test(utils): cover a lock that a waiting process takes after a release

When a process releases a lock that another process waits for, the
waiting process must take the lock. A third process must not take it
while the waiting process holds it.

xCAT-test/unit/utils_lock_release.t

@@ -0,0 +1,45 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+use FindBin;
+use lib "$FindBin::Bin/../lib";
+use lib "$FindBin::Bin/../../perl-xCAT";
+use File::Temp qw(tempdir);
+use IO::Handle;
+use POSIX ();
+use Test::More;
+
+require xCAT::Utils;
+
+# The lock files live in /var/lock/xcat, so the test keeps them in a directory of its own.
+my $lock_dir = tempdir(CLEANUP => 1);
+{
+    no warnings 'redefine';
+    *xCAT::Utils::lock_dir = sub { return $lock_dir };
+}
+
+# A process holds a lock, a second one waits for it, and a third tries it after the release.
+my $holder = xCAT::Utils->acquire_lock('kit', 1);
+ok($holder, 'the first process takes the lock');
+pipe(my $from_waiter, my $to_parent) or die "cannot make a pipe: $!";
+my $pid = fork() // die "cannot fork: $!";
+if (!$pid) {
+    close($from_waiter);
+    my $lock = xCAT::Utils->acquire_lock('kit', 0);
+    print {$to_parent} ($lock ? "held\n" : "failed\n");
+    $to_parent->flush;
+    sleep 60;
+    POSIX::_exit(0);
+}
+close($to_parent);
+
+# The second process waits in flock on the file of the first one.
+sleep 2;
+xCAT::Utils->release_lock($holder, 1);
+my $waiter = <$from_waiter>;
+is($waiter, "held\n", 'the waiting process takes the lock after its release');
+my $third = xCAT::Utils->acquire_lock('kit', 1);
+ok(!$third, 'a third process cannot take the lock while the waiting process holds it');
+kill('KILL', $pid);
+waitpid($pid, 0);
+done_testing();