core: add Ref()/Unref() bus calls for units

This adds two (privileged) bus calls Ref() and Unref() to the Unit interface.
The two calls may be used by clients to pin a unit into memory, so that various
runtime properties aren't flushed out by the automatic GC. This is necessary
to permit clients to race-freely acquire runtime results (such as process exit
status/code or accumulated CPU time) on successful service termination.

Ref() and Unref() are fully recursive, hence act like the usual reference
counting concept in C. Taking a reference is a privileged operation, as this
allows pinning units into memory which consumes resources.

Transient units may also gain a reference at the time of creation, via the new
AddRef property (that is only defined for transient units at the time of
creation).

1 files changed, 57 insertions, 0 deletions

diff --git a/src/core/dbus-manager.c b/src/core/dbus-manager.c
index ef05a75a8b..ea7ced2fd0 100644
--- a/src/core/dbus-manager.c
+++ b/src/core/dbus-manager.c
@@ -643,6 +643,54 @@ static int method_set_unit_properties(sd_bus_message *message, void *userdata, s
         return bus_unit_method_set_properties(message, u, error);
 }
 
+static int method_ref_unit(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Manager *m = userdata;
+        const char *name;
+        Unit *u;
+        int r;
+
+        assert(message);
+        assert(m);
+
+        r = sd_bus_message_read(message, "s", &name);
+        if (r < 0)
+                return r;
+
+        r = manager_load_unit(m, name, NULL, error, &u);
+        if (r < 0)
+                return r;
+
+        r = bus_unit_check_load_state(u, error);
+        if (r < 0)
+                return r;
+
+        return bus_unit_method_ref(message, u, error);
+}
+
+static int method_unref_unit(sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Manager *m = userdata;
+        const char *name;
+        Unit *u;
+        int r;
+
+        assert(message);
+        assert(m);
+
+        r = sd_bus_message_read(message, "s", &name);
+        if (r < 0)
+                return r;
+
+        r = manager_load_unit(m, name, NULL, error, &u);
+        if (r < 0)
+                return r;
+
+        r = bus_unit_check_load_state(u, error);
+        if (r < 0)
+                return r;
+
+        return bus_unit_method_unref(message, u, error);
+}
+
 static int reply_unit_info(sd_bus_message *reply, Unit *u) {
         _cleanup_free_ char *unit_path = NULL, *job_path = NULL;
         Unit *following;
@@ -781,6 +829,13 @@ static int transient_unit_from_message(
         if (r < 0)
                 return r;
 
+        /* If the client asked for it, automatically add a reference to this unit. */
+        if (u->bus_track_add) {
+                r = bus_unit_track_add_sender(u, message);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to watch sender: %m");
+        }
+
         /* Now load the missing bits of the unit we just created */
         unit_add_to_load_queue(u);
         manager_dispatch_load_queue(m);
@@ -2211,6 +2266,8 @@ const sd_bus_vtable bus_manager_vtable[] = {
         SD_BUS_METHOD("KillUnit", "ssi", NULL, method_kill_unit, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("ResetFailedUnit", "s", NULL, method_reset_failed_unit, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("SetUnitProperties", "sba(sv)", NULL, method_set_unit_properties, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("RefUnit", "s", NULL, method_ref_unit, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("UnrefUnit", "s", NULL, method_unref_unit, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("StartTransientUnit", "ssa(sv)a(sa(sv))", "o", method_start_transient_unit, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("GetUnitProcesses", "s", "a(sus)", method_get_unit_processes, SD_BUS_VTABLE_UNPRIVILEGED),
         SD_BUS_METHOD("GetJob", "u", "o", method_get_job, SD_BUS_VTABLE_UNPRIVILEGED),