Restore non-essential systemd unit cleanup for jammy warden

bosh-docker-cpi-release#60 moved the cleanup of non-essential systemd
units out of the Docker CPI and into the stemcell, on the basis that the
stemcell would mask anything that can't run in a container (resolving
bosh-docker-cpi-release#58 and #504). On the jammy line, only the
systemd-binfmt mask landed (#500); the rest of the CPI's allow-list was
never reproduced here.

As a result, warden stemcells started with exec /sbin/init now boot the
full stock systemd unit set. In a BOSH director the stock units contend
with the monit-managed bpm jobs and the director fails to converge (e.g.
the postgres role is never created). This regresses any consumer running
a Docker-CPI director on a jammy warden stemcell ≥ the build that pairs
with Docker CPI ≥ 0.2.9.

This restores the CPI's historical allow-list in base_warden, removing
the non-essential .wants entries at build time. It uses -delete (≡
systemctl disable) rather than mask, matching the CPI's original
semantics: units are dropped from the boot sequence but can still start
as dependencies of a kept unit. runit, ssh, dbus, journald, logrotate,
systemd-tmpfiles, systemd-user-sessions, and the bosh-agent are
preserved. Deriving the set at build time keeps it correct as the
package set changes.

Verified against ubuntu-jammy-stemcell:1.1250: 13 essential .wants
entries kept, ~75 non-essential removed. This is the same prune Docker
CPI 0.2.3 applied, which is green on cgroups-v1 workers. Adds a
warden_spec assertion that non-essential units are removed and essential
ones remain.

Related: bosh-docker-cpi-release#60, bosh-docker-cpi-release#58, #500,

ai-assisted=yes
[TNZ-88995]

Author: selzoc

bosh-stemcell/spec/stemcells/warden_spec.rb

@@ -58,4 +58,30 @@
       its(:content) { should include('"UseMonitIptablesFirewall": true') }
     end
   end
+
+  context "systemd unit cleanup for warden containers" do
+    # The Docker CPI runs warden stemcells with `exec /sbin/init`. base_warden
+    # strips non-essential stock systemd units from the boot sequence so they
+    # don't contend with the monit-managed bpm jobs in the BOSH director
+    # container (symptom: postgres role never created, bosh/0 never converges).
+    # Keep-list mirrors the historical Docker CPI allow-list. See
+    # base_warden/apply.sh and cloudfoundry/bosh-docker-cpi-release#60.
+    keep_patterns = %w[
+      *bosh-agent* *dbus* *journald* *logrotate* *runit* *ssh*
+      *systemd-user-sessions* *systemd-tmpfiles*
+    ]
+    not_name = keep_patterns.map { |g| "-not -name '#{g}'" }.join(" ")
+    wants = "find /etc/systemd/system /lib/systemd/system -path '*.wants/*'"
+
+    describe "non-essential units are removed from the boot sequence" do
+      describe command("#{wants} #{not_name}") do
+        its(:stdout) { should eq "" }
+      end
+    end
+
+    describe "essential units are preserved (guards against an over-broad prune)" do
+      subject { command(wants).stdout.split("\n").length }
+      it { should be > 0 }
+    end
+  end
 end

stemcell_builder/stages/base_warden/apply.sh

@@ -85,3 +85,32 @@ JSON
 
 # Mask systemd-binfmt.service which fails under Rosetta emulation
 run_in_chroot "$chroot" "systemctl mask systemd-binfmt.service"
+
+# Trim non-essential systemd units from the boot sequence.
+#
+# When the Docker CPI runs a warden stemcell with `exec /sbin/init`, full stock
+# systemd comes up and its units contend with the monit-managed bpm jobs in the
+# BOSH director container (symptom: the postgres role is never created and
+# bosh/0 never converges). The Docker CPI used to strip these units at
+# container-create time, but that allow-list prune was removed in
+# cloudfoundry/bosh-docker-cpi-release#60 on the assumption the stemcell would
+# take it over. The jammy stemcell only ever picked up the systemd-binfmt mask
+# above, so we reproduce the CPI's full allow-list here.
+#
+# This removes the `.wants` symlinks (equivalent to `systemctl disable`): units
+# are dropped from the boot sequence but can still start as dependencies of a
+# kept unit. The keep patterns mirror the historical CPI list exactly. Deriving
+# the set at build time keeps it correct as the stemcell's package set changes.
+run_in_chroot "$chroot" "
+find /etc/systemd/system /lib/systemd/system \
+  -path '*.wants/*' \
+  -not -name '*bosh-agent*' \
+  -not -name '*dbus*' \
+  -not -name '*journald*' \
+  -not -name '*logrotate*' \
+  -not -name '*runit*' \
+  -not -name '*ssh*' \
+  -not -name '*systemd-user-sessions*' \
+  -not -name '*systemd-tmpfiles*' \
+  -delete
+"