core: add a new unit file setting CollectMode= for tweaking the GC logic

Right now, the option only takes one of two possible values "inactive"
or "inactive-or-failed", the former being the default, and exposing same
behaviour as the status quo ante. If set to "inactive-or-failed" units
may be collected by the GC logic when in the "failed" state too.

This logic should be a nicer alternative to using the "-" modifier for
ExecStart= and friends, as the exit data is collected and logged about
and only removed when the GC comes along. This should be useful in
particular for per-connection socket-activated services, as well as
"systemd-run" command lines that shall leave no artifacts in the
system.

I was thinking about whether to expose this as a boolean, but opted for
an enum instead, as I have the suspicion other tweaks like this might be
a added later on, in which case we extend this setting instead of having
to add yet another one.

Also, let's add some documentation for the GC logic.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/core/unit.h b/src/core/unit.h
index b0df341a03..03dd88dcca 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -45,6 +45,13 @@ typedef enum KillOperation {
         _KILL_OPERATION_INVALID = -1
 } KillOperation;
 
+typedef enum CollectMode {
+        COLLECT_INACTIVE,
+        COLLECT_INACTIVE_OR_FAILED,
+        _COLLECT_MODE_MAX,
+        _COLLECT_MODE_INVALID = -1,
+} CollectMode;
+
 static inline bool UNIT_IS_ACTIVE_OR_RELOADING(UnitActiveState t) {
         return IN_SET(t, UNIT_ACTIVE, UNIT_RELOADING);
 }
@@ -282,6 +289,9 @@ struct Unit {
         /* How to start OnFailure units */
         JobMode on_failure_job_mode;
 
+        /* Tweaking the GC logic */
+        CollectMode collect_mode;
+
         /* The current invocation ID */
         sd_id128_t invocation_id;
         char invocation_id_string[SD_ID128_STRING_MAX]; /* useful when logging */
@@ -773,3 +783,6 @@ void unit_unlink_state_files(Unit *u);
 #define LOG_UNIT_MESSAGE(unit, fmt, ...) "MESSAGE=%s: " fmt, (unit)->id, ##__VA_ARGS__
 #define LOG_UNIT_ID(unit) (unit)->manager->unit_log_format_string, (unit)->id
 #define LOG_UNIT_INVOCATION_ID(unit) (unit)->manager->invocation_log_format_string, (unit)->invocation_id_string
+
+const char* collect_mode_to_string(CollectMode m) _const_;
+CollectMode collect_mode_from_string(const char *s) _pure_;