[lug] Interesting Common File Locking Problem

D. Stimits stimits at comcast.net
Fri Mar 5 19:20:47 MST 2004

Zan Lynx wrote:
> I spent some time figuring this one out.  I spent a few hours scratching
> my head over this one.  I thought I would share for any of you
> programmers out there.  Also, I just have to vent the frustration build
> up to _someone_, or it'll bother me all weekend!  Heh.
> Some programs appear to lock files using this method:
> - open the file, get a file descriptor.
> - lock the file descriptor.
> - write data into a temporary file.
> - optionally fsync the data to ensure it is really in there.
> - rename temporary into real file name, which deletes the original,
> which removes the lock.
> At first glance, this looks reasonable and safe.  What I discovered is
> that this can happen:
> - ProgA opens the file, get a file descriptor.
> - ProgB opens the file, gets a file descriptor.
> - ProgB locks the file descriptor.  Now ProgA is waiting...
> - ProgB creates temporary file, writes data into it.
> - ProgB now renames temporary to real name.
> - ProgB unlocks, closes, exits, etc.
> - ProgA finally gets its lock!  Yay for ProgA!
> But wait!  What lock is this!?
> Could it be a lock on the now removed original file!?  The file ProgB
> just deleted by renaming over it?
> Why Yes!  It could!
> Does anything stop ProgC from coming along and getting a lock on the
> file name?
> Why No, nothing does!  Because ProgA's lock is on a file that no longer
> has that name!  In fact, ProgA's locked file no longer has any name!
> And then does it stop there?  No indeed!  Because ProgC has the lock on
> the file name, it assumes no one else is using the file.  Now, using the
> same temporary file name ProgA is also using, ProgC goes ahead to
> truncate and write into the temporary file.  What does this lead to?  Me
> getting large chunks of zeros in my mail client spool file.

You just described a textbook example from a thread programming book. 
Sounds like you need a mutex/semaphore system built into the filesystem 
itself. It might be interesting to see how journals in various 
journaling filesystems do it.

D. Stimits, stimits AT comcast DOT net

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