执行结果：
In f
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc42000e23c)
/home/zenglin/Downloads/go1.8/src/runtime/sema.go:47 +0x34
sync.(*WaitGroup).Wait(0xc42000e230)
/home/zenglin/Downloads/go1.8/src/sync/waitgroup.go:131 +0x7a
main.main()
/mnt/hgfs/src/test/test1.go:10 +0x80
exit status 2
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