查下文档 https://golang.org/pkg/io/#ReadFull

ReadFull reads exactly len(buf) bytes from r into buf. It returns the number of bytes copied and an error if fewer bytes were read. The error is EOF only if no bytes were read. If an EOF happens after reading some but not all the bytes, ReadFull returns ErrUnexpectedEOF. On return, n == len(buf) if and only if err == nil. If r returns an error having read at least len(buf) bytes, the error is dropped.


// 第一次读没问题
head := make([]byte, 20)
_, err =io.ReadFull(r,head)
// 这时候 ReadFull 就全部 512 字节读取完了，然后只保留了 20 字节给你的 head 变量，后面全部舍弃了

// 第二次读一直阻塞
body := make([]byte, 6)
_, err =io.ReadFull(r,body)
// 当然就阻塞了，因为已经全部读完了都 EOF 了