core: add NOP jobs, job type collapsing

Two of our current job types are special:
JOB_TRY_RESTART, JOB_RELOAD_OR_START.

They differ from other job types by being sensitive to the unit active state.
They perform some action when the unit is active and some other action
otherwise. This raises a question: when exactly should the unit state be
checked to make the decision?

Currently the unit state is checked when the job becomes runnable. It's more
sensible to check the state immediately when the job is added by the user.
When the user types "systemctl try-restart foo.service", he really intends
to restart the service if it's running right now. If it isn't running right
now, the restart is pointless.

Consider the example (from Bugzilla[1]):

sleep.service takes some time to start.
hello.service has After=sleep.service.
Both services get started. Two jobs will appear:
 hello.service/start  waiting
 sleep.service/start  running
Then someone runs "systemctl try-restart hello.service".

Currently the try-restart operation will block and wait for
sleep.service/start to complete.

The correct result is to complete the try-restart operation immediately
with success, because hello.service is not running. The two original
jobs must not be disturbed by this.

To fix this we introduce two new concepts:
- a new job type: JOB_NOP
  A JOB_NOP job does not do anything to the unit. It does not pull in any
  dependencies. It is always immediately runnable. When installed to a unit,
  it sits in a special slot (u->nop_job) where it never conflicts with
  the installed job (u->job) of a different type. It never merges with jobs
  of other types, but it can merge into an already installed JOB_NOP job.

- "collapsing" of job types
  When a job of one of the two special types is added, the state of the unit
  is checked immediately and the job type changes:
  JOB_TRY_RESTART     -> JOB_RESTART or JOB_NOP
  JOB_RELOAD_OR_START -> JOB_RELOAD or JOB_START
  Should a job type JOB_RELOAD_OR_START appear later during job merging, it
  collapses immediately afterwards.
  Collapsing actually makes some things simpler, because there are now fewer
  job types that are allowed in the transaction.

[1] Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=753586

7 files changed, 233 insertions, 120 deletions

diff --git a/src/core/job.c b/src/core/job.c
index bd25a63d89..301d83a9dc 100644
--- a/src/core/job.c
+++ b/src/core/job.c
@@ -60,6 +60,7 @@ Job* job_new_raw(Unit *unit) {
 
         j->manager = unit->manager;
         j->unit = unit;
+        j->type = _JOB_TYPE_INVALID;
         j->timer_watch.type = WATCH_INVALID;
 
         return j;
@@ -115,15 +116,21 @@ void job_free(Job *j) {
 }
 
 void job_uninstall(Job *j) {
+        Job **pj;
+
         assert(j->installed);
-        assert(j->unit->job == j);
+
+        pj = (j->type == JOB_NOP) ? &j->unit->nop_job : &j->unit->job;
+        assert(*pj == j);
+
         /* Detach from next 'bigger' objects */
 
         /* daemon-reload should be transparent to job observers */
         if (j->manager->n_reloading <= 0)
                 bus_job_send_removed_signal(j);
 
-        j->unit->job = NULL;
+        *pj = NULL;
+
         unit_add_to_gc_queue(j->unit);
 
         hashmap_remove(j->manager->jobs, UINT32_TO_PTR(j->id));
@@ -144,31 +151,38 @@ static bool job_type_allows_late_merge(JobType t) {
          * patched into JOB_START after stopping the unit. So if we see a
          * JOB_RESTART running, it means the unit hasn't stopped yet and at
          * this time the merge is still allowed. */
-        return !(t == JOB_RELOAD || t == JOB_RELOAD_OR_START);
+        return t != JOB_RELOAD;
 }
 
 static void job_merge_into_installed(Job *j, Job *other) {
         assert(j->installed);
         assert(j->unit == other->unit);
 
-        j->type = job_type_lookup_merge(j->type, other->type);
-        assert(j->type >= 0);
+        if (j->type != JOB_NOP)
+                job_type_merge_and_collapse(&j->type, other->type, j->unit);
+        else
+                assert(other->type == JOB_NOP);
 
         j->override = j->override || other->override;
 }
 
 Job* job_install(Job *j) {
-        Job *uj = j->unit->job;
+        Job **pj;
+        Job *uj;
 
         assert(!j->installed);
+        assert(j->type < _JOB_TYPE_MAX_IN_TRANSACTION);
+
+        pj = (j->type == JOB_NOP) ? &j->unit->nop_job : &j->unit->job;
+        uj = *pj;
 
         if (uj) {
-                if (job_type_is_conflicting(uj->type, j->type))
+                if (j->type != JOB_NOP && job_type_is_conflicting(uj->type, j->type))
                         job_finish_and_invalidate(uj, JOB_CANCELED, true);
                 else {
                         /* not conflicting, i.e. mergeable */
 
