manager: split transaction.[ch]

manager.c takes care of the main loop, unit management, signal handling, ...
transaction.c computes transactions.

After split:
manager.c:     65 KB
transaction.c: 40 KB

5 files changed, 1139 insertions, 1106 deletions

diff --git a/src/core/job.h b/src/core/job.h
index 4c543f20b1..e25fc26235 100644
--- a/src/core/job.h
+++ b/src/core/job.h
@@ -34,6 +34,7 @@ typedef enum JobMode JobMode;
 typedef enum JobResult JobResult;
 
 #include "manager.h"
+#include "transaction.h"
 #include "unit.h"
 #include "hashmap.h"
 #include "list.h"
diff --git a/src/core/manager.c b/src/core/manager.c
index 52702f3b4c..f070295d88 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -44,6 +44,7 @@
 #include <systemd/sd-daemon.h>
 
 #include "manager.h"
+#include "transaction.h"
 #include "hashmap.h"
 #include "macro.h"
 #include "strv.h"
@@ -653,1105 +654,6 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds) {
         return r;
 }
 
-static void transaction_unlink_job(Transaction *tr, Job *j, bool delete_dependencies);
-
-static void transaction_delete_job(Transaction *tr, Job *j, bool delete_dependencies) {
-        assert(tr);
-        assert(j);
-
-        /* Deletes one job from the transaction */
-
-        transaction_unlink_job(tr, j, delete_dependencies);
-
-        if (!j->installed)
-                job_free(j);
-}
-
-static void transaction_delete_unit(Transaction *tr, Unit *u) {
-        Job *j;
-
-        /* Deletes all jobs associated with a certain unit from the
-         * transaction */
-
-        while ((j = hashmap_get(tr->jobs, u)))
-                transaction_delete_job(tr, j, true);
-}
-
-static void transaction_abort(Transaction *tr) {
-        Job *j;
-
-        assert(tr);
-
-        while ((j = hashmap_first(tr->jobs)))
-                transaction_delete_job(tr, j, true);
-
-        assert(hashmap_isempty(tr->jobs));
-
-        assert(!tr->anchor);
-}
-
-static void transaction_find_jobs_that_matter_to_anchor(Transaction *tr, Job *j, unsigned generation) {
-        JobDependency *l;
-
-        assert(tr);
-
-        /* A recursive sweep through the graph that marks all units
-         * that matter to the anchor job, i.e. are directly or
-         * indirectly a dependency of the anchor job via paths that
-         * are fully marked as mattering. */
-
-        if (j)
-                l = j->subject_list;
-        else
-                l = tr->anchor;
-
-        LIST_FOREACH(subject, l, l) {
-
-                /* This link does not matter */
-                if (!l->matters)
-                        continue;
-
-                /* This unit has already been marked */
-                if (l->object->generation == generation)
-                        continue;
-
-                l->object->matters_to_anchor = true;
-                l->object->generation = generation;
-
-                transaction_find_jobs_that_matter_to_anchor(tr, l->object, generation);
-        }
-}
-
-static void transaction_merge_and_delete_job(Transaction *tr, Job *j, Job *other, JobType t) {
-        JobDependency *l, *last;
-
-        assert(j);
-        assert(other);
-        assert(j->unit == other->unit);
-        assert(!j->installed);
-
-        /* Merges 'other' into 'j' and then deletes 'other'. */
-
-        j->type = t;
-        j->state = JOB_WAITING;
-        j->override = j->override || other->override;
-
-        j->matters_to_anchor = j->matters_to_anchor || other->matters_to_anchor;
-
-        /* Patch us in as new owner of the JobDependency objects */
-        last = NULL;
-        LIST_FOREACH(subject, l, other->subject_list) {
-                assert(l->subject == other);
-                l->subject = j;
-                last = l;
-        }
-
-        /* Merge both lists */
-        if (last) {
-                last->subject_next = j->subject_list;
-                if (j->subject_list)
-                        j->subject_list->subject_prev = last;
-                j->subject_list = other->subject_list;
-        }
-
-        /* Patch us in as new owner of the JobDependency objects */
-        last = NULL;
-        LIST_FOREACH(object, l, other->object_list) {
-                assert(l->object == other);
-                l->object = j;
-                last = l;
-        }
-
-        /* Merge both lists */
-        if (last) {
-                last->object_next = j->object_list;
-                if (j->object_list)
-                        j->object_list->object_prev = last;
-                j->object_list = other->object_list;
-        }
-
-        /* Kill the other job */
-        other->subject_list = NULL;
-        other->object_list = NULL;
-        transaction_delete_job(tr, other, true);
-}
-
-static bool job_is_conflicted_by(Job *j) {
-        JobDependency *l;
-
-        assert(j);
-
-        /* Returns true if this job is pulled in by a least one
-         * ConflictedBy dependency. */
-
-        LIST_FOREACH(object, l, j->object_list)
-                if (l->conflicts)
