#!/usr/bin/perl

use strict;
use warnings;
use autodie qw( open close );
use Fcntl qw( LOCK_EX LOCK_UN );

sub say
{
        print scalar localtime(),' pid=',$$,': ',@_,"\n";
}

my $mainpid=$$;
my $pid=fork() // die "Can't fork: $!";
# in both parent and child process, install signal handler
$SIG{'ALRM'}=sub { warn 'SIGALRM in ',$$==$mainpid ? 'main' : 'helper' };
if ($pid) {
        # parent process
        say "main starts";
        say "main waiting for helper to lock";
        sleep 1;
        open my $f,'>','tempfile.tmp';
        alarm(5);
        say "main flock";
        flock($f,LOCK_EX) or say "main can't lock: $! (error code ",0+$!,')';
        say "main flock done";
        alarm(0);
        close $f;
        say "main ends";
        wait;
} else {
        # child process
        say "helper starts";
        open my $f,'>','tempfile.tmp';
        flock($f,LOCK_EX) or die "Helper can't lock: $!";
        select(undef,undef,undef,10);
        flock($f,LOCK_UN) or die "Helper can't unlock: $!";
        close $f;
        say "helper ends";
        exit(0);
}


##</code><code>##

>env - perl interrupted.pl
Sun May 24 20:24:11 2015 pid=31389: main starts
Sun May 24 20:24:11 2015 pid=31389: main waiting for helper to lock
Sun May 24 20:24:11 2015 pid=31390: helper starts
Sun May 24 20:24:12 2015 pid=31389: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:24:17 2015 pid=31389: main can't lock: Interrupted system call (error code 4)
Sun May 24 20:24:17 2015 pid=31389: main flock done
Sun May 24 20:24:17 2015 pid=31389: main ends
Sun May 24 20:24:21 2015 pid=31390: helper ends

>env - PERL_SIGNALS=unsafe perl interrupted.pl
Sun May 24 20:24:39 2015 pid=31396: main starts
Sun May 24 20:24:39 2015 pid=31396: main waiting for helper to lock
Sun May 24 20:24:39 2015 pid=31397: helper starts
Sun May 24 20:24:40 2015 pid=31396: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:24:49 2015 pid=31397: helper ends
Sun May 24 20:24:49 2015 pid=31396: main flock done
Sun May 24 20:24:49 2015 pid=31396: main ends

alex@enterprise pts/0 20:24:49
/home/alex/tmp/lockdemo>


##</code><code>##

>env - PERL_SIGNALS=unsafe strace -o trace.txt /usr/bin/perl interrupted.pl
Sun May 24 20:39:02 2015 pid=31774: helper starts
Sun May 24 20:39:02 2015 pid=31773: main starts
Sun May 24 20:39:02 2015 pid=31773: main waiting for helper to lock
Sun May 24 20:39:03 2015 pid=31773: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:39:12 2015 pid=31774: helper ends
Sun May 24 20:39:12 2015 pid=31773: main flock done
Sun May 24 20:39:12 2015 pid=31773: main ends

>grep -C5 flock trace.txt
fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
alarm(5)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:39:03 2015 pid=317"..., 47) = 47
flock(3, LOCK_EX)                       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigaction(SIGALRM, NULL, {0x7fdcf683dde0, [], SA_RESTORER|SA_RESTART, 0x7fdcf5a2f670}, 8) = 0
write(2, "SIGALRM in main at interrupted.p"..., 43) = 43
rt_sigreturn()                          = 73
flock(3, LOCK_EX)                       = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:39:12 2015 pid=317"..., 52) = 52
alarm(0)                                = 0
close(3)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0

>env -  strace -o trace.txt /usr/bin/perl interrupted.pl
Sun May 24 20:43:14 2015 pid=31859: helper starts
Sun May 24 20:43:14 2015 pid=31858: main starts
Sun May 24 20:43:14 2015 pid=31858: main waiting for helper to lock
Sun May 24 20:43:15 2015 pid=31858: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:43:20 2015 pid=31858: main can't lock: Interrupted system call (error code 4)
Sun May 24 20:43:20 2015 pid=31858: main flock done
Sun May 24 20:43:20 2015 pid=31858: main ends
Sun May 24 20:43:24 2015 pid=31859: helper ends

>grep -C5 flock trace.txt
fstat(3, {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
alarm(5)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:43:15 2015 pid=318"..., 47) = 47
flock(3, LOCK_EX)                       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigreturn()                          = -1 EINTR (Interrupted system call)
rt_sigprocmask(SIG_BLOCK, [ALRM], [], 8) = 0
rt_sigaction(SIGALRM, NULL, {0x7f43b0eaede0, [], SA_RESTORER, 0x7f43b00a0670}, 8) = 0
write(2, "SIGALRM in main at interrupted.p"..., 43) = 43

