needs-restarting: Get boot time from systemd UnitsLoadStartTimestamp

Resolves https://issues.redhat.com/browse/RHEL-35577.

Get the boot time from UnitsLoadStartTimestamp if systemd is available,
but make sure to use the kernel boot time for calculating process start
times using data from procfs. The previous attempt [1] at this failed to
do so and introduced a regression [2].

Also, get the kernel boot time from the btime field of /proc/stat instead of
calculating it from /proc/uptime, to be consistent with what procps-ng
does.

[1] #527
[2] https://issues.redhat.com/browse/RHEL-39775

plugins/needs_restarting.py

@@ -206,36 +206,86 @@ def presumed_name(self):
                 return match.group(1)
         return self.name
 
-
 class ProcessStart(object):
     def __init__(self):
-        self.boot_time = self.get_boot_time()
-        self.sc_clk_tck = self.get_sc_clk_tck()
+        self.kernel_boot_time = ProcessStart.get_kernel_boot_time()
+        self.boot_time = ProcessStart.get_boot_time(self.kernel_boot_time)
+        self.sc_clk_tck = ProcessStart.get_sc_clk_tck()
+
+    @staticmethod
+    def get_kernel_boot_time():
+        try:
+            with open('/proc/stat', 'r') as file:
+                for line in file:
+                    if line.startswith("btime "):
+                        key, value = line.split()
+                        return float(value)
+        except OSError:
+            pass
+        logger.debug("Couldn't determine kernel boot time.")
+        return 0
 
     @staticmethod
-    def get_boot_time():
+    def get_boot_time(kernel_boot_time):
         """
-        We have two sources from which to derive the boot time. These values vary
+        We have three sources from which to derive the boot time. These values vary
         depending on containerization, existence of a Real Time Clock, etc.
-        For our purposes we want the latest derived value.
+        - UnitsLoadStartTimestamp property on /org/freedesktop/systemd1
+             The start time of the service manager, according to systemd itself.
+             Seems to be more reliable than UserspaceTimestamp when the RTC is
+             in local time. Works unless the system was not booted with systemd,
+             such as in (most) containers.
         - st_mtime of /proc/1
-             Reflects the time the first process was run after booting
-             This works for all known cases except machines without
-             a RTC - they awake at the start of the epoch.
-        - /proc/uptime
-             Seconds field of /proc/uptime subtracted from the current time
-             Works for machines without RTC iff the current time is reasonably correct.
-             Does not work on containers which share their kernel with the
-             host - there the host kernel uptime is returned
+             Reflects the time the first process was run after booting. This
+             works for all known cases except machines without a RTC---they
+             awake at the start of the epoch.
+        - btime field of /proc/stat
+             Reflects the time when the kernel started. Works for machines
+             without RTC iff the current time is reasonably correct. Does not
+             work on containers which share their kernel with the host---there,
+             the host kernel uptime is returned.
         """
 
-        proc_1_boot_time = int(os.stat('/proc/1').st_mtime)
-        if os.path.isfile('/proc/uptime'):
-            with open('/proc/uptime', 'rb') as f:
-                uptime = f.readline().strip().split()[0].strip()
-                proc_uptime_boot_time = int(time.time() - float(uptime))
-                return max(proc_1_boot_time, proc_uptime_boot_time)
-        return proc_1_boot_time
+        units_load_start_timestamp = None
+        try:
+            # systemd timestamps are the preferred method to determine boot
+            # time. max(proc_1_boot_time, kernel_boot_time) does not allow us
+            # to disambiguate between an unreliable RTC (e.g. the RTC is in
+            # UTC+1 instead of UTC) and a container with a proc_1_boot_time >
+            # kernel_boot_time. So we use UnitsLoadStartTimestamp if it's
+            # available, else fall back to the other methods.
+            bus = dbus.SystemBus()
+            systemd1 = bus.get_object(
+                'org.freedesktop.systemd1',
+                '/org/freedesktop/systemd1'
+            )
+            props = dbus.Interface(
+                systemd1,
+                dbus.PROPERTIES_IFACE
+            )
+            units_load_start_timestamp = props.Get(
+                'org.freedesktop.systemd1.Manager',
+                'UnitsLoadStartTimestamp'
+            )
+            if units_load_start_timestamp != 0:
+                systemd_boot_time = units_load_start_timestamp / (1000 * 1000)
+                logger.debug("Got boot time from systemd: %s", systemd_boot_time)
+                return systemd_boot_time
+        except dbus.exceptions.DBusException as e:
+            logger.debug("D-Bus error getting boot time from systemd: %s", e)
+
+        logger.debug("Couldn't get boot time from systemd, checking st_mtime of /proc/1 and btime field of /proc/stat.")
+
+        proc_1_boot_time = float(os.stat('/proc/1').st_mtime)
+        kernel_boot_time = kernel_boot_time
+
+        boot_time = max(proc_1_boot_time, kernel_boot_time)
+
+        logger.debug("st_mtime of /proc/1: %s", proc_1_boot_time)
+        logger.debug("btime field of /proc/stat: %s", kernel_boot_time)
+        logger.debug("Using %s as the system boot time.", boot_time)
+
+        return boot_time
 
     @staticmethod
     def get_sc_clk_tck():
@@ -244,10 +294,13 @@ def get_sc_clk_tck():
     def __call__(self, pid):
         stat_fn = '/proc/%d/stat' % pid
         with open(stat_fn) as stat_file:
-            stats = stat_file.read().strip().split()
-        ticks_after_boot = int(stats[21])
-        secs_after_boot = ticks_after_boot // self.sc_clk_tck
-        return self.boot_time + secs_after_boot
+            stats = stat_file.read().split()
+        ticks_after_kernel_boot = int(stats[21])
+        secs_after_kernel_boot = ticks_after_kernel_boot / self.sc_clk_tck
+
+        # The process's start time is always measured relative to the kernel
+        # start time, not either of the other methods we use to get "boot time".
+        return self.kernel_boot_time + secs_after_kernel_boot
 
 
 @dnf.plugin.register_command