Synchronize processes by locking a file

Question

One of my scripts is installing a component. When run in parallel, the same script tries to install the same component, so I thought about synchronizing the process by locking a file while the script is installing and wait while other script is installing something.

The code would look like this:

# this will create a file handler on a file from TEMP dir with the
# name of the component; if it doesn't exist in TEMP dir, it will create it

my $file = $ENV{"TEMP"}. "\\" . $componentName;
open (my $fh, ">", "$file") or die "Couldn't open file!";

# this will apply an exclusive lock  meaning that if another process
# already locked the file, it will wait until the lock is removed

flock($fh, 2) or die "Failed to lock the file";

# install the component..

# closing the file handle automatically removes the lock
close $fh;

I am concerned about the situation when a script locks the file and is starting the installation and the second script comes and tries to create a file handle on the locked file. I didn't see any errors, but I don't want to miss something.

Will there be a problem with this?

Answer 1

The thing that's important to remember is - the 'open' will work in either case, because that doesn't test the lock. It's the flock operation that will block until the lock is released.

And this should work just fine, although once the lock is released - you might want to check if you still need to run the install, unless you don't really care about doing it twice - e.g. if the rest of the script makes use of/relies upon it.

Also - are there other sources of 'installing' that aren't your script, that could cause the same problem? A lock is an advisory thing.

It would be a style improvement in your program to also:

• Test $ENV{'TEMP'} to see that it exists, and default (or fail) if it doesn't.
• use Fcntl qw ( :flock ); because then you can flock ( $fh, LOCK_EX ); to make it clear you're taking an exclusive lock.
• You appear to be using \\ as a file separator. That's probably better if you used something like File::Spec to do that, for portability reasons.
• You can use a LOCK_NB for nonblocking: flock ( $fh, LOCK_EX | LOCK_NB ) and then just skip if it's locked.