-                        return true;
-
-        return false;
-}
-
-static int delete_one_unmergeable_job(Transaction *tr, Job *j) {
-        Job *k;
-
-        assert(j);
-
-        /* Tries to delete one item in the linked list
-         * j->transaction_next->transaction_next->... that conflicts
-         * with another one, in an attempt to make an inconsistent
-         * transaction work. */
-
-        /* We rely here on the fact that if a merged with b does not
-         * merge with c, either a or b merge with c neither */
-        LIST_FOREACH(transaction, j, j)
-                LIST_FOREACH(transaction, k, j->transaction_next) {
-                        Job *d;
-
-                        /* Is this one mergeable? Then skip it */
-                        if (job_type_is_mergeable(j->type, k->type))
-                                continue;
-
-                        /* Ok, we found two that conflict, let's see if we can
-                         * drop one of them */
-                        if (!j->matters_to_anchor && !k->matters_to_anchor) {
-
-                                /* Both jobs don't matter, so let's
-                                 * find the one that is smarter to
-                                 * remove. Let's think positive and
-                                 * rather remove stops then starts --
-                                 * except if something is being
-                                 * stopped because it is conflicted by
-                                 * another unit in which case we
-                                 * rather remove the start. */
-
-                                log_debug("Looking at job %s/%s conflicted_by=%s", j->unit->id, job_type_to_string(j->type), yes_no(j->type == JOB_STOP && job_is_conflicted_by(j)));
-                                log_debug("Looking at job %s/%s conflicted_by=%s", k->unit->id, job_type_to_string(k->type), yes_no(k->type == JOB_STOP && job_is_conflicted_by(k)));
-
-                                if (j->type == JOB_STOP) {
-
-                                        if (job_is_conflicted_by(j))
-                                                d = k;
-                                        else
-                                                d = j;
-
-                                } else if (k->type == JOB_STOP) {
-
-                                        if (job_is_conflicted_by(k))
-                                                d = j;
-                                        else
-                                                d = k;
-                                } else
-                                        d = j;
-
-                        } else if (!j->matters_to_anchor)
-                                d = j;
-                        else if (!k->matters_to_anchor)
-                                d = k;
-                        else
-                                return -ENOEXEC;
-
-                        /* Ok, we can drop one, so let's do so. */
-                        log_debug("Fixing conflicting jobs by deleting job %s/%s", d->unit->id, job_type_to_string(d->type));
-                        transaction_delete_job(tr, d, true);
-                        return 0;
-                }
-
-        return -EINVAL;
-}
-
-static int transaction_merge_jobs(Transaction *tr, DBusError *e) {
-        Job *j;
-        Iterator i;
-        int r;
-
-        assert(tr);
-
-        /* First step, check whether any of the jobs for one specific
-         * task conflict. If so, try to drop one of them. */
-        HASHMAP_FOREACH(j, tr->jobs, i) {
-                JobType t;
-                Job *k;
-
-                t = j->type;
-                LIST_FOREACH(transaction, k, j->transaction_next) {
-                        if (job_type_merge(&t, k->type) >= 0)
-                                continue;
-
-                        /* OK, we could not merge all jobs for this
-                         * action. Let's see if we can get rid of one
-                         * of them */
-
-                        r = delete_one_unmergeable_job(tr, j);
-                        if (r >= 0)
-                                /* Ok, we managed to drop one, now
-                                 * let's ask our callers to call us
-                                 * again after garbage collecting */
-                                return -EAGAIN;
-
-                        /* We couldn't merge anything. Failure */
-                        dbus_set_error(e, BUS_ERROR_TRANSACTION_JOBS_CONFLICTING, "Transaction contains conflicting jobs '%s' and '%s' for %s. Probably contradicting requirement dependencies configured.",
-                                       job_type_to_string(t), job_type_to_string(k->type), k->unit->id);
-                        return r;
-                }
-        }
-
-        /* Second step, merge the jobs. */
-        HASHMAP_FOREACH(j, tr->jobs, i) {
-                JobType t = j->type;
-                Job *k;
-
