pid1: use a cache for all unit aliases

This reworks how we load units from disk. Instead of chasing symlinks every
time we are asked to load a unit by name, we slurp all symlinks from disk
and build two hashmaps:
1. from unit name to either alias target, or fragment on disk
   (if an alias, we put just the target name in the hashmap, if a fragment
    we put an absolute path, so we can distinguish both).
2. from a unit name to all aliases

Reading all this data can be pretty costly (40 ms) on my machine, so we keep it
around for reuse.

The advantage is that we can reliably know what all the aliases of a given unit
are. This means we can reliably load dropins under all names. This fixes #11972.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/core/unit.c b/src/core/unit.c
index fa89bd4a4d..7561f1e1a1 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -949,6 +949,9 @@ int unit_merge_by_name(Unit *u, const char *name) {
         Unit *other;
         int r;
 
+        /* Either add name to u, or if a unit with name already exists, merge it with u.
+         * If name is a template, do the same for name@instance, where instance is u's instance. */
+
         assert(u);
         assert(name);