前言
EPOLLRDHUP 和EPOLLHUP名称中的HUP的含义是hang up，表示挂断；它们是由shutdown系统调用或close系统调用所触发的；其实两者之间本质的差异在其名称中就已经体现出来了：RDHUP添加了前缀RD，表示读挂起，在EPOLL_CTL(2)中对其的解释是:
Stream socket peer closed connection, or shut down writing half of connection. (This flag is especially useful for writing simple code to detect peer shutdown when using Edge Triggered monitoring.)
EPOLLHUP的注解如下：
Hang up happened on the associated file descriptor.
epoll_wait(2)will always wait for this event; it is not necessary to set it in events.Note that when reading from a channel such as a pipe or a stream socket, this event merely indicates that the peer closed its end of the channel. Subsequent reads from the channel will return 0 (end of file) only after all outstanding data in the channel has been consumed.
EPOLLRDHUP vs EPOLLHUP
EPOLLRDHUP 表示**读关闭**
不是所有的内核版本都支持，没有查证。有两种场景：
1、对端发送 FIN (对端调用close 或者 shutdown(SHUT_WR)).
2、本端调用 shutdown(SHUT_RD). 当然，关闭 SHUT_RD 的场景很少。
SUMMARY : 对端是write， 本端是read
测试环境为 Linux localhost.localdomain 3.10.0-957.el7.x86_64 #1 SMP Thu Nov 8 23:39:32 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux。 ————————————————
使用 python 的服务端测试过程，客户端仅仅建立 TCP 连接，不发送任何数据。第 10 条指令 poll 超时，返回一个空的列表。通过关闭本端的读取，再次 poll 可以看到，返回 hex(8193) = 0x2001 表示EPOLLRDHUP 和 EPOLLIN 事件。
In [1]: import socket
In [2]: serv = socket.socket(socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_TCP)
In [3]: serv.bind(("0.0.0.0", 7777))
In [4]: serv.listen(5)
In [5]: import select
In [6]: epoll_fd = select.epoll()
In [7]: client,addr = serv.accept()
In [8]: client
Out[8]: <socket.socket fd=15, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=6, laddr=('192.168.1.168', 7777), raddr=('192.168.1.120', 8790)>
In [9]: epoll_fd.register(client, select.EPOLLIN | select.EPOLLRDHUP)
In [10]: epoll_fd.poll(5)
Out[10]: []
In [11]: client.shutdown(socket.SHUT_RD)
In [12]: epoll_fd.poll(5)
Out[12]: [(15, 8193)]
In [13]: for i in dir(select):
...: if "EPOLL" in i:
...: print(i, hex(getattr(select, i)))
...:
EPOLLERR 0x8
EPOLLET -0x80000000
EPOLLHUP 0x10
EPOLLIN 0x1
EPOLLMSG 0x400
EPOLLONESHOT 0x40000000
EPOLLOUT 0x4
EPOLLPRI 0x2
EPOLLRDBAND 0x80
EPOLLRDHUP 0x2000
EPOLLRDNORM 0x40
EPOLLWRBAND 0x200
EPOLLWRNORM 0x100
本端不动，客户端 shutdown(SHUT_WR) 得到一样的结果。