-                        if (uj->state == JOB_WAITING ||
+                        if (j->type == JOB_NOP || uj->state == JOB_WAITING ||
                             (job_type_allows_late_merge(j->type) && job_type_is_superset(uj->type, j->type))) {
                                 job_merge_into_installed(uj, j);
                                 log_debug("Merged into installed job %s/%s as %u",
@@ -189,23 +203,33 @@ Job* job_install(Job *j) {
         }
 
         /* Install the job */
-        j->unit->job = j;
+        *pj = j;
         j->installed = true;
         j->manager->n_installed_jobs ++;
         log_debug("Installed new job %s/%s as %u", j->unit->id, job_type_to_string(j->type), (unsigned) j->id);
         return j;
 }
 
-void job_install_deserialized(Job *j) {
+int job_install_deserialized(Job *j) {
+        Job **pj;
+
         assert(!j->installed);
 
-        if (j->unit->job) {
+        if (j->type < 0 || j->type >= _JOB_TYPE_MAX_IN_TRANSACTION) {
+                log_debug("Invalid job type %s in deserialization.", strna(job_type_to_string(j->type)));
+                return -EINVAL;
+        }
+
+        pj = (j->type == JOB_NOP) ? &j->unit->nop_job : &j->unit->job;
+
+        if (*pj) {
                 log_debug("Unit %s already has a job installed. Not installing deserialized job.", j->unit->id);
-                return;
+                return -EEXIST;
         }
-        j->unit->job = j;
+        *pj = j;
         j->installed = true;
         log_debug("Reinstalled deserialized job %s/%s as %u", j->unit->id, job_type_to_string(j->type), (unsigned) j->id);
+        return 0;
 }
 
 JobDependency* job_dependency_new(Job *subject, Job *object, bool matters, bool conflicts) {
@@ -268,6 +292,10 @@ void job_dump(Job *j, FILE*f, const char *prefix) {
  * its lower triangle to avoid duplication. We don't store the main diagonal,
  * because A merged with A is simply A.
  *
+ * If the resulting type is collapsed immediately afterwards (to get rid of
+ * the JOB_RELOAD_OR_START, which lies outside the lookup function's domain),
+ * the following properties hold:
+ *
  * Merging is associative! A merged with B merged with C is the same as
  * A merged with C merged with B.
  *
@@ -278,21 +306,19 @@ void job_dump(Job *j, FILE*f, const char *prefix) {
  * be merged with C either.
  */
 static const JobType job_merging_table[] = {
-/* What \ With       *  JOB_START         JOB_VERIFY_ACTIVE  JOB_STOP JOB_RELOAD   JOB_RELOAD_OR_START  JOB_RESTART JOB_TRY_RESTART */
-/************************************************************************************************************************************/
+/* What \ With       *  JOB_START         JOB_VERIFY_ACTIVE  JOB_STOP JOB_RELOAD */
+/*********************************************************************************/
 /*JOB_START          */
 /*JOB_VERIFY_ACTIVE  */ JOB_START,
 /*JOB_STOP           */ -1,                  -1,
 /*JOB_RELOAD         */ JOB_RELOAD_OR_START, JOB_RELOAD,          -1,
-/*JOB_RELOAD_OR_START*/ JOB_RELOAD_OR_START, JOB_RELOAD_OR_START, -1, JOB_RELOAD_OR_START,
-/*JOB_RESTART        */ JOB_RESTART,         JOB_RESTART,         -1, JOB_RESTART,         JOB_RESTART,
-/*JOB_TRY_RESTART    */ JOB_RESTART,         JOB_TRY_RESTART,     -1, JOB_TRY_RESTART,     JOB_RESTART, JOB_RESTART,
+/*JOB_RESTART        */ JOB_RESTART,         JOB_RESTART,         -1, JOB_RESTART,
 };
 
