非阻塞先进先出

Question

如何在两个 python 进程之间创建一个 fifo，如果阅读器无法处理输入，则允许删除行？

• 如果读者尝试read或readline比作者写得更快，它应该阻塞。
• 如果读者不能像作者那样快地工作，那么作者不应该阻塞。不应该缓冲行（一次一行除外），并且读取器在下次readline尝试时只应接收写入的最后一行。

使用命名的 fifo 是否可以做到这一点，或者还有其他简单的方法可以实现这一目标吗？

Answer 1

以下代码使用命名的 FIFO 来允许两个脚本之间进行通信。

• 如果读者试图read比作者更快，就会阻塞。
• 如果读者无法跟上作者，则作者不会阻塞。
• 操作是面向缓冲区的。目前尚未实现面向线路的操作。
• 该代码应被视为概念验证。延迟和缓冲区大小是任意的。

代码

import argparse
import errno
import os
from select import select
import time

class OneFifo(object):
    def __init__(self, name):
        self.name = name

    def __enter__(self):
        if os.path.exists(self.name):
            os.unlink(self.name)
        os.mkfifo(self.name)
        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):
        if os.path.exists(self.name):
            os.unlink(self.name)

    def write(self, data):
        print "Waiting for client to open FIFO..."
        try:
            server_file = os.open(self.name, os.O_WRONLY | os.O_NONBLOCK)
        except OSError as exc:
            if exc.errno == errno.ENXIO:
                server_file = None
            else:
                raise
        if server_file is not None:
            print "Writing line to FIFO..."
            try:
                os.write(server_file, data)
                print "Done."
            except OSError as exc:
                if exc.errno == errno.EPIPE:
                    pass
                else:
                    raise
            os.close(server_file)

    def read_nonblocking(self):
        result = None
        try:
            client_file = os.open(self.name, os.O_RDONLY | os.O_NONBLOCK)
        except OSError as exc:
            if exc.errno == errno.ENOENT:
                client_file = None
            else:
                raise
        if client_file is not None:
            try:
                rlist = [client_file]
                wlist = []
                xlist = []
                rlist, wlist, xlist = select(rlist, wlist, xlist, 0.01)
                if client_file in rlist:
                    result = os.read(client_file, 1024)
            except OSError as exc:
                if exc.errno == errno.EAGAIN or exc.errno == errno.EWOULDBLOCK:
                    result = None
                else:
                    raise
            os.close(client_file)
        return result

    def read(self):
        try:
            with open(self.name, 'r') as client_file:
                result = client_file.read()
        except OSError as exc:
            if exc.errno == errno.ENOENT:
                result = None
            else:
                raise
        if not len(result):
            result = None
        return result

def parse_argument():
    parser = argparse.ArgumentParser()
    parser.add_argument('-c', '--client', action='store_true',
                        help='Set this flag for the client')
    parser.add_argument('-n', '--non-blocking', action='store_true',
                        help='Set this flag to read without blocking')
    result = parser.parse_args()
    return result

if __name__ == '__main__':
    args = parse_argument()
    if not args.client:
        with OneFifo('known_name') as one_fifo:
            while True:
                one_fifo.write('one line')
                time.sleep(0.1)
    else:
        one_fifo = OneFifo('known_name')
        while True:
            if args.non_blocking:
                result = one_fifo.read_nonblocking()
            else:
                result = one_fifo.read()
            if result is not None:
                print result

检查server是否已client打开 FIFO。如果client已打开 FIFO，则server写入一行。否则，server继续运行。我已经实现了非阻塞读取，因为阻塞读取会导致一个问题：如果server重新启动，大多数时候client会保持阻塞状态并且永远不会恢复。使用非阻塞client，server重新启动更容易被容忍。

输出

[user@machine:~] python onefifo.py
Waiting for client to open FIFO...
Waiting for client to open FIFO...
Writing line to FIFO...           
Done.
Waiting for client to open FIFO...
Writing line to FIFO...
Done.

[user@machine:~] python onefifo.py -c
one line
one line

笔记

启动时，如果server检测到 FIFO 已存在，则会将其删除。clients这是通知已重新启动的最简单方法server。该通知通常会被阻止版本的client.