Preceptor has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks, I'm hoping this isn't too stupid a question. I have been trying to test (for a new system) locking files on NFS. My problem is - I'm having a real difficulty untangling what is a good way of accomplishing it - I've tracked down fcntl, and flock.

So is anyone able to give me a pointer in how this is done 'right'? All I really need is to be able to create and lock a test file (twice, so I can check if the lock is 'working'), and can run on multiple instances/hosts. I am aware that this is implementation dependent - but this is as much to test if I _can_ on our implementation, as anything else.

Thus far, I've got as my tests:

#!/usr/bin/env perl
use strict;
use warnings;

use Fcntl qw ( :flock ); 

open ( my $lock_this, '>', 'lockfile_flock' ) or die $!;

print "Enter to lock\n"; <>; 
print "Requesting EX lock\n"; 
flock ( $lock_this, LOCK_EX ); 
print "Lock acquired\n"; 
print "Enter to release\n"; <>; 
flock ( $lock_this, LOCK_UN ); 
print "Lock released\n";

[download]

I'm trying to do the same with fcntl, but am being flummoxed a bit by references to pack and flags. Is anyone able to give me an example of how using "fcntl" to lock a file is supposed to look in perl?

Replies are listed 'Best First'.
Re: NFS locking with Fcntl
by Preceptor (Deacon) on May 10, 2016 at 11:25 UTC

    Of course, shortly after posting - I found the answer, in this answer

    use strict;
use Fcntl qw(SEEK_SET F_WRLCK F_UNLCK F_SETLKW);

my($pack);


open(FILE,">a");
$pack = pack('s s l l s', F_WRLCK, SEEK_SET, 0, 1, 0);
print(fcntl(FILE, F_SETLKW, $pack) . "\n");
sleep(20);
$pack = pack('s s l l s', F_UNLCK, SEEK_SET, 0, 1, 0);
print(fcntl(FILE, F_SETLKW, $pack) . "\n");
close(FILE);

    [download]

    This worked for me on my Linux implementation, to test whether locking over NFS works.

[reply]
[d/l]
[reply]

        Nor I. However, my requirement isn't to use it, it's to test whether it actually works at all. Further test may follow.

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Re: NFS locking with Fcntl
by taint (Chaplain) on May 10, 2016 at 13:19 UTC
    Greetings, Preceptor.

    While I've never been particularly excited about the way the nfs protocol is implemented. It's your project. :)

    So I asked cpan about fcntl. Given the description: "Set / reset locks using fcntl", IPC::SRLock::Fcntl might be a good candidate. If you're implementing this on UNIX, or a UNIX-like system, POSIX::1003::Fcntl might also be of value. If for no other reason, than to provide some insight into the inner workings of the related functions. Lastly, perlfunc would be a good reference for you. :)

    --Chris

    ¡λɐp ʇɑəɹ⅁ ɐ əʌɐɥ puɐ ʻꜱdləɥ ꜱᴉɥʇ ədoH

[reply]

      Well, no, it isn't. It's my storage array, and I'm being asked to test locking functionality. I know locking in general (and NFS specifically) are a bit of a 'fuzzy area' when it comes to file locking. And there's huge caveats on the relative interoperability. I'm trying to avoid specific perl modules, simply because this _should_ be language independent. I don't mind knocking up my test cases in C, if that's necessary - but I thought it _should_ be possible using perl built in hooks into C calls.

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        It's only going to ever be, as good as the OS's implementation of it. So you're mileage will vary greatly, depending on what, and where it's implemented. Version is also a variable to consider. They're all based on the BSD/SUN implementation (links to follow(1, 2)) which dates back to the late 70's to the early 80's, and hasn't changed much, since then. My point being; your results won't have absolute returns -- they all pretty much suck. The client side/implementation is also a factor. Are/will you be testing on/for that, as well?

        The references I provided were just as good for use as modules, as they were for references for a better understanding of the underlying protocol(s). You could have just as easily "cobbled" up something that suites your needs using the code/references therein.

        You also might be interested in mmap for more efficient use of reading/writing of data from/to memory, from disk.

        --Chris

        1) unix history repo
        2) berkeley software distribution

        ¡λɐp ʇɑəɹ⅁ ɐ əʌɐɥ puɐ ʻꜱdləɥ ꜱᴉɥʇ ədoH

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Re: NFS locking with Fcntl
by mr_mischief (Monsignor) on May 10, 2016 at 22:04 UTC

    What exactly is wrong with File::NFSLock that you have to find an alternative?

    What type of locking do you need or want? Do you want atomic, mandatory locking? Do you want advisory locking for a single application?

    Why are you using a filesystem that's long been known to have problems with this sort of thing when there are many shared filesystems and clustered, more reliable filesystems that don't?

[reply]
      +1, mr_mischief.

      Thanks for saying that, as it got me to thinking that maybe Preceptor might be even better off trying something like Fuse. I think it matches his requirements to-the-letter. Albeit completely without nfs; it's written in Perl, and is OS agnostic. Well tested, and requires little effort on his part.

      Seems a perfect fit! :)

      --Chris

      ¡λɐp ʇɑəɹ⅁ ɐ əʌɐɥ puɐ ʻꜱdləɥ ꜱᴉɥʇ ədoH

[reply]

        There's alternatives, but my actual requirement was to verify whether 'filesystem locking' works on NFS. Which is really vague, I know. But that was why I was seeking a system call based option, as that gives me a basis to say "it works" in at least an approximately language agnostic way.

        FWIW - on a 2.6.12+ Linux kernel, both flock and fcntl work. You will still have platform specific issues doing it regardless, but that isn't a new storage when it comes to NFS.

[reply]

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