在Go语言的crypto/rand包中的Prime函数（生成可能的素数）中，它从crypto/internal/randutil包中调用MaybeReadByte函数（如下所示）。根据函数描述，我可以理解为什么使用它，但我不明白这个实现为什么有50%的机会阅读字节。难道不应该保证其中一个case在另一个之前运行吗？

var (
    closedChanOnce sync.Once
    closedChan     chan struct{}
)

// MaybeReadByte reads a single byte from r with ~50% probability. This is used
// to ensure that callers do not depend on non-guaranteed behaviour, e.g.
// assuming that rsa.GenerateKey is deterministic w.r.t. a given random stream.
//
// This does not affect tests that pass a stream of fixed bytes as the random
// source (e.g. a zeroReader).
func MaybeReadByte(r io.Reader) {
    closedChanOnce.Do(func() {
        closedChan = make(chan struct{})
        close(closedChan)
    })

    select {
    case <-closedChan:
        return
    case <-closedChan:
        var buf [1]byte
        r.Read(buf[:])
    }
}

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