Release of flock in case of errors? - linux

Question

Imagine the following Perl code (here in pseudo code):
successfully acquired flock for FILEHANDLER # line 1
some error or maybe simply a call to exit() # line 2
close FILEHANDLER (which also releases the lock) # line 3
In this case I wouldn't release the lock, as the Perl script ends in line 2. In that case, is the lock ever released by the operating system? Does it see "hey, the script that acquired the lock crashed" and release the lock? Does it release the lock immediately? Also, is there one Perl instance running for each script, so that it's clear which script crashed/stopped without releasing the lock?

Answer 1

In that case, is the lock ever released by the operating system?
Does it see "hey, the script that acquired the lock crashed" and release the lock?
Does it release the lock immediately?
All of these questions are system dependent. Perl 5 does not implement a file locking function, it just provides a common interface to flock(2), fcntl(2) locking, or lockf(3) (depending on what is available in the OS). There may also be a difference between what happens when a program exits, segfaults, or is killed with a sigkill.
A quick test under Linux shows that a lock is removed under normal exit conditions:
$ perl -le 'open my $fh, ">", "f" or die $!; print flock($fh, 6) ? "got lock" : "was already locked", "\n"'
got lock
$ perl -le 'open my $fh, ">", "f" or die $!; print flock($fh, 6) ? "got lock" : "was already locked", "\n"'
got lock
Let's see what happens when we die:
$ perl -le 'open my $fh, ">", "f" or die $!; print flock($fh, 6) ? "got lock" : "was already locked", "\n"; die "died"'
got lock
died at -e line 1.
$ perl -le 'open my $fh, ">", "f" or die $!; print flock($fh, 6) ? "got lock" : "was already locked", "\n"; die "died"'
got lock
died at -e line 1.
To get a segfault, we will need access to C, I am using Inline to get it:
$ cat segfault.pl
#!/usr/bin/perl
use strict;
use warnings;
use Inline "C";
open my $fh, ">", "f" or die $!;
print flock($fh, 6) ? "got lock" : "was already locked", "\n";
crash();
__DATA__
__C__
void crash() {
int* ptr = NULL;
*ptr = 5;
}
$ perl segfault.pl
got lock
Segmentation fault
$ perl segfault.pl
got lock
Segmentation fault
And finally, here is what happens when a program is sent SIGKILL:
$ cat fork.pl
#!/usr/bin/perl
use strict;
use warnings;
$SIG{CHLD} = "IGNORE"; #auto-reap children
die "could not fork: $!" unless defined(my $pid = fork);
unless ($pid) {
#child
open my $fh, ">", "f" or die $!;
print flock($fh, 6) ? "got lock" : "was already locked", "\n";
sleep(100);
exit;
}
kill 9, $pid;
die "could not fork: $!" unless defined($pid = fork);
unless ($pid) {
#child
open my $fh, ">", "f" or die $!;
print flock($fh, 6) ? "got lock" : "was already locked", "\n";
exit;
}
$ perl fork.pl
got lock
got lock
From these experiments, we can see that the lock is released in Linux for each of the cases you were concerned with.
Also, is there one perl instance running for each script, so that it's clear which script crashed/stopped without releasing the lock?
Yes, Perl 5 has one perl process per script. Even if you fork, the child gets its own perl process. Threading does not provide a separate perl process.
Note: if a parent process gets a lock and does not give it up before locking, then the child will have the same lock even if the parent exits.

Answer 2

When the program exits, the OS automatically releases all locks acquired by the program and closes all files opened by the program.