feat: report a dpkg left halfway and when the host's own automatic updates last succeeded

An unattended upgrade cut off while it built an initrd left a kernel package
half-configured on an Ubuntu host. For three months apt refused every upgrade,
while apt.systemd.daily exited quietly and the timers reported success
(NetOrk/netork#738).

HOST_STATUS_COMMAND now also reads, without root:

- dpkg's state the way apt checks it: numbered files left in
  /var/lib/dpkg/updates, or a package half-installed, unpacked,
  half-configured, waiting for triggers or flagged for reinstallation.
  While dpkg, apt, aptitude or unattended-upgrade runs, packages are
  halfway on purpose, so the answer is then None.
- /var/lib/apt/periodic/upgrade-stamp, which apt.systemd.daily touches only
  after unattended-upgrade succeeded, and the interval of
  APT::Periodic::Unattended-Upgrade in apt's units. update-success-stamp is
  not read: every apt-get update touches it, whoever runs it.

HostStatusDict gains package_manager_interrupted, interrupted_packages,
auto_updates_last_success and auto_updates_interval, all NotRequired.

tests/test_host_status.py

@@ -29,8 +29,12 @@ def _wire(
     modules=("6.8.0-139-generic", "6.8.0-142-generic"),
     needs_restarting=None,
     periodic=None,
+    stamp=None,
     timer="enabled",
     dnf_timers=("not-found", "not-found"),
+    dpkg=None,
+    journal=(),
+    busy=False,
 ):
     lines = ["HSTAT_BEGIN"]
     if reboot_file:
@@ -39,7 +43,11 @@ def _wire(
         lines += ["[needs-restarting]", str(needs_restarting)]
     lines += ["[kernel]", running, "[modules]", *modules]
     if periodic is not None:
-        lines += ["[apt-config]", *periodic]
+        lines += ["[apt-config]", *periodic, "[upgrade-stamp]", *([str(stamp)] if stamp else [])]
+    if dpkg is not None:
+        if busy:
+            lines.append("[dpkg-busy]")
+        lines += ["[dpkg-journal]", *journal, "[dpkg-audit]", *dpkg]
     lines += [
         "[timers]",
         f"apt-daily-upgrade.timer {timer}",
@@ -243,3 +251,127 @@ class TestFreeBSD:
         assert "command -v freebsd-version" in HOST_STATUS_COMMAND
         assert "freebsd-version -k" in HOST_STATUS_COMMAND
         assert "freebsd-version -r" in HOST_STATUS_COMMAND
+
+
+class TestInterruptedDpkg:
+    """dpkg left halfway: apt refuses to install anything until
+    ``dpkg --configure -a`` has run, and the daily apt timers report success
+    all the same (NetOrk/netork#738).
+
+    The fixture is aris (Ubuntu 24.04): unattended-upgrades was cut off on
+    2026-07-03 while update-initramfs built the initrd for 6.8.0-134, and the
+    kernel package stayed half-configured for three months.
+    """
+
+    ARIS = ("iF  linux-image-6.8.0-134-generic",)
+
+    def test_a_half_configured_kernel_is_an_interrupted_dpkg(self):
+        status = parse_host_status(_wire(dpkg=self.ARIS))
+
+        assert status["package_manager_interrupted"] is True
+        assert status["interrupted_packages"] == [
+            {"name": "linux-image-6.8.0-134-generic", "state": "half-configured"}
+        ]
+
+    def test_a_clean_dpkg_is_not_interrupted(self):
+        status = parse_host_status(_wire(dpkg=()))
+
+        assert status["package_manager_interrupted"] is False
+        assert status["interrupted_packages"] == []
+
+    @pytest.mark.parametrize(
+        ("line", "state"),
+        [
+            ("iH  base-files", "half-installed"),
+            ("iU  base-files", "unpacked"),
+            ("iF  base-files", "half-configured"),
+            ("iW  base-files", "triggers-awaited"),
