tests: add t/180-fd-leaks to ensure i3 does not leak fds on exec

testcases/t/180-fd-leaks.t

@@ -0,0 +1,49 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Verifies that i3 does not leak any file descriptors in 'exec'.
+#
+use i3test;
+use POSIX qw(mkfifo);
+use File::Temp qw(:POSIX tempfile);
+
+my $i3 = i3(get_socket_path());
+
+my $tmp = tmpnam();
+mkfifo($tmp, 0600) or die "Could not create FIFO in $tmp";
+my ($outfh, $outname) = tempfile('/tmp/i3-ls-output.XXXXXX', UNLINK => 1);
+
+cmd qq|exec ls -l /proc/self/fd >$outname && echo done >$tmp|;
+
+open(my $fh, '<', $tmp);
+# Block on the FIFO, this will return exactly when the command is done.
+<$fh>;
+close($fh);
+unlink($tmp);
+
+# Get the ls /proc/self/fd output
+my $output;
+{
+    local $/;
+    $output = <$outfh>;
+}
+close($outfh);
+
+# Split lines, keep only those which are symlinks.
+my @lines = grep { /->/ } split("\n", $output);
+
+my %fds = map { /([0-9]+) -> (.+)$/; ($1, $2) } @lines;
+
+# Filter out 0, 1, 2 (stdin, stdout, stderr).
+delete $fds{0};
+delete $fds{1};
+delete $fds{2};
+
+# Filter out the fd which is caused by ls calling readdir().
+for my $fd (keys %fds) {
+    delete $fds{$fd} if $fds{$fd} =~ m,^/proc/\d+/fd$,;
+}
+
+is(scalar keys %fds, 0, 'No file descriptors leaked');
+
+done_testing;