core: introduce a "control" unit file directory

This patch adds a concept of a "control" unit file directory, which is supposed
to be used as place for unit file drop-ins created by "systemctl set-property"
(note that this directory is not actually hooked up to "systemctl set-property"
yet, that's coming in a later patch).

The rationale for this: previously changes made by the user and by "systemctl
set-property" were done in the same directory, which made semantics very
unclear: the changes made by "systemctl set-property" were applied instantly,
and their drop-ins only written to not lose settings on a later "systemctl
daemon-reload", while drop-ins made by the user would only be in effect after
"systemctl daemon-reload". This is particular problematic as the changes made
by "systemctl set-property" would really apply immediately without any respect
for the unit search path. This meant that using "set-property" could have an
effect that is lsot as soon as "daemon-reload" is issued, in case there was a
"later" drop-in already in place.

With this change the directories are seperated, and the "control" directory
always takes the highest priority of all, to avoid any confusion.

1 files changed, 16 insertions, 3 deletions

diff --git a/src/shared/path-lookup.h b/src/shared/path-lookup.h
index 29f74cf211..89254cbac5 100644
--- a/src/shared/path-lookup.h
+++ b/src/shared/path-lookup.h
@@ -27,20 +27,33 @@ typedef struct LookupPaths LookupPaths;
 #include "macro.h"
 
 struct LookupPaths {
+        /* Where we look for unit files. This includes the individual special paths below, but also any vendor
+         * supplied, static unit file paths. */
         char **search_path;
 
-        /* Where we shall create or remove our installation symlinks, aka "configuration". */
+        /* Where we shall create or remove our installation symlinks, aka "configuration", and where the user/admin
+         * shall place his own unit files. */
         char *persistent_config;
         char *runtime_config;
 
-        /* Where to place generated unit files */
+        /* Where to place generated unit files (i.e. those a "generator" tool generated). Note the special semantics of
+         * this directory: the generators are flushed each time a "systemctl daemon-reload" is issued. The user should
+         * not alter these directories directly. */
         char *generator;
         char *generator_early;
         char *generator_late;
 
-        /* Where to place transient unit files */
+        /* Where to place transient unit files (i.e. those created dynamically via the bus API). Note the special
+         * semantics of this directory: all units created transiently have their unit files removed as the transient
+         * unit is unloaded. The user should not alter this directory directly. */
         char *transient;
 
+        /* Where the snippets created by "systemctl set-property" are placed. Note that for transient units, the
+         * snippets are placed in the transient directory though (see above). The user should not alter this directory
+         * directly. */
+        char *persistent_control;
+        char *runtime_control;
+
         /* The root directory prepended to all items above, or NULL */
         char *root_dir;
 };