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1#encoding=utf-8
2import os
3import time
4import errno
5
6class FileLockException(Exception):
7    pass
8
9class FileLock(object):
10    """ A file locking mechanism that has context-manager support so
11        you can use it in a with statement. This should be relatively cross
12        compatible as it doesn't rely on msvcrt or fcntl for the locking.
13    """
14
15
16    def __init__(self, timeout=7200, delay=30):
17        """ Prepare the file locker. Specify the file to lock and optionally
18            the maximum timeout and the delay between each attempt to lock.
19        """
20        self.is_locked = False
21        self.timeout = timeout
22        self.delay = delay
23
24    def _setLockFileName(self, file_name):
25        self.lockfile = file_name
26
27    def acquire(self):
28        """ Acquire the lock, if possible. If the lock is in use, it check again
29            every `wait` seconds. It does this until it either gets the lock or
30            exceeds `timeout` number of seconds, in which case it throws
31            an exception.
32        """
33        start_time = time.time()
34        if os.path.isfile(self.lockfile):
35            try:
36                mark_file_mtime = os.path.getmtime(self.lockfile)
37                if (start_time - mark_file_mtime > self.timeout):
38                    os.remove(self.lockfile)
39            except Exception as e:
40                print("the lock file is locked by other process")
41
42        while True:
43            try:
44                #open file , other application can't open it
45                self.fd = os.open(self.lockfile, os.O_CREAT|os.O_EXCL|os.O_RDWR)
46                break
47            except OSError as e:
48                if e.errno != errno.EEXIST:
49                    raise
50                if (time.time() - start_time) >= self.timeout:
51                    raise FileLockException("Timeout occured.")
52                time.sleep(self.delay)
53        self.is_locked = True
54
55
56    def release(self):
57        """ Get rid of the lock by deleting the lockfile.
58            When working in a `with` statement, this gets automatically
59            called at the end.
60        """
61        #
62        if self.is_locked :
63            os.close(self.fd)
64            os.unlink(self.lockfile)
65            self.is_locked = False
66
67    def lockFile(self, file_name):
68        """ Activated when used in the with statement.
69            Should automatically acquire a lock to be used in the with block.
70        """
71        self._setLockFileName(file_name)
72        if not self.is_locked:
73            self.acquire()
74        return self
75
76    def releaseFile(self):
77        """ Activated at the end of the with statement.
78            It automatically releases the lock if it isn't locked.
79        """
80        if self.is_locked:
81            self.release()
82
83    def __del__(self):
84        """ Make sure that the FileLock instance doesn't leave a lockfile
85            lying around.
86        """
87        self.release()
88
89
90
91#用法比较有意思，使用with关键字。对with关键字来说，FileLock类先执行__enter__函数，然后，执行with块里的那些代码，执行完了之后，再执行__exit__函数，等价于相当于如下形式：
92#try:
93#    执行 __enter__的内容
94#    执行 with_block.
95#finally:
96#    执行 __exit__内容
97#FileLock在__enter__函数独占式创建或打开一个文件，这个文件不会被其他程序或者进程再次创建或者打开，由此形成lock，执行完代码，在__exit__里，关闭并删除文件