core: rework how transient unit files and property drop-ins work

With this change the logic for placing transient unit files and drop-ins
generated via "systemctl set-property" is reworked.

The latter are now placed in the newly introduced "control" unit file
directory. The fomer are now placed in the "transient" unit file directory.

Note that the properties originally set when a transient unit was created will
be written to and stay in the transient unit file directory, while later
changes are done via drop-ins.

This is preparation for a later "systemctl revert" addition, where existing
drop-ins are flushed out, but the original transient definition is restored.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 601e763ce2..cfdac852a5 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -95,6 +95,9 @@ struct Unit {
         usec_t source_mtime;
         usec_t dropin_mtime;
 
+        /* If this is a transient unit we are currently writing, this is where we are writing it to */
+        FILE *transient_file;
+
         /* If there is something to do with this unit, then this is the installed job for it */
         Job *job;