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annotate lib/python/lock.py @ 134:d3ef00af2064

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add usage/help info
author Henry S. Thompson <ht@inf.ed.ac.uk>
date Wed, 14 Jul 2021 16:49:54 +0000
parents b6a5999d8e06
children
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1 # Courtesy of https://stackoverflow.com/a/46407326
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2 import fcntl, os
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3 def lock_file(f):
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4 if f.writable(): fcntl.lockf(f, fcntl.LOCK_EX)
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5 def unlock_file(f):
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6 if f.writable(): fcntl.lockf(f, fcntl.LOCK_UN)
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7
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8 # Class for ensuring that all file operations are atomic, treat
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9 # initialization like a standard call to 'open' that happens to be atomic.
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10 # This file opener *must* be used in a "with" block.
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11 class AtomicOpen:
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12 # Open the file with arguments provided by user. Then acquire
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13 # a lock on that file object (WARNING: Advisory locking).
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14 def __init__(self, path, *args, **kwargs):
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15 # Open the file and acquire a lock on the file before operating
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16 self.file = open(path,*args, **kwargs)
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17 # Lock the opened file
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18 lock_file(self.file)
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19
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20 # Return the opened file object (knowing a lock has been obtained).
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21 def __enter__(self, *args, **kwargs): return self.file
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22
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23 # Unlock the file and close the file object.
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24 def __exit__(self, exc_type=None, exc_value=None, traceback=None):
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25 # Flush to make sure all buffered contents are written to file.
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26 self.file.flush()
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27 os.fsync(self.file.fileno())
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28 # Release the lock on the file.
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29 unlock_file(self.file)
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30 self.file.close()
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31 # Handle exceptions that may have come up during execution, by
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32 # default any exceptions are raised to the user.
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33 if (exc_type != None): return False
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34 else: return True