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comparison lib/python/lock.py @ 87:b6a5999d8e06

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working with locking and copying
author Henry S. Thompson <ht@inf.ed.ac.uk>
date Tue, 16 Mar 2021 16:20:02 +0000
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86:b5fef78cbb26 87:b6a5999d8e06
1 # Courtesy of https://stackoverflow.com/a/46407326
2 import fcntl, os
3 def lock_file(f):
4 if f.writable(): fcntl.lockf(f, fcntl.LOCK_EX)
5 def unlock_file(f):
6 if f.writable(): fcntl.lockf(f, fcntl.LOCK_UN)
7
8 # Class for ensuring that all file operations are atomic, treat
9 # initialization like a standard call to 'open' that happens to be atomic.
10 # This file opener *must* be used in a "with" block.
11 class AtomicOpen:
12 # Open the file with arguments provided by user. Then acquire
13 # a lock on that file object (WARNING: Advisory locking).
14 def __init__(self, path, *args, **kwargs):
15 # Open the file and acquire a lock on the file before operating
16 self.file = open(path,*args, **kwargs)
17 # Lock the opened file
18 lock_file(self.file)
19
20 # Return the opened file object (knowing a lock has been obtained).
21 def __enter__(self, *args, **kwargs): return self.file
22
23 # Unlock the file and close the file object.
24 def __exit__(self, exc_type=None, exc_value=None, traceback=None):
25 # Flush to make sure all buffered contents are written to file.
26 self.file.flush()
27 os.fsync(self.file.fileno())
28 # Release the lock on the file.
29 unlock_file(self.file)
30 self.file.close()
31 # Handle exceptions that may have come up during execution, by
32 # default any exceptions are raised to the user.
33 if (exc_type != None): return False
34 else: return True