 JobType job_type_lookup_merge(JobType a, JobType b) {
-        assert_cc(ELEMENTSOF(job_merging_table) == _JOB_TYPE_MAX * (_JOB_TYPE_MAX - 1) / 2);
-        assert(a >= 0 && a < _JOB_TYPE_MAX);
-        assert(b >= 0 && b < _JOB_TYPE_MAX);
+        assert_cc(ELEMENTSOF(job_merging_table) == _JOB_TYPE_MAX_MERGING * (_JOB_TYPE_MAX_MERGING - 1) / 2);
+        assert(a >= 0 && a < _JOB_TYPE_MAX_MERGING);
+        assert(b >= 0 && b < _JOB_TYPE_MAX_MERGING);
 
         if (a == b)
                 return a;
@@ -328,24 +354,50 @@ bool job_type_is_redundant(JobType a, UnitActiveState b) {
                 return
                         b == UNIT_RELOADING;
 
-        case JOB_RELOAD_OR_START:
-                return
-                        b == UNIT_ACTIVATING ||
-                        b == UNIT_RELOADING;
-
         case JOB_RESTART:
                 return
                         b == UNIT_ACTIVATING;
 
+        default:
+                assert_not_reached("Invalid job type");
+        }
+}
+
+void job_type_collapse(JobType *t, Unit *u) {
+        UnitActiveState s;
+
+        switch (*t) {
+
         case JOB_TRY_RESTART:
-                return
-                        b == UNIT_ACTIVATING;
+                s = unit_active_state(u);
+                if (UNIT_IS_INACTIVE_OR_DEACTIVATING(s))
+                        *t = JOB_NOP;
+                else
+                        *t = JOB_RESTART;
+                break;
+
+        case JOB_RELOAD_OR_START:
+                s = unit_active_state(u);
+                if (UNIT_IS_INACTIVE_OR_DEACTIVATING(s))
+                        *t = JOB_START;
+                else
+                        *t = JOB_RELOAD;
+                break;
 
         default:
-                assert_not_reached("Invalid job type");
+                ;
         }
 }
 
+int job_type_merge_and_collapse(JobType *a, JobType b, Unit *u) {
+        JobType t = job_type_lookup_merge(*a, b);
+        if (t < 0)
+                return -EEXIST;
+        *a = t;
+        job_type_collapse(a, u);
+        return 0;
+}
+
 bool job_is_runnable(Job *j) {
         Iterator i;
         Unit *other;
@@ -362,10 +414,12 @@ bool job_is_runnable(Job *j) {
         if (j->ignore_order)
                 return true;
 
+        if (j->type == JOB_NOP)
+                return true;
+
         if (j->type == JOB_START ||
             j->type == JOB_VERIFY_ACTIVE ||
-            j->type == JOB_RELOAD ||
-            j->type == JOB_RELOAD_OR_START) {
+            j->type == JOB_RELOAD) {
 
                 /* Immediate result is that the job is or might be
                  * started. In this case lets wait for the
@@ -383,8 +437,7 @@ bool job_is_runnable(Job *j) {
         SET_FOREACH(other, j->unit->dependencies[UNIT_BEFORE], i)
                 if (other->job &&
                     (other->job->type == JOB_STOP ||
-                     other->job->type == JOB_RESTART ||
-                     other->job->type == JOB_TRY_RESTART))
+                     other->job->type == JOB_RESTART))
                         return false;
 
         /* This means that for a service a and a service b where b
@@ -416,6 +469,7 @@ int job_run_and_invalidate(Job *j) {
 
         assert(j);
         assert(j->installed);
+        assert(j->type < _JOB_TYPE_MAX_IN_TRANSACTION);
 
         if (j->in_run_queue) {
                 LIST_REMOVE(Job, run_queue, j->manager->run_queue, j);
@@ -441,15 +495,6 @@ int job_run_and_invalidate(Job *j) {
 
         switch (j->type) {
 
-                case JOB_RELOAD_OR_START:
-                        if (unit_active_state(j->unit) == UNIT_ACTIVE) {
-                                job_change_type(j, JOB_RELOAD);
-                                r = unit_reload(j->unit);
-                                break;
-                        }
-                        job_change_type(j, JOB_START);
-                        /* fall through */
-
                 case JOB_START:
                         r = unit_start(j->unit);
 
