core: rework how we match mount units against each other

Previously to automatically create dependencies between mount units we
matched every mount unit agains all others resulting in O(n^2)
complexity. On setups with large amounts of mount units this might make
things slow.

This change replaces the matching code to use a hashtable that is keyed
by a path prefix, and points to a set of units that require that path to
be around. When a new mount unit is installed it is hence sufficient to
simply look up this set of units via its own file system paths to know
which units to order after itself.

This patch also changes all unit types to only create automatic mount
dependencies via the RequiresMountsFor= logic, and this is exposed to
the outside to make things more transparent.

With this change we still have some O(n) complexities in place when
handling mounts, but that's currently unavoidable due to kernel APIs,
and still substantially better than O(n^2) as before.

https://bugs.freedesktop.org/show_bug.cgi?id=69740

1 files changed, 42 insertions, 2 deletions

diff --git a/src/core/manager.c b/src/core/manager.c
index f70ff03033..bc57e4a1fd 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -770,6 +770,9 @@ void manager_free(Manager *m) {
         for (i = 0; i < RLIMIT_NLIMITS; i++)
                 free(m->rlimit[i]);
 
+        assert(hashmap_isempty(m->units_requiring_mounts_for));
+        hashmap_free(m->units_requiring_mounts_for);
+
         free(m);
 }
 
@@ -782,9 +785,11 @@ int manager_enumerate(Manager *m) {
         /* Let's ask every type to load all units from disk/kernel
          * that it might know */
         for (c = 0; c < _UNIT_TYPE_MAX; c++)
-                if (unit_vtable[c]->enumerate)
-                        if ((q = unit_vtable[c]->enumerate(m)) < 0)
+                if (unit_vtable[c]->enumerate) {
+                        q = unit_vtable[c]->enumerate(m);
+                        if (q < 0)
                                 r = q;
+                }
 
         manager_dispatch_load_queue(m);
         return r;
@@ -2764,6 +2769,41 @@ void manager_status_printf(Manager *m, bool ephemeral, const char *status, const
         va_end(ap);
 }
 
+int manager_get_unit_by_path(Manager *m, const char *path, const char *suffix, Unit **_found) {
+        _cleanup_free_ char *p = NULL;
+        Unit *found;
+
+        assert(m);
+        assert(path);
+        assert(suffix);
+        assert(_found);
+
+        p = unit_name_from_path(path, suffix);
+        if (!p)
+                return -ENOMEM;
+
+        found = manager_get_unit(m, p);
+        if (!found) {
+                *_found = NULL;
+                return 0;
+        }
+
+        *_found = found;
+        return 1;
+}
+
+Set *manager_get_units_requiring_mounts_for(Manager *m, const char *path) {
+        char p[strlen(path)+1];
+
+        assert(m);
+        assert(path);
+
+        strcpy(p, path);
+        path_kill_slashes(p);
+
+        return hashmap_get(m->units_requiring_mounts_for, streq(p, "/") ? "" : p);
+}
+
 void watch_init(Watch *w) {
         assert(w);