+            ("it  base-files", "triggers-pending"),
+            ("iUR base-files", "reinstall-required"),
+            ("hF  base-files", "half-configured"),
+        ],
+    )
+    def test_every_state_an_interrupted_run_leaves(self, line, state):
+        status = parse_host_status(_wire(dpkg=(line,)))
+
+        assert status["interrupted_packages"] == [{"name": "base-files", "state": state}]
+
+    def test_a_journal_left_behind_is_interrupted_on_its_own(self):
+        """What apt itself checks: numbered files in /var/lib/dpkg/updates."""
+        status = parse_host_status(_wire(dpkg=(), journal=("0000", "0001")))
+
+        assert status["package_manager_interrupted"] is True
+        assert status["interrupted_packages"] == []
+
+    def test_only_numbered_journal_files_count(self):
+        """apt ignores anything else there, such as dpkg's tmp.i."""
+        status = parse_host_status(_wire(dpkg=(), journal=("tmp.i",)))
+
+        assert status["package_manager_interrupted"] is False
+
+    def test_a_running_package_manager_makes_it_unknown(self):
+        """Mid-upgrade, packages are unpacked and not yet configured, also
+        between two of apt's dpkg calls. That is work in progress, not an
+        interruption."""
+        status = parse_host_status(_wire(dpkg=("iU  base-files",), busy=True))
+
+        assert status["package_manager_interrupted"] is None
+        assert status["interrupted_packages"] == []
+
+    def test_without_dpkg_it_is_unknown(self):
+        status = parse_host_status(_wire())
+
+        assert status["package_manager_interrupted"] is None
+        assert status["interrupted_packages"] == []
+
+    def test_the_command_asks_dpkg_only_where_it_exists(self):
+        assert "command -v dpkg-query" in HOST_STATUS_COMMAND
+        assert "/var/lib/dpkg/updates" in HOST_STATUS_COMMAND
+
+    def test_it_never_repairs(self):
+        assert "--configure" not in HOST_STATUS_COMMAND
+
+
+class TestAutoUpdatesLastSuccess:
+    """apt.systemd.daily touches upgrade-stamp only after unattended-upgrade
+    succeeded. update-success-stamp says nothing: netOrk's own daily
+    ``apt-get update`` touches it as well (NetOrk/netork#738)."""
+
+    def test_the_upgrade_stamp_is_the_last_successful_run(self):
+        status = parse_host_status(_wire(periodic=UNATTENDED, stamp=1751525820))
+
+        assert status["auto_updates_last_success"] == 1751525820
+        assert status["auto_updates_interval"] == 86400
+
+    def test_no_stamp_is_unknown(self):
+        status = parse_host_status(_wire(periodic=UNATTENDED))
+
+        assert status["auto_updates_last_success"] is None
+
+    def test_without_apt_both_are_unknown(self):
+        status = parse_host_status(_wire())
+
+        assert status["auto_updates_last_success"] is None
+        assert status["auto_updates_interval"] is None
+
+    @pytest.mark.parametrize(
+        ("value", "seconds"),
+        [
+            ('"1"', 86400),
+            ('"7d"', 7 * 86400),
+            ('"12h"', 43200),
+            ('"30m"', 1800),
+            ('"90s"', 90),
+            ('"always"', 0),
+            ('"0"', None),
+            ('"weekly"', None),
+        ],
+    )
+    def test_the_interval_in_apts_units(self, value, seconds):
+        periodic = [f"APT::Periodic::Unattended-Upgrade {value};"]
+
+        assert parse_host_status(_wire(periodic=periodic))["auto_updates_interval"] == seconds
+
+    def test_the_command_reads_the_upgrade_stamp_only(self):
+        assert "/var/lib/apt/periodic/upgrade-stamp" in HOST_STATUS_COMMAND
+        assert "update-success-stamp" not in HOST_STATUS_COMMAND