lxc: extract blocked_service_startup() context manager into basedeploy

Move the policy-rc.d install/remove boilerplate into a shared context
manager in basedeploy.py so both UnboundDeployer and DovecotDeployer use
the same abstraction, and the DNS container's _install_powerdns() inline
shell uses the same pattern.

DovecotDeployer now wraps its three package installs in
blocked_service_startup() to prevent Dovecot from auto-starting on
initial install — avoiding bind conflicts on IPv4-only systems.

cmdeploy/src/cmdeploy/basedeploy.py

@@ -1,6 +1,7 @@
 import importlib.resources
 import io
 import os
+from contextlib import contextmanager
 
 from pyinfra.operations import files, server, systemd
 
@@ -10,6 +11,28 @@ def has_systemd():
     return os.path.isdir("/run/systemd/system")
 
 
+@contextmanager
+def blocked_service_startup():
+    """Prevent services from auto-starting during package installation.
+
+    Installs a ``/usr/sbin/policy-rc.d`` that exits 101, blocking any
+    service from being started by the package manager.  This avoids bind
+    conflicts and CPU/RAM spikes during initial setup.  The file is removed
+    when the context exits.
+    """
+    # For documentation about policy-rc.d, see:
+    # https://people.debian.org/~hmh/invokerc.d-policyrc.d-specification.txt
+    files.put(
+        src=get_resource("policy-rc.d"),
+        dest="/usr/sbin/policy-rc.d",
+        user="root",
+        group="root",
+        mode="755",
+    )
+    yield
+    files.file("/usr/sbin/policy-rc.d", present=False)
+
+
 def get_resource(arg, pkg=__package__):
     return importlib.resources.files(pkg).joinpath(arg)
 

cmdeploy/src/cmdeploy/deployers.py

@@ -22,6 +22,7 @@ from .basedeploy import (
     Deployer,
     Deployment,
     activate_remote_units,
+    blocked_service_startup,
     configure_remote_units,
     get_resource,
     has_systemd,
@@ -148,33 +149,16 @@ class UnboundDeployer(Deployer):
         self.need_restart = False
 
     def install(self):
-        # Run local DNS resolver `unbound`.
-        # `resolvconf` takes care of setting up /etc/resolv.conf
-        # to use 127.0.0.1 as the resolver.
+        # Run local DNS resolver `unbound`. `resolvconf` takes care of
+        # setting up /etc/resolv.conf to use 127.0.0.1 as the resolver.
 
-        #
-        # On an IPv4-only system, if unbound is started but not
-        # configured, it causes subsequent steps to fail to resolve hosts.
-        # Here, we use policy-rc.d to prevent unbound from starting up
-        # on initial install.  Later, we will configure it and start it.
-        #
-        # For documentation about policy-rc.d, see:
-        # https://people.debian.org/~hmh/invokerc.d-policyrc.d-specification.txt
-        #
-        files.put(
-            src=get_resource("policy-rc.d"),
-            dest="/usr/sbin/policy-rc.d",
-            user="root",
-            group="root",
-            mode="755",
-        )
-
-        apt.packages(
-            name="Install unbound",
-            packages=["unbound", "unbound-anchor", "dnsutils"],
-        )
-
-        files.file("/usr/sbin/policy-rc.d", present=False)
+        # On an IPv4-only system, if unbound is started but not configured,
+        # it causes subsequent steps to fail to resolve hosts.
+        with blocked_service_startup():
+            apt.packages(
+                name="Install unbound",
+                packages=["unbound", "unbound-anchor", "dnsutils"],
+            )
 
     def configure(self):
         server.shell(

cmdeploy/src/cmdeploy/dovecot/deployer.py

@@ -10,6 +10,7 @@ from pyinfra.operations import apt, files, server, systemd
 from cmdeploy.basedeploy import (
     Deployer,
     activate_remote_units,
+    blocked_service_startup,
     configure_remote_units,
     get_resource,
     has_systemd,
@@ -28,9 +29,11 @@ class DovecotDeployer(Deployer):
         arch = host.get_fact(Arch)
         if has_systemd() and "dovecot.service" in host.get_fact(SystemdEnabled):
             return  # already installed and running
-        _install_dovecot_package("core", arch)
-        _install_dovecot_package("imapd", arch)
-        _install_dovecot_package("lmtpd", arch)
+
+        with blocked_service_startup():
+            _install_dovecot_package("core", arch)
+            _install_dovecot_package("imapd", arch)
+            _install_dovecot_package("lmtpd", arch)
 
     def configure(self):
         configure_remote_units(self.config.mail_domain, self.units)

cmdeploy/src/cmdeploy/lxc/incus.py

@@ -605,9 +605,18 @@ class DNSContainer(Container):
             systemctl disable --now systemd-resolved 2>/dev/null || true
             rm -f /etc/resolv.conf
             echo 'nameserver 9.9.9.9' > /etc/resolv.conf
+
+            # Block automatic service startup during package installation
+            printf '#!/bin/sh\\nexit 101\\n' > /usr/sbin/policy-rc.d
+            chmod +x /usr/sbin/policy-rc.d
+
             apt-get -o DPkg::Lock::Timeout=60 update
             DEBIAN_FRONTEND=noninteractive apt-get install -y \
                 pdns-server pdns-backend-sqlite3 sqlite3 pdns-recursor dnsutils
+
+            # Remove the startup block
+            rm /usr/sbin/policy-rc.d
+
             systemctl stop pdns pdns-recursor || true
             mkdir -p /var/lib/powerdns
             sqlite3 /var/lib/powerdns/pdns.sqlite3 \