[PATCH v2] t/spawn: Find invalid PID to try to join its process group

[Thiago Jung Bauermann <bauermann@kolabnow.com>]

In the container used to build packages of the GNU Guix distribution, PID 1
runs as the same user as the test so this spawn that should fail actually
succeeds.

Fix the problem by going through different PIDs and picking one that
either doesn't exist or we aren't allowed to signal.
---

Hello,

This version incorporates Eric's feedback.

I also took the opportunity to
add a comment explaining what the purpose of that particular spawn
invocation is. I wasn't familiar with the code so it wasn't immediately
obvious to me. I hope I got the comment right.

Changes since v1:
- Start PID search at 0x7fffffff.
- Changed indentation to all tabs.
- Added comment explaining purpose of the spawn invocation.

 t/spawn.t | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/t/spawn.t b/t/spawn.t
index 6168c1f6171c..5fc99a2a101c 100644
--- a/t/spawn.t
+++ b/t/spawn.t
@@ -24,7 +24,18 @@ SKIP: {
 	is(waitpid($pid, 0), $pid, 'waitpid succeeds on spawned process');
 	is($?, 0, 'true exited successfully');
 	pipe(my ($r, $w)) or BAIL_OUT;
-	$pid = eval { spawn(['true'], undef, { pgid => 1, 2 => $w }) };
+
+	# Find invalid PID to try to join its process group.
+	my $wrong_pgid = 1;
+	for (my $i=0x7fffffff; $i >= 2; $i--) {
+		if (kill(0, $i) == 0) {
+			$wrong_pgid = $i;
+			last;
+		}
+	}
+
+	# Test spawn behavior when it can't join the requested process group.
+	$pid = eval { spawn(['true'], undef, { pgid => $wrong_pgid, 2 => $w }) };
 	close $w;
 	my $err = do { local $/; <$r> };
 	# diag "$err ($@)";

base-commit: 930d2dc63e04c652e3b64cc7f3b3a7d377637065