test(xcat-core): capture the Genesis root home directory left at /root

mknb writes the management node key to /.ssh/authorized_keys for the legacy
Genesis image. sshd reads that file only while the home directory of root is
/. The dracut cmdline hook makes it / by deleting the root entry the image
ships and appending its own.

The test runs that rewrite against both root entry shapes dracut writes, for
all three hooks, and reads back /etc/passwd. It also keeps a user name that
starts with root in the file, so a wider delete cannot pass unnoticed.

Six of the eighteen assertions fail today.

Signed-off-by: Daniel Hilst <392820+dhilst@users.noreply.github.com>

xCAT-test/unit/genesis_root_home.t

@@ -0,0 +1,90 @@
+#!/usr/bin/env perl
+# Drive the /etc/passwd rewrite out of the Genesis dracut cmdline hooks.
+#
+# mknb writes the management node key to /.ssh/authorized_keys for the legacy Genesis
+# image, so sshd finds it only while the home directory of root is /. The hook makes it /
+# by deleting the root entry the image ships and appending its own. Run that rewrite
+# against every root entry shape dracut writes and read back the result.
+use strict;
+use warnings;
+
+use File::Slurper qw(read_text write_text);
+use File::Temp qw(tempdir);
+use FindBin;
+use lib "$FindBin::Bin/../lib";
+use Test::More;
+
+use XCAT::Test::File qw(repo_path);
+
+my @HOOKS = (
+    'xCAT-genesis-builder/xcat-cmdline.sh',
+    'xCAT-genesis-builder/dracut_105/el/xcat-cmdline.sh',
+    'xCAT-genesis-builder/dracut_105/ubuntu/xcat-cmdline.sh',
+);
+
+# dracut 99base writes the root entry itself. Up to dracut 057 the password field is
+# always x. From dracut 060 the x arrives only with --hostonly, and the Genesis image is
+# built with -N, so el10 (dracut 107) ships an empty password field.
+my %SHIPPED = (
+    'dracut 049/057 (el8, el9)' => "root:x:0:0::/root:/bin/sh\n",
+    'dracut 107 (el10)'         => "root::0:0::/root:/bin/sh\n",
+);
+
+# A user name that starts with root but is not root. The delete must keep this line.
+my $DECOY = "rootfsadm:x:501:501::/home/rootfsadm:/sbin/nologin\n";
+
+plan tests => 3 * @HOOKS * scalar(keys %SHIPPED);
+
+my $tmpdir = tempdir(CLEANUP => 1);
+
+foreach my $hook (@HOOKS) {
+    my $block = extract_passwd_block(repo_path($hook), $hook);
+    foreach my $shape (sort keys %SHIPPED) {
+        my $passwd = run_rewrite($block, $SHIPPED{$shape} . $DECOY, $hook);
+        my @root = grep { /^root:/ } split(/\n/, $passwd);
+
+        is(scalar @root, 1,
+            "$hook / $shape: one root entry is left in /etc/passwd");
+        is($root[0], 'root:x:0:0::/:/bin/bash',
+            "$hook / $shape: the home directory of root is /");
+        like($passwd, qr/^\Qrootfsadm:x:501:501::\/home\/rootfsadm:\/sbin\/nologin\E$/m,
+            "$hook / $shape: a user name that starts with root is kept");
+    }
+}
+
+#---
+# extract_passwd_block: lift the /etc/passwd rewrite out of a hook that cannot be sourced.
+# The hook mounts filesystems, starts udev and ends in an endless loop.
+# Bails out when the block stops being extractable, so a rewrite fails loudly instead of
+# leaving the test asserting nothing.
+#---
+sub extract_passwd_block {
+    my ($path, $label) = @_;
+    my $text = read_text($path);
+    my ($block) = $text =~ m{^(sed [^\n]*/etc/passwd\ncat >>/etc/passwd <<"__ENDL"\n.*?^__ENDL)$}ms;
+    BAIL_OUT("$label: the /etc/passwd rewrite was not found") unless defined $block;
+    return $block;
+}
+
+#---
+# run_rewrite: run the extracted block against a scratch passwd file and return it.
+# The block names /etc/passwd literally, so the path is redirected into the scratch tree
+# first. Bails out if any reference to the real file survives, because the block runs as
+# root under CI.
+#---
+sub run_rewrite {
+    my ($block, $shipped, $label) = @_;
+    my $dir    = tempdir(DIR => $tmpdir, CLEANUP => 1);
+    my $passwd = "$dir/passwd";
+    write_text($passwd, $shipped);
+
+    my $script = $block;
+    my $hits = ($script =~ s{/etc/passwd}{$passwd}g);
+    BAIL_OUT("$label: expected 2 references to /etc/passwd, found $hits") unless $hits == 2;
+    BAIL_OUT("$label: a reference to the real /etc/passwd survived") if index($script, '/etc/passwd') >= 0;
+
+    write_text("$dir/rewrite.sh", "set -e\n$script\n");
+    system('/bin/bash', "$dir/rewrite.sh") == 0
+      or BAIL_OUT("$label: the /etc/passwd rewrite failed to run");
+    return read_text($passwd);
+}