subprocess.popen阻塞

subprocess.Popen在读取子进程的标准输出时，如果子进程没有输出，使用stdout.read()会导致程序阻塞。 为了避免阻塞，可以采用两种方法。一种是在读取输出之前，将文件描述符设置为非阻塞模式，例如使用fcntl库的fcntl函数进行设置。具体实现可以参考下面的代码示例：

import select
import os
import subprocess
import time
import fcntl

args = ['python','./fetch_file2.py',ip,path]
proc = subprocess.Popen(args, stdout=subprocess.PIPE,stderr=subprocess.PIPE,close_fds=True)

def non_block_read(output): # 避免阻塞
    fd = output.fileno()
    fl = fcntl.fcntl(fd, fcntl.F_GETFL)
    fcntl.fcntl(fd, fcntl.F_SETFL, fl | os.O_NONBLOCK)
    try:
        return output.read()
    except:
        return ""

while proc.poll() is None: #fetch中rsync结束。但是fetch没有结束(怀疑输出过大) 导致这里一直是None
    pass

print proc.poll() # 杀死fetch进程 返回-9
print proc.stderr.read() #阻塞#方法1：
#non_block_read(proc.stderr) #防止阻塞#方法2：
select_rfds = [ proc.stdout, proc.stderr]
(rfds, wfds, efds) = select.select(select_rfds, [],[])
if proc.stderr in rfds: #不存在。若select_rfds=[stderr],则阻塞在select上
    len = proc.stderr.read(10)
    if len == 0:
        print "empty"
    else:
        print "proc.stderr"
if proc.stdout in rfds:
    print "proc.stdout"

通过设置文件描述符为非阻塞模式，可以避免阻塞问题，并且使用select.select来检查子进程的标准输出是否可读。 这样即可解决subprocess.Popen阻塞的问题。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
#### 引用[.reference_title]
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