Process based locks for files

ruby-talk
1

Hi,

I have two programs accessing the same file. Both programs open and
close the file multiple times while running (for example reading it
first, and then writing it back using global functions).

In order to avoid race conditions i was wondering if it's possible to
lock the file at the start of the running process and then unlock it at
the end. During that time the running process should be able to open and
close the file without problems.

Thanks,
Radu Spineanu

2

Radu Spineanu wrote:

Hi,

I have two programs accessing the same file. Both programs open and
close the file multiple times while running (for example reading it
first, and then writing it back using global functions).

In order to avoid race conditions i was wondering if it's possible to
lock the file at the start of the running process and then unlock it at
the end. During that time the running process should be able to open and
close the file without problems.

There's File#flock, but keep in mind that flock is only an advisory lock
(i.e. it only notifies cooperating processes that try and get a lock
themselves) and is notoriously flakey on NFS mounts.

···

--
Posted via http://www.ruby-forum.com/\.

3

Radu Spineanu wrote:

Hi,

I have two programs accessing the same file. Both programs open and
close the file multiple times while running (for example reading it
first, and then writing it back using global functions).

In order to avoid race conditions i was wondering if it's possible to
lock the file at the start of the running process and then unlock it at
the end. During that time the running process should be able to open and
close the file without problems.

If both programs are willing to cooperate with each other, why not use a
flag file, as is often done to lock and unlock various resources in Linux?
This approach has the advantage of being platform-neutral and doesn't
require any explicit file-locking facilities.

···

-----------------------------------------

#!/usr/bin/ruby -w

flag_file = "FLAGFILE"

puts "Waiting for access ..."
while FileTest.exists? flag_file
   sleep 1
end

puts "Creating flag file ..."
ff = File.open(flag_file,"w") {}

puts "Processing ..."
sleep 5

puts "Removing flag file ..."
File.delete flag_file

-----------------------------------------

There is an obvious collision possibility in this arrangement, that may or
may not be an issue.

--
Paul Lutus
http://www.arachnoid.com

4

I don't see a mention of your platform. However, you might consider Ara Howard's lockfile.rb. If is an NFS safe file lock library that works well. It just doesn't work on Windows.

If you need something that can work on both Windows and not-windows, email me. I have a hacked and a bit simplified version that will autodetect whether the primary locking scheme will work or not, and will fall back to flock based locking. It remembers the result of the autodetection so subsequent locks opened in the same process later don't incur the cost of detection again.

Ara's library works well, and I believe he uses it extensively.

My hacked version probably works less well because it is an undertested ugly hack, but in my (less extensive) use of it, I haven't encountered any problems so far.

Find his lockfile.rb here:

http://codeforpeople.com/lib/ruby/lockfile/

Kirk Haines

···

On Wed, 30 Aug 2006, Mike Fletcher wrote:

Radu Spineanu wrote:

Hi,

I have two programs accessing the same file. Both programs open and
close the file multiple times while running (for example reading it
first, and then writing it back using global functions).

In order to avoid race conditions i was wondering if it's possible to
lock the file at the start of the running process and then unlock it at
the end. During that time the running process should be able to open and
close the file without problems.

There's File#flock, but keep in mind that flock is only an advisory lock
(i.e. it only notifies cooperating processes that try and get a lock
themselves) and is notoriously flakey on NFS mounts.

5

plus, it fails horriblly on nfs. this does all that and much, much more -
including preventing stale lock starvation

   http://codeforpeople.com/lib/ruby/lockfile/lockfile-1.4.0/README
   http://codeforpeople.com/lib/ruby/lockfile/lockfile-1.4.0/
   http://rubyforge.org/frs/?group_id=1024&release_id=3473

gen install lockfile

regards.

-a

···

On Wed, 30 Aug 2006, Paul Lutus wrote:

Radu Spineanu wrote:

Hi,

I have two programs accessing the same file. Both programs open and
close the file multiple times while running (for example reading it
first, and then writing it back using global functions).

In order to avoid race conditions i was wondering if it's possible to
lock the file at the start of the running process and then unlock it at
the end. During that time the running process should be able to open and
close the file without problems.

If both programs are willing to cooperate with each other, why not use a
flag file, as is often done to lock and unlock various resources in Linux?
This approach has the advantage of being platform-neutral and doesn't
require any explicit file-locking facilities.

-----------------------------------------

#!/usr/bin/ruby -w

flag_file = "FLAGFILE"

puts "Waiting for access ..."
while FileTest.exists? flag_file
  sleep 1
end

puts "Creating flag file ..."
ff = File.open(flag_file,"w") {}

puts "Processing ..."
sleep 5

puts "Removing flag file ..."
File.delete flag_file

-----------------------------------------

There is an obvious collision possibility in this arrangement, that may or
may not be an issue.

--
what science finds to be nonexistent, we must accept as nonexistent; but what
science merely does not find is a completely different matter... it is quite
clear that there are many, many mysterious things.
- h.h. the 14th dalai lama

6

Hi ara,

···

ara.t.howard@noaa.gov wrote:

plus, it fails horriblly on nfs. this does all that and much, much more -
including preventing stale lock starvation

  http://codeforpeople.com/lib/ruby/lockfile/lockfile-1.4.0/README
  http://codeforpeople.com/lib/ruby/lockfile/lockfile-1.4.0/
  http://rubyforge.org/frs/?group_id=1024&release_id=3473

gen install lockfile

I'm looking into this, it seems to be what I am looking for.

Thanks,
Radu Spineanu