python 文件锁simpleflock fcntl

1、假设场景：

执行某段代码时，产生了一些不想要的结果（称为A），我们会在之后将A修改成想要的结果B；同时，有一个定期监控的流程，一直在读取产生的结果，我们的意图是读取实时读取最新的B，但是有那么一个时刻将A读走，导致错误发生

2、解决办法：

使用文件锁，将产生A以及修改成B的过程加写锁，定期监控加读锁，资源被修改成B的代码拿走时，无法监控读取取

3、代码实现

3.1. simpleflock.py文件锁实现

import time
import os
import fcntl
import errno

class SimpleFlock:
    """Provides the simplest possible interface to flock-based file locking. Intended for use with the `with` syntax. It will create/truncate/delete the lock file as necessary."""

    def __init__(self, path, timeout = None):
        self._path = path
        self._timeout = timeout
        self._fd = None

    def __enter__(self):
        self._fd = os.open(self._path, os.O_CREAT)
        start_lock_search = time.time()
        while True:
            try:
                fcntl.flock(self._fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
                # Lock acquired!
                return
            except IOError, ex:
                if ex.errno != errno.EAGAIN: # Resource temporarily unavailable
                    raise
                elif self._timeout is not None and time.time() > (start_lock_search + self._timeout):
                    # Exceeded the user-specified timeout.
                    raise

            # TODO It would be nice to avoid an arbitrary sleep here, but spinning
            # without a delay is also undesirable.
            time.sleep(0.1)

    def __exit__(self, *args):
        fcntl.flock(self._fd, fcntl.LOCK_UN)
        os.close(self._fd)
        self._fd = None
        # print 'come to exit'
        # Try to remove the lock file, but don't try too hard because it is
        # unnecessary. This is mostly to help the user see whether a lock
        # exists by examining the filesystem.
        try:
            os.unlink(self._path)
        except:
            pass

if __name__ == "__main__":
    print "Acquiring lock..."
    with SimpleFlock("locktest", 2):
        print "Lock acquired."
        time.sleep(3)
    print "Lock released."

3.2 文件锁使用

print "Acquiring lock..."
    with SimpleFlock("locktest", 2):
        print "Lock acquired."
        time.sleep(3)
    print "Lock released."

最后

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