@@ -469,14 +514,6 @@ int job_run_and_invalidate(Job *j) {
                         break;
                 }
 
-                case JOB_TRY_RESTART:
-                        if (UNIT_IS_INACTIVE_OR_DEACTIVATING(unit_active_state(j->unit))) {
-                                r = -ENOEXEC;
-                                break;
-                        }
-                        job_change_type(j, JOB_RESTART);
-                        /* fall through */
-
                 case JOB_STOP:
                 case JOB_RESTART:
                         r = unit_stop(j->unit);
@@ -490,11 +527,16 @@ int job_run_and_invalidate(Job *j) {
                         r = unit_reload(j->unit);
                         break;
 
+                case JOB_NOP:
+                        r = -EALREADY;
+                        break;
+
                 default:
                         assert_not_reached("Unknown job type");
         }
 
-        if ((j = manager_get_job(m, id))) {
+        j = manager_get_job(m, id);
+        if (j) {
                 if (r == -EALREADY)
                         r = job_finish_and_invalidate(j, JOB_DONE, true);
                 else if (r == -ENOEXEC)
@@ -564,6 +606,7 @@ int job_finish_and_invalidate(Job *j, JobResult result, bool recursive) {
 
         assert(j);
         assert(j->installed);
+        assert(j->type < _JOB_TYPE_MAX_IN_TRANSACTION);
 
         job_add_to_dbus_queue(j);
 
@@ -597,29 +640,25 @@ int job_finish_and_invalidate(Job *j, JobResult result, bool recursive) {
         if (result != JOB_DONE && recursive) {
 
                 if (t == JOB_START ||
-                    t == JOB_VERIFY_ACTIVE ||
-                    t == JOB_RELOAD_OR_START) {
+                    t == JOB_VERIFY_ACTIVE) {
 
                         SET_FOREACH(other, u->dependencies[UNIT_REQUIRED_BY], i)
                                 if (other->job &&
                                     (other->job->type == JOB_START ||
-                                     other->job->type == JOB_VERIFY_ACTIVE ||
-                                     other->job->type == JOB_RELOAD_OR_START))
+                                     other->job->type == JOB_VERIFY_ACTIVE))
                                         job_finish_and_invalidate(other->job, JOB_DEPENDENCY, true);
 
                         SET_FOREACH(other, u->dependencies[UNIT_BOUND_BY], i)
                                 if (other->job &&
                                     (other->job->type == JOB_START ||
-                                     other->job->type == JOB_VERIFY_ACTIVE ||
-                                     other->job->type == JOB_RELOAD_OR_START))
+                                     other->job->type == JOB_VERIFY_ACTIVE))
                                         job_finish_and_invalidate(other->job, JOB_DEPENDENCY, true);
 
                         SET_FOREACH(other, u->dependencies[UNIT_REQUIRED_BY_OVERRIDABLE], i)
                                 if (other->job &&
                                     !other->job->override &&
                                     (other->job->type == JOB_START ||
-                                     other->job->type == JOB_VERIFY_ACTIVE ||
-                                     other->job->type == JOB_RELOAD_OR_START))
+                                     other->job->type == JOB_VERIFY_ACTIVE))
                                         job_finish_and_invalidate(other->job, JOB_DEPENDENCY, true);
 
                 } else if (t == JOB_STOP) {
@@ -627,8 +666,7 @@ int job_finish_and_invalidate(Job *j, JobResult result, bool recursive) {
                         SET_FOREACH(other, u->dependencies[UNIT_CONFLICTED_BY], i)
                                 if (other->job &&
                                     (other->job->type == JOB_START ||
-                                     other->job->type == JOB_VERIFY_ACTIVE ||
-                                     other->job->type == JOB_RELOAD_OR_START))
+                                     other->job->type == JOB_VERIFY_ACTIVE))
                                         job_finish_and_invalidate(other->job, JOB_DEPENDENCY, true);
                 }
         }
@@ -808,6 +846,8 @@ int job_deserialize(Job *j, FILE *f, FDSet *fds) {
                         JobType t = job_type_from_string(v);
                         if (t < 0)
                                 log_debug("Failed to parse job type %s", v);
+                        else if (t >= _JOB_TYPE_MAX_IN_TRANSACTION)
+                                log_debug("Cannot deserialize job of type %s", v);
                         else
                                 j->type = t;
                 } else if (streq(l, "job-state")) {
@@ -887,6 +927,7 @@ static const char* const job_type_table[_JOB_TYPE_MAX] = {
         [JOB_RELOAD_OR_START] = "reload-or-start",
         [JOB_RESTART] = "restart",
         [JOB_TRY_RESTART] = "try-restart",
+        [JOB_NOP] = "nop",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(job_type, JobType);
diff --git a/src/core/job.h b/src/core/job.h
index 2b679735fc..2e10143d2b 100644
--- a/src/core/job.h
+++ b/src/core/job.h
@@ -46,14 +46,34 @@ enum JobType {
 
