core: store unit aliases in a separate set

We allocated the names set for each unit, but in the majority of cases, we'd
put only one name in the set:

$ systemctl show --value -p Names '*'|grep .|grep -v ' '|wc -l
564
$ systemctl show --value -p Names '*'|grep .|grep ' '|wc -l
16

So let's add a separate .id field, and only store aliases in the set, and only
create the set if there's at least one alias. This requires a bit of gymnastics
in the code, but I think this optimization is worth the trouble, because we
save one object for many loaded units.

In particular set_complete_move() wasn't very useful because the target
unit would always have at least one name defined, i.e. the optimization to
move the whole set over would never fire.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 6659406c2d..ffa003da6b 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -117,10 +117,10 @@ typedef struct Unit {
         FreezerState freezer_state;
         sd_bus_message *pending_freezer_message;
 
-        char *id; /* One name is special because we use it for identification. Points to an entry in the names set */
+        char *id;   /* The one special name that we use for identification */
         char *instance;
 
-        Set *names;
+        Set *aliases; /* All the other names. */
 
         /* For each dependency type we maintain a Hashmap whose key is the Unit* object, and the value encodes why the
          * dependency exists, using the UnitDependencyInfo type */