> I just thought of a clever way to do it without alarm!
So clever, it doesn't work!
> lock_file($foo);
> open(FH, $foo);
> ok( !flock(FH, LOCK_NB | LOCK_EX) );
Seriously, on most unix systems, the following:
flock(FH, LOCK_EX);
flock(FH, LOCK_EX|LOCK_NB) or die;
doe *not* die. A process is allowed to ask for (and obtain) an
exclusive lock on a file if it already has a lock on that same file.
There's no deadlock at all.
I just checked this under linux and solaris with:
use Fcntl ':flock';
open F, "+< $0: or die $!;
flock F, LOCK_EX or die $!;
flock F, LOCK_EX|LOCK_NB or die $!;
print "OK\n";
and it OK's on both. Similarly,
use Fcntl ':flock';
open F, "+< $0" or die $!;
open G, "+< $0" or die $!;
flock F, LOCK_EX or die $!;
flock G, LOCK_EX|LOCK_NB or die $!;
print "OK\n";
This failus under Linux (which I think is wrong) but OK's on Solaris.
"OK" here means "What you suggested won't work."
> The traditional approaches require causing a deadlock and breaking it
> with alarm.
Perhaps the traditional approach doesn't work. :)
> So here's the other way. We cause a deadlock using another process
> which kills itself after a certain amount of time.
>
> my $start = time;
> lock_file($foo);
> system($^X, '-e', 'use Whatever; alarm 5; lock_file($foo)');
> cmp_ok( time, '>', $start + 3, 'Locking works' );
>
> again, these all rely on alarm().
This on also relies on system creating a new process. I wasn't sure
what this would do on Win32, VMS, OS390, etc. Hence my original
question:
>> What is an easy way to check that the file is actually being locked?
>> I put off writing the tests because i did not want to fork.
Sorry to be so elliptical. What I should have said was that I can
think of lots of ways to do it but they all involve starting a new
process, and I'm reluctant to start a new prcoess unless I have to,
because I have no idea what it does on Weird Platform X.
