I wrote a subclass of the file that: a) provides methods for conveniently blocking it (using fcntl, so it only supports unix, which however suits me atm) and b) when reading or writing claims that the file is properly locked.
Now I am not an expert in such things (I just read one document [de] about this) and some feedback would be appreciated: is it safe, are there race conditions, are there other things that could be done better ... Here the code:
from fcntl import flock, LOCK_EX, LOCK_SH, LOCK_UN, LOCK_NB
class LockedFile(file):
"""
A wrapper around `file` providing locking. Requires a shared lock to read
and a exclusive lock to write.
Main differences:
* Additional methods: lock_ex, lock_sh, unlock
* Refuse to read when not locked, refuse to write when not locked
exclusivly.
* mode cannot be `w` since then the file would be truncated before
it could be locked.
You have to lock the file yourself, it won't be done for you implicitly.
Only you know what lock you need.
Example usage::
def get_config():
f = LockedFile(CONFIG_FILENAME, 'r')
f.lock_sh()
config = parse_ini(f.read())
f.close()
def set_config(key, value):
f = LockedFile(CONFIG_FILENAME, 'r+')
f.lock_ex()
config = parse_ini(f.read())
config[key] = value
f.truncate()
f.write(make_ini(config))
f.close()
"""
def __init__(self, name, mode='r', *args, **kwargs):
if 'w' in mode:
raise ValueError('Cannot open file in `w` mode')
super(LockedFile, self).__init__(name, mode, *args, **kwargs)
self.locked = None
def lock_sh(self, **kwargs):
"""
Acquire a shared lock on the file. If the file is already locked
exclusively, do nothing.
:returns: Lock status from before the call (one of 'sh', 'ex', None).
:param nonblocking: Don't wait for the lock to be available.
"""
if self.locked == 'ex':
return
return self._lock(LOCK_SH, **kwargs)
def lock_ex(self, **kwargs):
"""
Acquire an exclusive lock on the file.
:returns: Lock status from before the call (one of 'sh', 'ex', None).
:param nonblocking: Don't wait for the lock to be available.
"""
return self._lock(LOCK_EX, **kwargs)
def unlock(self):
"""
Release all locks on the file.
Flushes if there was an exclusive lock.
:returns: Lock status from before the call (one of 'sh', 'ex', None).
"""
if self.locked == 'ex':
self.flush()
return self._lock(LOCK_UN)
def _lock(self, mode, nonblocking=False):
flock(self, mode | bool(nonblocking) * LOCK_NB)
before = self.locked
self.locked = {LOCK_SH: 'sh', LOCK_EX: 'ex', LOCK_UN: None}[mode]
return before
def _assert_read_lock(self):
assert self.locked, "File is not locked"
def _assert_write_lock(self):
assert self.locked == 'ex', "File is not locked exclusively"
def read(self, *args):
self._assert_read_lock()
return super(LockedFile, self).read(*args)
def readline(self, *args):
self._assert_read_lock()
return super(LockedFile, self).readline(*args)
def readlines(self, *args):
self._assert_read_lock()
return super(LockedFile, self).readlines(*args)
def xreadlines(self, *args):
self._assert_read_lock()
return super(LockedFile, self).xreadlines(*args)
def __iter__(self):
self._assert_read_lock()
return super(LockedFile, self).__iter__()
def next(self):
self._assert_read_lock()
return super(LockedFile, self).next()
def write(self, *args):
self._assert_write_lock()
return super(LockedFile, self).write(*args)
def writelines(self, *args):
self._assert_write_lock()
return super(LockedFile, self).writelines(*args)
def flush(self):
self._assert_write_lock()
return super(LockedFile, self).flush()
def truncate(self, *args):
self._assert_write_lock()
return super(LockedFile, self).truncate(*args)
def close(self):
self.unlock()
return super(LockedFile, self).close()
(the example in docstring is also my current use case)
Thank you for reading until here below, and may not even have answered :)