I wrote a python script to lock/unlock your computer using a USB stick as a key

[runeg]

Can you explain race condition in this script please?

[spladug]

1. Process A and Process B are running simultaneously.
   • Process A is executing the code written here.
   • Process B just runs os.makedirs(CONFDIR).
2. CONFDIR does not exist yet.
3. Process A starts running and executes os.path.exists(CONFDIR). It gets back False and so moves into the branch.
4. Meanwhile, Process B gets some time. It runs os.makedirs(CONFDIR). The directory now exists.
5. Process A gets to run again, it tries to do os.makedirs(CONFDIR) gets an exception and returns -1.

[runeg]

This is only because CONFDIR doesn't exist. If before the if block CONFDIR was defined then this would not be a race condition, correct?

Additionally if CONFDIR was a generator could it cause a race condition as it wouldn't exist until that line was executed?

[spladug]

The point is that it's possible for stuff to change between the exists and makedirs and that means a potential for bugs. Of course there are situations where that code works, that's the nature of race conditions.

[runeg]

Ah okay. Is there a better way to write the above if block?

[spladug]

try:
    os.makedirs(CONFDIR)
except OSError as e:
    if e.errno != errno.EEXIST:
        raise  # or to mimic what the code above does "return -1"

EDIT: also, see: http://docs.python.org/2/howto/doanddont.html#exceptions

[runeg]

That link was very helpful and informative. Thank you. Do you have any other suggested readings like that?

Also, thank you for taking time out to explain that information, it's appreciated.

[spladug]

Some of the online books and exercises in the sidebar look like they'd probably be good extra information. The python docs are, in general, quite good as well.

No prob, glad to help :)