         JOB_STOP,
 
-        JOB_RELOAD,                 /* if running reload */
-        JOB_RELOAD_OR_START,        /* if running reload, if not running start */
+        JOB_RELOAD,                 /* if running, reload */
 
         /* Note that restarts are first treated like JOB_STOP, but
          * then instead of finishing are patched to become
          * JOB_START. */
-        JOB_RESTART,                /* if running stop, then start unconditionally */
-        JOB_TRY_RESTART,            /* if running stop and then start */
+        JOB_RESTART,                /* If running, stop. Then start unconditionally. */
+
+        _JOB_TYPE_MAX_MERGING,
+
+        /* JOB_NOP can enter into a transaction, but as it won't pull in
+         * any dependencies, it won't have to merge with anything.
+         * job_install() avoids the problem of merging JOB_NOP too (it's
+         * special-cased, only merges with other JOB_NOPs). */
+        JOB_NOP = _JOB_TYPE_MAX_MERGING, /* do nothing */
+
+        _JOB_TYPE_MAX_IN_TRANSACTION,
+
+        /* JOB_TRY_RESTART can never appear in a transaction, because
+         * it always collapses into JOB_RESTART or JOB_NOP before entering.
+         * Thus we never need to merge it with anything. */
+        JOB_TRY_RESTART = _JOB_TYPE_MAX_IN_TRANSACTION, /* if running, stop and then start */
+
+        /* JOB_RELOAD_OR_START won't enter into a transaction and cannot result
+         * from transaction merging (there's no way for JOB_RELOAD and
+         * JOB_START to meet in one transaction). It can result from a merge
+         * during job installation, but then it will immediately collapse into
+         * one of the two simpler types. */
+        JOB_RELOAD_OR_START,        /* if running, reload, otherwise start */
 
         _JOB_TYPE_MAX,
         _JOB_TYPE_INVALID = -1
@@ -150,7 +170,7 @@ Job* job_new(Unit *unit, JobType type);
 Job* job_new_raw(Unit *unit);
 void job_free(Job *job);
 Job* job_install(Job *j);
-void job_install_deserialized(Job *j);
+int job_install_deserialized(Job *j);
 void job_uninstall(Job *j);
 void job_dump(Job *j, FILE*f, const char *prefix);
 int job_serialize(Job *j, FILE *f, FDSet *fds);
@@ -164,14 +184,6 @@ int job_merge(Job *j, Job *other);
 
 JobType job_type_lookup_merge(JobType a, JobType b);
 
-static inline int job_type_merge(JobType *a, JobType b) {
-        JobType t = job_type_lookup_merge(*a, b);
-        if (t < 0)
-                return -EEXIST;
-        *a = t;
-        return 0;
-}
-
 static inline bool job_type_is_mergeable(JobType a, JobType b) {
         return job_type_lookup_merge(a, b) >= 0;
 }
@@ -187,6 +199,12 @@ static inline bool job_type_is_superset(JobType a, JobType b) {
 
 bool job_type_is_redundant(JobType a, UnitActiveState b);
 
+/* Collapses a state-dependent job type into a simpler type by observing
+ * the state of the unit which it is going to be applied to. */
+void job_type_collapse(JobType *t, Unit *u);
+
+int job_type_merge_and_collapse(JobType *a, JobType b, Unit *u);
+
 bool job_is_runnable(Job *j);
 
 void job_add_to_run_queue(Job *j);
diff --git a/src/core/manager.c b/src/core/manager.c
index a6013668b8..62222e6c7a 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -678,6 +678,8 @@ int manager_add_job(Manager *m, JobType type, Unit *unit, JobMode mode, bool ove
 
         log_debug("Trying to enqueue job %s/%s/%s", unit->id, job_type_to_string(type), job_mode_to_string(mode));
 