>


##</code><code>##

>env - PERL_SIGNALS=unsafe strace -o trace.txt /usr/bin/perl interrupted.pl
Sun May 24 20:51:14 2015 pid=32083: helper starts
Sun May 24 20:51:14 2015 pid=32082: main starts
Sun May 24 20:51:14 2015 pid=32082: main waiting for helper to lock
Sun May 24 20:51:15 2015 pid=32082: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:51:24 2015 pid=32083: helper ends
Sun May 24 20:51:24 2015 pid=32082: main flock done
Sun May 24 20:51:24 2015 pid=32082: main ends

>grep -C5 SIGALRM trace.txt
close(3)                                = 0
rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], [], 8) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fdbc5fc2a10) = 32083
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
rt_sigprocmask(SIG_BLOCK, [ALRM], [], 8) = 0
rt_sigaction(SIGALRM, {0x7fdbc5ad9de0, [], SA_RESTORER|SA_RESTART, 0x7fdbc4ccb670}, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
open("/etc/localtime", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fdbc5fe6000
--
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
alarm(5)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:51:15 2015 pid=320"..., 47) = 47
flock(3, LOCK_EX)                       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigaction(SIGALRM, NULL, {0x7fdbc5ad9de0, [], SA_RESTORER|SA_RESTART, 0x7fdbc4ccb670}, 8) = 0
write(2, "SIGALRM in main at interrupted.p"..., 43) = 43
rt_sigreturn()                          = 73
flock(3, LOCK_EX)                       = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:51:24 2015 pid=320"..., 52) = 52
alarm(0)                                = 0
--
rt_sigaction(SIGKILL, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGUSR1, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGSEGV, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGUSR2, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGPIPE, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGALRM, NULL, {0x7fdbc5ad9de0, [], SA_RESTORER|SA_RESTART, 0x7fdbc4ccb670}, 8) = 0
rt_sigaction(SIGALRM, {SIG_DFL, [], SA_RESTORER|SA_RESTART, 0x7fdbc4ccb670}, {0x7fdbc5ad9de0, [], SA_RESTORER|SA_RESTART, 0x7fdbc4ccb670}, 8) = 0
rt_sigaction(SIGTERM, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGSTKFLT, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGCONT, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGSTOP, NULL, {SIG_DFL, [], 0}, 8) = 0

>env - strace -o trace.txt /usr/bin/perl interrupted.pl
Sun May 24 20:51:35 2015 pid=32103: helper starts
Sun May 24 20:51:35 2015 pid=32102: main starts
Sun May 24 20:51:35 2015 pid=32102: main waiting for helper to lock
Sun May 24 20:51:36 2015 pid=32102: main flock
SIGALRM in main at interrupted.pl line 16.
Sun May 24 20:51:41 2015 pid=32102: main can't lock: Interrupted system call (error code 4)
Sun May 24 20:51:41 2015 pid=32102: main flock done
Sun May 24 20:51:41 2015 pid=32102: main ends
Sun May 24 20:51:45 2015 pid=32103: helper ends

>grep -C5 SIGALRM trace.txt
close(3)                                = 0
rt_sigprocmask(SIG_SETMASK, ~[RTMIN RT_1], [], 8) = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f45f31f1a10) = 32103
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
rt_sigprocmask(SIG_BLOCK, [ALRM], [], 8) = 0
rt_sigaction(SIGALRM, {0x7f45f2d08de0, [], SA_RESTORER, 0x7f45f1efa670}, {SIG_DFL, [], 0}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
open("/etc/localtime", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f45f3215000
--
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
alarm(5)                                = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:51:36 2015 pid=321"..., 47) = 47
flock(3, LOCK_EX)                       = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} ---
rt_sigreturn()                          = -1 EINTR (Interrupted system call)
rt_sigprocmask(SIG_BLOCK, [ALRM], [], 8) = 0
rt_sigaction(SIGALRM, NULL, {0x7f45f2d08de0, [], SA_RESTORER, 0x7f45f1efa670}, 8) = 0
write(2, "SIGALRM in main at interrupted.p"..., 43) = 43
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:51:41 2015 pid=321"..., 92) = 92
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=2335, ...}) = 0
write(1, "Sun May 24 20:51:41 2015 pid=321"..., 52) = 52
--
rt_sigaction(SIGKILL, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGUSR1, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGSEGV, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGUSR2, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGPIPE, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGALRM, NULL, {0x7f45f2d08de0, [], SA_RESTORER, 0x7f45f1efa670}, 8) = 0
rt_sigaction(SIGALRM, {SIG_DFL, [], SA_RESTORER, 0x7f45f1efa670}, {0x7f45f2d08de0, [], SA_RESTORER, 0x7f45f1efa670}, 8) = 0
rt_sigaction(SIGTERM, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGSTKFLT, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGCHLD, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGCONT, NULL, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGSTOP, NULL, {SIG_DFL, [], 0}, 8) = 0

>


##</code><code>##

#ifdef SA_RESTART
    if (PL_signals & PERL_SIGNALS_UNSAFE_FLAG)
        act.sa_flags |= SA_RESTART; /* SVR4, 4.3+BSD */
#endif