleep (0.02s)
    futex_test.go:65: futexsleep test "before the year 2038" finished early after 20.627855ms
FAIL
FAIL	runtime	14.092s
FAIL
2019/11/04 18:30:12 Failed: exit status 1
2019/11/04 18:30:12 FAILED

/cc @ianlancetaylor

[mdempsky]

Got the same flake on the linux-amd64 trybot just now: https://storage.googleapis.com/go-build-log/dec1040d/linux-amd64_babdf1c2.log

[ianlancetaylor]

My first guess is that the test is failing because the futex sleep is being interrupted by a preemption signal. The test works by making a direct call to futexsleep, which on GNU/Linux calls futex and on FreeBSD calls sys_umtx_op. The intent is to sleep for a long time. However, these sleeps are interruptible; if a preemption interrupt is delivered, the sleep will wake up and return without error.

CC @aclements @cherrymui

[gopherbot]

Change https://golang.org/cl/205378 mentions this issue: runtime: disable preemption during test calls to futexsleep