+        job_type_collapse(&type, unit);
+
         tr = transaction_new();
         if (!tr)
                 return -ENOMEM;
diff --git a/src/core/transaction.c b/src/core/transaction.c
index 09ed80737d..44fdc06580 100644
--- a/src/core/transaction.c
+++ b/src/core/transaction.c
@@ -236,7 +236,7 @@ static int transaction_merge_jobs(Transaction *tr, DBusError *e) {
 
                 t = j->type;
                 LIST_FOREACH(transaction, k, j->transaction_next) {
-                        if (job_type_merge(&t, k->type) >= 0)
+                        if (job_type_merge_and_collapse(&t, k->type, j->unit) >= 0)
                                 continue;
 
                         /* OK, we could not merge all jobs for this
@@ -262,9 +262,9 @@ static int transaction_merge_jobs(Transaction *tr, DBusError *e) {
                 JobType t = j->type;
                 Job *k;
 
-                /* Merge all transactions */
+                /* Merge all transaction jobs for j->unit */
                 LIST_FOREACH(transaction, k, j->transaction_next)
-                        assert_se(job_type_merge(&t, k->type) == 0);
+                        assert_se(job_type_merge_and_collapse(&t, k->type, j->unit) == 0);
 
                 while ((k = j->transaction_next)) {
                         if (tr->anchor_job == k) {
@@ -809,6 +809,7 @@ int transaction_add_job_and_dependencies(
 
         assert(tr);
         assert(type < _JOB_TYPE_MAX);
+        assert(type < _JOB_TYPE_MAX_IN_TRANSACTION);
         assert(unit);
 
         /* log_debug("Pulling in %s/%s from %s/%s", */
@@ -859,7 +860,8 @@ int transaction_add_job_and_dependencies(
                 assert(!tr->anchor_job);
                 tr->anchor_job = ret;
         }
-        if (is_new && !ignore_requirements) {
+
+        if (is_new && !ignore_requirements && type != JOB_NOP) {
                 Set *following;
 
                 /* If we are following some other unit, make sure we
@@ -879,7 +881,7 @@ int transaction_add_job_and_dependencies(
                 }
 
                 /* Finally, recursively add in all dependencies. */
-                if (type == JOB_START || type == JOB_RELOAD_OR_START || type == JOB_RESTART) {
+                if (type == JOB_START || type == JOB_RESTART) {
                         SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUIRES], i) {
                                 r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, true, override, false, false, ignore_order, e);
                                 if (r < 0) {
@@ -969,7 +971,7 @@ int transaction_add_job_and_dependencies(
 
                 }
 
-                if (type == JOB_STOP || type == JOB_RESTART || type == JOB_TRY_RESTART) {
+                if (type == JOB_STOP || type == JOB_RESTART) {
 
                         SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUIRED_BY], i) {
                                 r = transaction_add_job_and_dependencies(tr, type, dep, ret, true, override, false, false, ignore_order, e);
@@ -994,7 +996,7 @@ int transaction_add_job_and_dependencies(
                         }
                 }
 
-                if (type == JOB_RELOAD || type == JOB_RELOAD_OR_START) {
+                if (type == JOB_RELOAD) {
 
                         SET_FOREACH(dep, ret->unit->dependencies[UNIT_PROPAGATE_RELOAD_TO], i) {
                                 r = transaction_add_job_and_dependencies(tr, JOB_RELOAD, dep, ret, false, override, false, false, ignore_order, e);
@@ -1007,7 +1009,7 @@ int transaction_add_job_and_dependencies(
                         }
                 }
 
-                /* JOB_VERIFY_STARTED, JOB_RELOAD require no dependency handling */
+                /* JOB_VERIFY_STARTED require no dependency handling */
         }
 
         return 0;
diff --git a/src/core/unit.c b/src/core/unit.c
index c5fde63afb..491ba3f33f 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -249,6 +249,9 @@ bool unit_check_gc(Unit *u) {
         if (u->job)
                 return true;
 
+        if (u->nop_job)
+                return true;
+
         if (unit_active_state(u) != UNIT_INACTIVE)
                 return true;
 
@@ -358,6 +361,12 @@ void unit_free(Unit *u) {
                 job_free(j);
         }
 
