core: add ReloadCount to Manager and bump on successful reload

Introduce a counter that tracks how many configuration reloads have
been successfully completed by the manager. The increment lives in
manager_reload() right after the "point of no return", so failed
reload attempts that bail out earlier (e.g. during serialization)
do not bump the counter.

It is accessible as a new ReloadCount property to
org.freedesktop.systemd1.Manager (D-Bus) and ReloadCount to
io.systemd.Manager.Describe (Varlink).

Also add an integration test for ReloadCount
that verifies that the new ReloadCount property increments by one per
daemon-reload, accumulates correctly across multiple reloads, and that
D-Bus and Varlink return identical values. Also tests that the counter
reset after a reexec.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>
Signed-off-by: Simon Lucido <simonlucido@meta.com>

src/core/dbus-manager.c

@@ -2978,6 +2978,7 @@ const sd_bus_vtable bus_manager_vtable[] = {
         SD_BUS_PROPERTY("DefaultRestrictSUIDSGID", "b", bus_property_get_bool, offsetof(Manager, defaults.restrict_suid_sgid), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("CtrlAltDelBurstAction", "s", bus_property_get_emergency_action, offsetof(Manager, cad_burst_action), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("SoftRebootsCount", "u", bus_property_get_unsigned, offsetof(Manager, soft_reboots_count), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("ReloadCount", "t", NULL, offsetof(Manager, reload_count), 0),
         SD_BUS_PROPERTY("DefaultMemoryZSwapWriteback", "b", bus_property_get_bool, offsetof(Manager, defaults.memory_zswap_writeback), SD_BUS_VTABLE_PROPERTY_CONST),
 
         /* deprecated cgroup v1 property */

src/core/manager.c

@@ -3650,6 +3650,10 @@ int manager_reload(Manager *m) {
         /* 💀 This is the point of no return, from here on there is no way back. 💀 */
         reloading = NULL;
 
+        /* Bump before sending the Reloading signal, so any client that reads
+         * ReloadCount in response to that signal observes the new value. */
+        m->reload_count = saturate_add(m->reload_count, 1, UINT64_MAX);
+
         bus_manager_send_reloading(m, true);
 
         /* Start by flushing out all jobs and units, all generated units, all runtime environments, all dynamic users

src/core/manager.h

@@ -492,6 +492,9 @@ typedef struct Manager {
 
         unsigned soft_reboots_count;
 
+        /* The number of successfully completed configuration reloads. */
+        uint64_t reload_count;
+
         /* Original ambient capabilities when we were initialized */
         uint64_t saved_ambient_set;
 } Manager;

src/core/varlink-manager.c

@@ -193,7 +193,8 @@ static int manager_runtime_build_json(sd_json_variant **ret, const char *name, v
                 JSON_BUILD_PAIR_DUAL_TIMESTAMP_NON_NULL("WatchdogLastPingTimestamp", watchdog_get_last_ping_as_dual_timestamp(&watchdog_last_ping)),
                 SD_JSON_BUILD_PAIR_STRING("SystemState", manager_state_to_string(manager_state(m))),
                 SD_JSON_BUILD_PAIR_UNSIGNED("ExitCode", m->return_value),
-                SD_JSON_BUILD_PAIR_UNSIGNED("SoftRebootsCount", m->soft_reboots_count));
+                SD_JSON_BUILD_PAIR_UNSIGNED("SoftRebootsCount", m->soft_reboots_count),
+                SD_JSON_BUILD_PAIR_UNSIGNED("ReloadCount", m->reload_count));
 }
 
 int vl_method_describe_manager(sd_varlink *link, sd_json_variant *parameters, sd_varlink_method_flags_t flags, void *userdata) {