TestWatchEventType: WaitGroup is reused before previous Wait has returned

[heyitsanthony]

via semaphore (https://semaphoreci.com/coreos/etcd/branches/master/builds/995)

panic: sync: WaitGroup is reused before previous Wait has returned

goroutine 57085 [running]:
panic(0xf5aa20, 0xc420937640)
    /usr/local/golang/1.7/go/src/runtime/panic.go:500 +0x1ae
sync.(*WaitGroup).Wait(0xc4281f9ae8)
    /usr/local/golang/1.7/go/src/sync/waitgroup.go:133 +0x13e
github.com/coreos/etcd/etcdserver.(*EtcdServer).run.func1(0xc4281f98c0, 0x1568880, 0xc42214e8a0)
    /home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdserver/server.go:580 +0x82
github.com/coreos/etcd/etcdserver.(*EtcdServer).run(0xc4281f98c0)
    /home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdserver/server.go:645 +0xad7
created by github.com/coreos/etcd/etcdserver.(*EtcdServer).start
    /home/runner/workspace/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdserver/server.go:503 +0x5aa

[gyuho]

@heyitsanthony @xiang90 Go doc says

    // This goroutine has set counter to 0 when waiters > 0.
    // Now there can't be concurrent mutations of state:
    // - Adds must not happen concurrently with Wait,
    // - Wait does not increment waiters if it sees counter == 0.

We are calling Wait in defer statement, while concurrent Add call (from goAttach) happens.