+        if (u->nop_job) {
+                Job *j = u->nop_job;
+                job_uninstall(j);
+                job_free(j);
+        }
+
         for (d = 0; d < _UNIT_DEPENDENCY_MAX; d++)
                 bidi_set_free(u, u->dependencies[d]);
 
@@ -501,6 +510,9 @@ int unit_merge(Unit *u, Unit *other) {
         if (other->job)
                 return -EEXIST;
 
+        if (other->nop_job)
+                return -EEXIST;
+
         if (!UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(other)))
                 return -EEXIST;
 
@@ -729,6 +741,9 @@ void unit_dump(Unit *u, FILE *f, const char *prefix) {
         if (u->job)
                 job_dump(u->job, f, prefix2);
 
+        if (u->nop_job)
+                job_dump(u->nop_job, f, prefix2);
+
         free(p2);
 }
 
@@ -1507,6 +1522,7 @@ bool unit_job_is_applicable(Unit *u, JobType j) {
         case JOB_VERIFY_ACTIVE:
         case JOB_START:
         case JOB_STOP:
+        case JOB_NOP:
                 return true;
 
         case JOB_RESTART:
@@ -2293,6 +2309,11 @@ int unit_serialize(Unit *u, FILE *f, FDSet *fds) {
                 job_serialize(u->job, f, fds);
         }
 
+        if (u->nop_job) {
+                fprintf(f, "job\n");
+                job_serialize(u->nop_job, f, fds);
+        }
+
         dual_timestamp_serialize(f, "inactive-exit-timestamp", &u->inactive_exit_timestamp);
         dual_timestamp_serialize(f, "active-enter-timestamp", &u->active_enter_timestamp);
         dual_timestamp_serialize(f, "active-exit-timestamp", &u->active_exit_timestamp);
@@ -2382,12 +2403,18 @@ int unit_deserialize(Unit *u, FILE *f, FDSet *fds) {
                                         return r;
                                 }
 
-                                job_install_deserialized(j);
                                 r = hashmap_put(u->manager->jobs, UINT32_TO_PTR(j->id), j);
                                 if (r < 0) {
                                         job_free(j);
                                         return r;
                                 }
+
+                                r = job_install_deserialized(j);
+                                if (r < 0) {
+                                        hashmap_remove(u->manager->jobs, UINT32_TO_PTR(j->id));
+                                        job_free(j);
+                                        return r;
+                                }
                         } else {
                                 /* legacy */
                                 JobType type = job_type_from_string(v);
diff --git a/src/core/unit.h b/src/core/unit.h
index 1b777bfaab..c2bae0a45c 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -158,10 +158,12 @@ struct Unit {
         char *fragment_path; /* if loaded from a config file this is the primary path to it */
         usec_t fragment_mtime;
 
-        /* If there is something to do with this unit, then this is
-         * the job for it */
+        /* If there is something to do with this unit, then this is the installed job for it */
         Job *job;
 
+        /* JOB_NOP jobs are special and can be installed without disturbing the real job. */
+        Job *nop_job;
+
         usec_t job_timeout;
 
         /* References to this */
diff --git a/src/test/test-job-type.c b/src/test/test-job-type.c
index ba8b307dd3..a60be24090 100644
--- a/src/test/test-job-type.c
+++ b/src/test/test-job-type.c
@@ -25,59 +25,80 @@
 #include <unistd.h>
 
 #include "job.h"
+#include "unit.h"
+#include "service.h"
 
