core: GC redundant device jobs from the run queue

In contrast to all other unit types device units when queued just track
external state, they cannot effect state changes on their own. Hence unless a
client or other job waits for them there's no reason to keep them in the job
queue. This adds a concept of GC'ing jobs of this type as soon as no client or
other job waits for them anymore.

To ensure this works correctly we need to track which clients actually
reference a job (i.e. which ones enqueued it). Unfortunately that's pretty
nasty to do for direct connections, as sd_bus_track doesn't work for
them. For now, work around this, by simply remembering in a boolean that a job
was requested by a direct connection, and reset it when we notice the direct
connection is gone. This means the GC logic works fine, except that jobs are
not immediately removed when direct connections disconnect.

In the longer term, a rework of the bus logic should fix this properly. For now
this should be good enough, as GC works for fine all cases except this one, and
thus is a clear improvement over the previous behaviour.

Fixes: #1921

1 files changed, 7 insertions, 0 deletions

diff --git a/src/core/job.h b/src/core/job.h
index ccfc7def4d..6fdec9f226 100644
--- a/src/core/job.h
+++ b/src/core/job.h
@@ -107,6 +107,7 @@ enum JobResult {
         JOB_INVALID,             /* JOB_RELOAD of inactive unit */
         JOB_ASSERT,              /* Couldn't start a unit, because an assert didn't hold */
         JOB_UNSUPPORTED,         /* Couldn't start a unit, because the unit type is not supported on the system */
+        JOB_COLLECTED,           /* Job was garbage collected, since nothing needed it anymore */
         _JOB_RESULT_MAX,
         _JOB_RESULT_INVALID = -1
 };
@@ -133,6 +134,7 @@ struct Job {
         LIST_FIELDS(Job, transaction);
         LIST_FIELDS(Job, run_queue);
         LIST_FIELDS(Job, dbus_queue);
+        LIST_FIELDS(Job, gc_queue);
 
         LIST_HEAD(JobDependency, subject_list);
         LIST_HEAD(JobDependency, object_list);
@@ -168,6 +170,8 @@ struct Job {
         bool sent_dbus_new_signal:1;
         bool ignore_order:1;
         bool irreversible:1;
+        bool in_gc_queue:1;
+        bool ref_by_private_bus:1;
 };
 
 Job* job_new(Unit *unit, JobType type);
@@ -227,6 +231,9 @@ void job_shutdown_magic(Job *j);
 
 int job_get_timeout(Job *j, usec_t *timeout) _pure_;
 
+bool job_check_gc(Job *j);
+void job_add_to_gc_queue(Job *j);
+
 const char* job_type_to_string(JobType t) _const_;
 JobType job_type_from_string(const char *s) _pure_;