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, 7 insertions, 0 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 53875653d7..e5a2a77b7b 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -108,6 +108,10 @@ struct Unit {
         /* The slot used for watching NameOwnerChanged signals */
         sd_bus_slot *match_bus_slot;
 
+        /* References to this unit from clients */
+        sd_bus_track *bus_track;
+        char **deserialized_refs;
+
         /* Job timeout and action to take */
         usec_t job_timeout;
         FailureAction job_timeout_action;
@@ -247,6 +251,9 @@ struct Unit {
 
         /* Did we already invoke unit_coldplug() for this unit? */
         bool coldplugged:1;
+
+        /* For transient units: whether to add a bus track reference after creating the unit */
+        bool bus_track_add:1;
 };
 
 struct UnitStatusMessageFormats {