 int main(int argc, char*argv[]) {
-        JobType a, b, c, d, e, f, g;
-
-        for (a = 0; a < _JOB_TYPE_MAX; a++)
-                for (b = 0; b < _JOB_TYPE_MAX; b++) {
-
-                        if (!job_type_is_mergeable(a, b))
-                                printf("Not mergeable: %s + %s\n", job_type_to_string(a), job_type_to_string(b));
-
-                        for (c = 0; c < _JOB_TYPE_MAX; c++) {
+        JobType a, b, c, ab, bc, ab_c, bc_a, a_bc;
+        const ServiceState test_states[] = { SERVICE_DEAD, SERVICE_RUNNING };
+        unsigned i;
+        bool merged_ab;
+
+        /* fake a unit */
+        static Service s = {
+                .meta.load_state = UNIT_LOADED,
+                .type = UNIT_SERVICE
+        };
+        Unit *u = UNIT(&s);
+
+        for (i = 0; i < ELEMENTSOF(test_states); i++) {
+                s.state = test_states[i];
+                printf("\nWith collapsing for service state %s\n"
+                       "=========================================\n", service_state_to_string(s.state));
+                for (a = 0; a < _JOB_TYPE_MAX_MERGING; a++) {
+                        for (b = 0; b < _JOB_TYPE_MAX_MERGING; b++) {
+
+                                ab = a;
+                                merged_ab = (job_type_merge_and_collapse(&ab, b, u) >= 0);
+
+                                if (!job_type_is_mergeable(a, b)) {
+                                        assert(!merged_ab);
+                                        printf("Not mergeable: %s + %s\n", job_type_to_string(a), job_type_to_string(b));
+                                        continue;
+                                }
 
-                                /* Verify transitivity of mergeability
-                                 * of job types */
-                                assert(!job_type_is_mergeable(a, b) ||
-                                       !job_type_is_mergeable(b, c) ||
-                                       job_type_is_mergeable(a, c));
+                                assert(merged_ab);
+                                printf("%s + %s = %s\n", job_type_to_string(a), job_type_to_string(b), job_type_to_string(ab));
 
-                                d = a;
-                                if (job_type_merge(&d, b) >= 0) {
+                                for (c = 0; c < _JOB_TYPE_MAX_MERGING; c++) {
 
-                                        printf("%s + %s = %s\n", job_type_to_string(a), job_type_to_string(b), job_type_to_string(d));
+                                        /* Verify transitivity of mergeability of job types */
+                                        assert(!job_type_is_mergeable(a, b) ||
+                                               !job_type_is_mergeable(b, c) ||
+                                               job_type_is_mergeable(a, c));
 
-                                        /* Verify that merged entries can be
-                                         * merged with the same entries they
-                                         * can be merged with separately */
-                                        assert(!job_type_is_mergeable(a, c) || job_type_is_mergeable(d, c));
-                                        assert(!job_type_is_mergeable(b, c) || job_type_is_mergeable(d, c));
+                                        /* Verify that merged entries can be merged with the same entries
+                                         * they can be merged with separately */
+                                        assert(!job_type_is_mergeable(a, c) || job_type_is_mergeable(ab, c));
+                                        assert(!job_type_is_mergeable(b, c) || job_type_is_mergeable(ab, c));
 
-                                        /* Verify that if a merged
-                                         * with b is not mergeable with
-                                         * c then either a or b is not
-                                         * mergeable with c either. */
-                                        assert(job_type_is_mergeable(d, c) || !job_type_is_mergeable(a, c) || !job_type_is_mergeable(b, c));
+                                        /* Verify that if a merged with b is not mergeable with c, then
+                                         * either a or b is not mergeable with c either. */
+                                        assert(job_type_is_mergeable(ab, c) || !job_type_is_mergeable(a, c) || !job_type_is_mergeable(b, c));
 
-                                        e = b;
-                                        if (job_type_merge(&e, c) >= 0) {
+                                        bc = b;
+                                        if (job_type_merge_and_collapse(&bc, c, u) >= 0) {
 
                                                 /* Verify associativity */
 
-                                                f = d;
-                                                assert(job_type_merge(&f, c) == 0);
+                                                ab_c = ab;
+                                                assert(job_type_merge_and_collapse(&ab_c, c, u) == 0);
+
+                                                bc_a = bc;
+                                                assert(job_type_merge_and_collapse(&bc_a, a, u) == 0);
 
-                                                g = e;
-                                                assert(job_type_merge(&g, a) == 0);
+                                                a_bc = a;
+                                                assert(job_type_merge_and_collapse(&a_bc, bc, u) == 0);
 
-                                                assert(f == g);
+                                                assert(ab_c == bc_a);
+                                                assert(ab_c == a_bc);
 
-                                                printf("%s + %s + %s = %s\n", job_type_to_string(a), job_type_to_string(b), job_type_to_string(c), job_type_to_string(d));
+                                                printf("%s + %s + %s = %s\n", job_type_to_string(a), job_type_to_string(b), job_type_to_string(c), job_type_to_string(ab_c));
                                         }
                                 }
                         }
                 }
+        }
 
 
         return 0;