core: implement Uphold= dependency type

This is like a really strong version of Wants=, that keeps starting the
specified unit if it is ever found inactive.

This is an alternative to Restart= inside a unit, acknowledging the fact
that whether to keep restarting the unit is sometimes not a property of
the unit itself but the state of the system.

This implements a part of what #4263 requests. i.e. there's no
distinction between "always" and "opportunistic". We just dumbly
implement "always" and become active whenever we see no job queued for
an inactive unit that is supposed to be upheld.

1 files changed, 9 insertions, 3 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index 1a3aaa5020..67367d5e4c 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -229,9 +229,12 @@ typedef struct Unit {
         /* Target dependencies queue */
         LIST_FIELDS(Unit, target_deps_queue);
 
-        /* Queue of units with StopWhenUnneeded set that shell be checked for clean-up. */
+        /* Queue of units with StopWhenUnneeded= set that shall be checked for clean-up. */
         LIST_FIELDS(Unit, stop_when_unneeded_queue);
 
+        /* Queue of units that have an Uphold= dependency from some other unit, and should be checked for starting */
+        LIST_FIELDS(Unit, start_when_upheld_queue);
+
         /* PIDs we keep an eye on. Note that a unit might have many
          * more, but these are the ones we care enough about to
          * process SIGCHLD for */
@@ -260,8 +263,8 @@ typedef struct Unit {
         int success_action_exit_status, failure_action_exit_status;
         char *reboot_arg;
 
-        /* Make sure we never enter endless loops with the check unneeded logic, or the BindsTo= logic */
-        RateLimit auto_stop_ratelimit;
+        /* Make sure we never enter endless loops with the StopWhenUnneeded=, BindsTo=, Uphold= logic */
+        RateLimit auto_start_stop_ratelimit;
 
         /* Reference to a specific UID/GID */
         uid_t ref_uid;
@@ -383,6 +386,7 @@ typedef struct Unit {
         bool in_cgroup_oom_queue:1;
         bool in_target_deps_queue:1;
         bool in_stop_when_unneeded_queue:1;
+        bool in_start_when_upheld_queue:1;
 
         bool sent_dbus_new_signal:1;
 
@@ -740,6 +744,7 @@ void unit_add_to_cleanup_queue(Unit *u);
 void unit_add_to_gc_queue(Unit *u);
 void unit_add_to_target_deps_queue(Unit *u);
 void unit_submit_to_stop_when_unneeded_queue(Unit *u);
+void unit_submit_to_start_when_upheld_queue(Unit *u);
 
 int unit_merge(Unit *u, Unit *other);
 int unit_merge_by_name(Unit *u, const char *other);
@@ -869,6 +874,7 @@ bool unit_type_supported(UnitType t);
 bool unit_is_pristine(Unit *u);
 
 bool unit_is_unneeded(Unit *u);
+bool unit_is_upheld_by_active(Unit *u, Unit **ret_culprit);
 
 pid_t unit_control_pid(Unit *u);
 pid_t unit_main_pid(Unit *u);