x/sync/singleflight: occasional failures in TestForget

[bcmills]

--- FAIL: TestForget (0.00s)
    singleflight_test.go:160: Third call should not be started because was started during second execution
    singleflight_test.go:163: We should receive result produced by second call, expected: 2, got 3
FAIL
FAIL	golang.org/x/sync/singleflight	61.958s

The failing test is the regression test added for #31420 (CC @cuonglm @bradfitz).

The bug appears to be in the test:
https://github.com/golang/sync/blob/b3e1573b75205f2905c4b27986413d2e2be9803c/singleflight/singleflight_test.go#L153-L157

In that section, nothing guarantees that the second invocation is still running when g.Do is evaluated. (The second goroutine can complete immediately after close(secondCh), causing the third invocation to be evaluated.)

2020-10-08T14:14:35-b3e1573/android-386-emu
2019-09-11T18:51:00-cd5d95a/freebsd-amd64-race
2019-04-23T02:48:10-1122301/freebsd-amd64-race
2019-04-23T02:48:10-1122301/windows-amd64-race

[gopherbot]

Change https://golang.org/cl/263877 mentions this issue: singleflight: fix TestForget