-                /* Merge all transactions */
-                LIST_FOREACH(transaction, k, j->transaction_next)
-                        assert_se(job_type_merge(&t, k->type) == 0);
-
-                /* If an active job is mergeable, merge it too */
-                if (j->unit->job)
-                        job_type_merge(&t, j->unit->job->type); /* Might fail. Which is OK */
-
-                while ((k = j->transaction_next)) {
-                        if (j->installed) {
-                                transaction_merge_and_delete_job(tr, k, j, t);
-                                j = k;
-                        } else
-                                transaction_merge_and_delete_job(tr, j, k, t);
-                }
-
-                if (j->unit->job && !j->installed)
-                        transaction_merge_and_delete_job(tr, j, j->unit->job, t);
-
-                assert(!j->transaction_next);
-                assert(!j->transaction_prev);
-        }
-
-        return 0;
-}
-
-static void transaction_drop_redundant(Transaction *tr) {
-        bool again;
-
-        assert(tr);
-
-        /* Goes through the transaction and removes all jobs that are
-         * a noop */
-
-        do {
-                Job *j;
-                Iterator i;
-
-                again = false;
-
-                HASHMAP_FOREACH(j, tr->jobs, i) {
-                        bool changes_something = false;
-                        Job *k;
-
-                        LIST_FOREACH(transaction, k, j) {
-
-                                if (!job_is_anchor(k) &&
-                                    (k->installed || job_type_is_redundant(k->type, unit_active_state(k->unit))) &&
-                                    (!k->unit->job || !job_type_is_conflicting(k->type, k->unit->job->type)))
-                                        continue;
-
-                                changes_something = true;
-                                break;
-                        }
-
-                        if (changes_something)
-                                continue;
-
-                        /* log_debug("Found redundant job %s/%s, dropping.", j->unit->id, job_type_to_string(j->type)); */
-                        transaction_delete_job(tr, j, false);
-                        again = true;
-                        break;
-                }
-
-        } while (again);
-}
-
-static bool unit_matters_to_anchor(Unit *u, Job *j) {
-        assert(u);
-        assert(!j->transaction_prev);
-
-        /* Checks whether at least one of the jobs for this unit
-         * matters to the anchor. */
-
-        LIST_FOREACH(transaction, j, j)
-                if (j->matters_to_anchor)
-                        return true;
-
-        return false;
-}
-
-static int transaction_verify_order_one(Transaction *tr, Job *j, Job *from, unsigned generation, DBusError *e) {
-        Iterator i;
-        Unit *u;
-        int r;
-
-        assert(tr);
-        assert(j);
-        assert(!j->transaction_prev);
-
-        /* Does a recursive sweep through the ordering graph, looking
-         * for a cycle. If we find cycle we try to break it. */
-
-        /* Have we seen this before? */
-        if (j->generation == generation) {
-                Job *k, *delete;
-
-                /* If the marker is NULL we have been here already and
-                 * decided the job was loop-free from here. Hence
-                 * shortcut things and return right-away. */
-                if (!j->marker)
-                        return 0;
-
-                /* So, the marker is not NULL and we already have been
-                 * here. We have a cycle. Let's try to break it. We go
-                 * backwards in our path and try to find a suitable
-                 * job to remove. We use the marker to find our way
-                 * back, since smart how we are we stored our way back
-                 * in there. */
-                log_warning("Found ordering cycle on %s/%s", j->unit->id, job_type_to_string(j->type));
-
-                delete = NULL;
-                for (k = from; k; k = ((k->generation == generation && k->marker != k) ? k->marker : NULL)) {
-
-                        log_info("Walked on cycle path to %s/%s", k->unit->id, job_type_to_string(k->type));
-
-                        if (!delete &&
-                            !k->installed &&
-                            !unit_matters_to_anchor(k->unit, k)) {
-                                /* Ok, we can drop this one, so let's
-                                 * do so. */
-                                delete = k;
-                        }
-
-                        /* Check if this in fact was the beginning of
-                         * the cycle */
-                        if (k == j)
-                                break;
-                }
-
-
-                if (delete) {
-                        log_warning("Breaking ordering cycle by deleting job %s/%s", delete->unit->id, job_type_to_string(delete->type));
