core: add a concept of "dynamic" user ids, that are allocated as long as a service is running

This adds a new boolean setting DynamicUser= to service files. If set, a new
user will be allocated dynamically when the unit is started, and released when
it is stopped. The user ID is allocated from the range 61184..65519. The user
will not be added to /etc/passwd (but an NSS module to be added later should
make it show up in getent passwd).

For now, care should be taken that the service writes no files to disk, since
this might result in files owned by UIDs that might get assigned dynamically to
a different service later on. Later patches will tighten sandboxing in order to
ensure that this cannot happen, except for a few selected directories.

A simple way to test this is:

        systemd-run -p DynamicUser=1 /bin/sleep 99999

1 files changed, 17 insertions, 1 deletions

diff --git a/src/core/manager.c b/src/core/manager.c
index a0181e2138..a4d027f0fc 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -1004,6 +1004,9 @@ Manager* manager_free(Manager *m) {
 
         bus_done(m);
 
+        dynamic_user_vacuum(m, false);
+        hashmap_free(m->dynamic_users);
+
         hashmap_free(m->units);
         hashmap_free(m->jobs);
         hashmap_free(m->watch_pids1);
@@ -1227,6 +1230,9 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds) {
         /* Third, fire things up! */
         manager_coldplug(m);
 
+        /* Release any dynamic users no longer referenced */
+        dynamic_user_vacuum(m, true);
+
         if (serialization) {
                 assert(m->n_reloading > 0);
                 m->n_reloading--;
@@ -2403,6 +2409,10 @@ int manager_serialize(Manager *m, FILE *f, FDSet *fds, bool switching_root) {
 
         bus_track_serialize(m->subscribed, f);
 
+        r = dynamic_user_serialize(m, f, fds);
+        if (r < 0)
+                return r;
+
         fputc('\n', f);
 
         HASHMAP_FOREACH_KEY(u, t, m->units, i) {
@@ -2579,7 +2589,9 @@ int manager_deserialize(Manager *m, FILE *f, FDSet *fds) {
                                 m->kdbus_fd = fdset_remove(fds, fd);
                         }
 
-                } else {
+                } else if (startswith(l, "dynamic-user="))
+                        dynamic_user_deserialize_one(m, l + 13, fds);
+                else {
                         int k;
 
                         k = bus_track_deserialize_item(&m->deserialized_subscribed, l);
@@ -2660,6 +2672,7 @@ int manager_reload(Manager *m) {
         manager_clear_jobs_and_units(m);
         lookup_paths_flush_generator(&m->lookup_paths);
         lookup_paths_free(&m->lookup_paths);
+        dynamic_user_vacuum(m, false);
 
         q = lookup_paths_init(&m->lookup_paths, m->unit_file_scope, 0, NULL);
         if (q < 0 && r >= 0)
@@ -2696,6 +2709,9 @@ int manager_reload(Manager *m) {
         /* Third, fire things up! */
         manager_coldplug(m);
 
+        /* Release any dynamic users no longer referenced */
+        dynamic_user_vacuum(m, true);
+
         /* Sync current state of bus names with our set of listening units */
         if (m->api_bus)
                 manager_sync_bus_names(m, m->api_bus);