internal/singleflight: flaky TestDoDupSuppress

[mikioh]

See http://build.golang.org/log/683c92f5ad9b397c36b9e6bcd3d457e70a2eca59.
The singleflight.Group.Do didn't return?

panic: test timed out after 2m0s

goroutine 33 [running]:
testing.startAlarm.func1()
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/testing/testing.go:702 +0x132
created by time.goFunc
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/time/sleep.go:129 +0x3a

goroutine 1 [chan receive]:
testing.RunTests(0x5d7908, 0x6606c0, 0x3, 0x3, 0x1)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/testing/testing.go:561 +0x8ad
testing.(*M).Run(0xc82003fef8, 0xc82000e640)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/testing/testing.go:493 +0x70
main.main()
    internal/singleflight/_test/_testmain.go:58 +0x116

goroutine 8 [semacquire]:
sync.runtime_Semacquire(0xc82000e7ac)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/runtime/sema.go:43 +0x26
sync.(*WaitGroup).Wait(0xc82000e7a0)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/sync/waitgroup.go:126 +0xb4
internal/singleflight.TestDoDupSuppress(0xc8200101b0)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/internal/singleflight/singleflight_test.go:69 +0x1c6
testing.tRunner(0xc8200101b0, 0x6606f0)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/testing/testing.go:455 +0x98
created by testing.RunTests
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/testing/testing.go:560 +0x86d

goroutine 10 [chan receive]:
internal/singleflight.TestDoDupSuppress.func1(0x0, 0x0, 0x0, 0x0)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/internal/singleflight/singleflight_test.go:49 +0x7d
internal/singleflight.(*Group).doCall(0xc82000e780, 0xc8200bc000, 0x58b150, 0x3, 0xc820016060)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/internal/singleflight/singleflight.go:93 +0x2c
internal/singleflight.(*Group).Do(0xc82000e780, 0x58b150, 0x3, 0xc820016060, 0x0, 0x0, 0x0, 0x0, 0x0)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/internal/singleflight/singleflight.go:63 +0x284
internal/singleflight.TestDoDupSuppress.func2(0xc82000e780, 0xc820016060, 0xc8200101b0, 0xc82000e7a0)
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/internal/singleflight/singleflight_test.go:57 +0x81
created by internal/singleflight.TestDoDupSuppress
    /tmp/gobuilder/dragonfly-amd64-69f0d4c6bebc/go/src/internal/singleflight/singleflight_test.go:65 +0x156
FAIL    internal/singleflight   120.020s

[bradfitz]

I can't reproduce this (tried many runs, and varying timing, GOMAXPROCS), the code looks fine, and this has been testing for years inside Google's build system for dl.google.com. I'm inclined to blame dragonfly somehow, even though that's lazy of me.

Hey, @dvyukov, have you ever reviewed this code? Maybe it is buggy.

[dvyukov]

@bradfitz I guess one of the goroutines in test did not block in 100ms.
It's easy to reproduce if you comment out the sleep.
The following sequence does not do what it is meant to do. If calls > 1, then wg.Wait won't return.

    wg.Wait()
    if got := atomic.LoadInt32(&calls); got != 1 {
        t.Errorf("number of calls = %d; want 1", got)
    }

[gopherbot]

CL https://golang.org/cl/11770 mentions this issue.