-                        transaction_delete_unit(tr, delete->unit);
-                        return -EAGAIN;
-                }
-
-                log_error("Unable to break cycle");
-
-                dbus_set_error(e, BUS_ERROR_TRANSACTION_ORDER_IS_CYCLIC, "Transaction order is cyclic. See system logs for details.");
-                return -ENOEXEC;
-        }
-
-        /* Make the marker point to where we come from, so that we can
-         * find our way backwards if we want to break a cycle. We use
-         * a special marker for the beginning: we point to
-         * ourselves. */
-        j->marker = from ? from : j;
-        j->generation = generation;
-
-        /* We assume that the the dependencies are bidirectional, and
-         * hence can ignore UNIT_AFTER */
-        SET_FOREACH(u, j->unit->dependencies[UNIT_BEFORE], i) {
-                Job *o;
-
-                /* Is there a job for this unit? */
-                o = hashmap_get(tr->jobs, u);
-                if (!o) {
-                        /* Ok, there is no job for this in the
-                         * transaction, but maybe there is already one
-                         * running? */
-                        o = u->job;
-                        if (!o)
-                                continue;
-                }
-
-                r = transaction_verify_order_one(tr, o, j, generation, e);
-                if (r < 0)
-                        return r;
-        }
-
-        /* Ok, let's backtrack, and remember that this entry is not on
-         * our path anymore. */
-        j->marker = NULL;
-
-        return 0;
-}
-
-static int transaction_verify_order(Transaction *tr, unsigned *generation, DBusError *e) {
-        Job *j;
-        int r;
-        Iterator i;
-        unsigned g;
-
-        assert(tr);
-        assert(generation);
-
-        /* Check if the ordering graph is cyclic. If it is, try to fix
-         * that up by dropping one of the jobs. */
-
-        g = (*generation)++;
-
-        HASHMAP_FOREACH(j, tr->jobs, i)
-                if ((r = transaction_verify_order_one(tr, j, NULL, g, e)) < 0)
-                        return r;
-
-        return 0;
-}
-
-static void transaction_collect_garbage(Transaction *tr) {
-        bool again;
-
-        assert(tr);
-
-        /* Drop jobs that are not required by any other job */
-
-        do {
-                Iterator i;
-                Job *j;
-
-                again = false;
-
-                HASHMAP_FOREACH(j, tr->jobs, i) {
-                        if (j->object_list) {
-                                /* log_debug("Keeping job %s/%s because of %s/%s", */
-                                /*           j->unit->id, job_type_to_string(j->type), */
-                                /*           j->object_list->subject ? j->object_list->subject->unit->id : "root", */
-                                /*           j->object_list->subject ? job_type_to_string(j->object_list->subject->type) : "root"); */
-                                continue;
-                        }
-
-                        /* log_debug("Garbage collecting job %s/%s", j->unit->id, job_type_to_string(j->type)); */
-                        transaction_delete_job(tr, j, true);
-                        again = true;
-                        break;
-                }
-
-        } while (again);
-}
-
-static int transaction_is_destructive(Transaction *tr, DBusError *e) {
-        Iterator i;
-        Job *j;
-
-        assert(tr);
-
-        /* Checks whether applying this transaction means that
-         * existing jobs would be replaced */
-
-        HASHMAP_FOREACH(j, tr->jobs, i) {
-
-                /* Assume merged */
-                assert(!j->transaction_prev);
-                assert(!j->transaction_next);
-
-                if (j->unit->job &&
-                    j->unit->job != j &&
-                    !job_type_is_superset(j->type, j->unit->job->type)) {
-
-                        dbus_set_error(e, BUS_ERROR_TRANSACTION_IS_DESTRUCTIVE, "Transaction is destructive.");
-                        return -EEXIST;
-                }
-        }
-
-        return 0;
-}
-
-static void transaction_minimize_impact(Transaction *tr) {
-        bool again;
-        assert(tr);
-
-        /* Drops all unnecessary jobs that reverse already active jobs
-         * or that stop a running service. */
-
-        do {
-                Job *j;
-                Iterator i;
-
-                again = false;
-
-                HASHMAP_FOREACH(j, tr->jobs, i) {
-                        LIST_FOREACH(transaction, j, j) {
-                                bool stops_running_service, changes_existing_job;
-
-                                /* If it matters, we shouldn't drop it */
-                                if (j->matters_to_anchor)
-                                        continue;
-
-                                /* Would this stop a running service?
-                                 * Would this change an existing job?
