Amir Michail wrote:
Hi,
Trying to open a file for writing that is already open for writing
should result in an exception.
Look at fcntl module, I use it in a class to control access from within my processes.
I don't think this functionality should be inherent to python though.
Keep in mind only my processes open the shelve db so your mileage may vary.
get and set methods are just for convenience
This works under linux, don't know about windows.
#!/usr/bin/env python
import fcntl, shelve, time, bsddb
from os.path import exists
class fLocked:
def __init__(self, fname):
if exists(fname):
#verify it is not corrupt
bsddb.db.DB().verify(fname)
self.fname = fname
self.have_lock = False
self.db = shelve.open(self.fname)
self.fileno = self.db.dict.db.fd()
def __del__(self):
try: self.db.close()
except: pass
def aquire_lock(self, timeout = 5):
if self.have_lock: return True
started = time.time()
while not self.have_lock and (time.time() - started < timeout):
try:
fcntl.flock(self.fileno, fcntl.LOCK_EX + fcntl.LOCK_NB)
self.have_lock = True
except IOError:
# wait for it to become available
time.sleep(.5)
return self.have_lock
def release_lock(self):
if self.have_lock:
fcntl.flock(self.fileno, fcntl.LOCK_UN)
self.have_lock = False
return not self.have_lock
def get(self, key, default = {}):
if self.aquire_lock():
record = self.db.get(key, default)
self.release_lock()
else:
raise IOError, "Unable to lock %s" % self.fname
return record
def set(self, key, value):
if self.aquire_lock():
self.db[key] = value
self.release_lock()
else:
raise IOError, "Unable to lock %s" % self.fname
if __name__ == '__main__':
fname = 'test.db'
dbs = []
for i in range(2): dbs.append(fLocked(fname))
print dbs[0].aquire_lock()
print dbs[1].aquire_lock(1) #should fail getting flock
dbs[0].release_lock()
print dbs[1].aquire_lock() #should be able to get lock
--Tim
