Python fcntl未能按预期锁定 [英] Python fcntl does not lock as expected

问题描述

在基于Debian的操作系统(Ubuntu，Debian Squeeze)上，我正在使用Python(2.7，3.2)fcntl锁定文件.据我了解，fnctl.flock以某种方式锁定文件，如果另一个客户端想要锁定同一文件，则会抛出异常.

我构建了一个小示例，由于我首先锁定了文件，然后紧接着尝试再次将其锁定，因此我希望抛出一个exepiton:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import fcntl
fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX)
try:
    fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError:
    print("can't immediately write-lock the file ($!), blocking ...")
else:
    print("No error")

但是该示例仅显示无错误".

如果我将此代码拆分为同时运行的两个客户端(一个锁定然后等待，另一个在第一个锁定已激活后尝试锁定)，我将得到相同的行为-完全无效./p>

对此行为有何解释?

编辑:

根据Nightcracker的要求进行更改，该版本也显示无错误"，尽管我不希望这样:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import fcntl
import time
fcntl.flock(open('/tmp/locktest', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB)
try:
    fcntl.flock(open('/tmp/locktest', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError:
    print("can't immediately write-lock the file ($!), blocking ...")
else:
    print("No error")

解决方案

知道了.我的脚本中的错误是我在每次调用时都创建了一个新的文件描述符:

fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB)
(...)
fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB)

相反，我必须将文件对象分配给变量，然后尝试锁定:

f = open('/tmp/locktest', 'r')
fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
(...)
fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)

比我也想看到的异常:IOError: [Errno 11] Resource temporarily unavailable.现在，我必须考虑在哪种情况下使用fcntl才有意义.

On a Debian-based OS (Ubuntu, Debian Squeeze), I'm using Python (2.7, 3.2) fcntl to lock a file. As I understand from what I read, fnctl.flock locks a file in a way, that an exception will be thrown if another client wants to lock the same file.

I built a little example, which I would expect to throw an excepiton, since I first lock the file, and then, immediately after, I try to lock it again:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import fcntl
fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX)
try:
    fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError:
    print("can't immediately write-lock the file ($!), blocking ...")
else:
    print("No error")

But the example just prints "No error".

If I split this code up to two clients running at the same time (one locking and then waiting, the other trying to lock after the first lock is already active), I get the same behavior - no effect at all.

Whats the explanation for this behavior?

EDIT:

Changes as requested by nightcracker, this version also prints "No error", although I would not expect that:

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import fcntl
import time
fcntl.flock(open('/tmp/locktest', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB)
try:
    fcntl.flock(open('/tmp/locktest', 'w'), fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError:
    print("can't immediately write-lock the file ($!), blocking ...")
else:
    print("No error")

解决方案

Got it. The error in my script is that I create a new file descriptor on each call:

fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB)
(...)
fcntl.flock(open('/tmp/locktest', 'r'), fcntl.LOCK_EX | fcntl.LOCK_NB)

Instead, I have to assign the file object to a variable and than try to lock:

f = open('/tmp/locktest', 'r')
fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
(...)
fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)

Than I'm also getting the exception I wanted to see: IOError: [Errno 11] Resource temporarily unavailable. Now I have to think about in which cases it makes sense at all to use fcntl.

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