-                                 * If so, let's drop this entry */
-
-                                stops_running_service =
-                                        j->type == JOB_STOP && UNIT_IS_ACTIVE_OR_ACTIVATING(unit_active_state(j->unit));
-
-                                changes_existing_job =
-                                        j->unit->job &&
-                                        job_type_is_conflicting(j->type, j->unit->job->type);
-
-                                if (!stops_running_service && !changes_existing_job)
-                                        continue;
-
-                                if (stops_running_service)
-                                        log_debug("%s/%s would stop a running service.", j->unit->id, job_type_to_string(j->type));
-
-                                if (changes_existing_job)
-                                        log_debug("%s/%s would change existing job.", j->unit->id, job_type_to_string(j->type));
-
-                                /* Ok, let's get rid of this */
-                                log_debug("Deleting %s/%s to minimize impact.", j->unit->id, job_type_to_string(j->type));
-
-                                transaction_delete_job(tr, j, true);
-                                again = true;
-                                break;
-                        }
-
-                        if (again)
-                                break;
-                }
-
-        } while (again);
-}
-
-static int transaction_apply(Transaction *tr, Manager *m, JobMode mode) {
-        Iterator i;
-        Job *j;
-        int r;
-
-        /* Moves the transaction jobs to the set of active jobs */
-
-        if (mode == JOB_ISOLATE) {
-
-                /* When isolating first kill all installed jobs which
-                 * aren't part of the new transaction */
-        rescan:
-                HASHMAP_FOREACH(j, m->jobs, i) {
-                        assert(j->installed);
-
-                        if (hashmap_get(tr->jobs, j->unit))
-                                continue;
-
-                        /* 'j' itself is safe to remove, but if other jobs
-                           are invalidated recursively, our iterator may become
-                           invalid and we need to start over. */
-                        if (job_finish_and_invalidate(j, JOB_CANCELED) > 0)
-                                goto rescan;
-                }
-        }
-
-        HASHMAP_FOREACH(j, tr->jobs, i) {
-                /* Assume merged */
-                assert(!j->transaction_prev);
-                assert(!j->transaction_next);
-
-                if (j->installed)
-                        continue;
-
-                r = hashmap_put(m->jobs, UINT32_TO_PTR(j->id), j);
-                if (r < 0)
-                        goto rollback;
-        }
-
-        while ((j = hashmap_steal_first(tr->jobs))) {
-                Job *uj;
-                if (j->installed) {
-                        /* log_debug("Skipping already installed job %s/%s as %u", j->unit->id, job_type_to_string(j->type), (unsigned) j->id); */
-                        continue;
-                }
-
-                uj = j->unit->job;
-                if (uj) {
-                        job_uninstall(uj);
-                        job_free(uj);
-                }
-
-                j->unit->job = j;
-                j->installed = true;
-                m->n_installed_jobs ++;
-
-                /* We're fully installed. Now let's free data we don't
-                 * need anymore. */
-
-                assert(!j->transaction_next);
-                assert(!j->transaction_prev);
-
-                /* Clean the job dependencies */
-                transaction_unlink_job(tr, j, false);
-
-                job_add_to_run_queue(j);
-                job_add_to_dbus_queue(j);
-                job_start_timer(j);
-
-                log_debug("Installed new job %s/%s as %u", j->unit->id, job_type_to_string(j->type), (unsigned) j->id);
-        }
-
-        assert(!tr->anchor);
-
-        return 0;
-
-rollback:
-
-        HASHMAP_FOREACH(j, tr->jobs, i) {
-                if (j->installed)
-                        continue;
-
-                hashmap_remove(m->jobs, UINT32_TO_PTR(j->id));
-        }
-
-        return r;
-}
-
-static int transaction_activate(Transaction *tr, Manager *m, JobMode mode, DBusError *e) {
-        int r;
-        unsigned generation = 1;
-
-        assert(tr);
-
-        /* This applies the changes recorded in tr->jobs to
-         * the actual list of jobs, if possible. */
-
-        /* First step: figure out which jobs matter */
-        transaction_find_jobs_that_matter_to_anchor(tr, NULL, generation++);
-
-        /* Second step: Try not to stop any running services if
-         * we don't have to. Don't try to reverse running
-         * jobs if we don't have to. */
-        if (mode == JOB_FAIL)
-                transaction_minimize_impact(tr);
-
-        /* Third step: Drop redundant jobs */
-        transaction_drop_redundant(tr);
-
-        for (;;) {
-                /* Fourth step: Let's remove unneeded jobs that might
-                 * be lurking. */
-                if (mode != JOB_ISOLATE)
-                        transaction_collect_garbage(tr);
-
-                /* Fifth step: verify order makes sense and correct
-                 * cycles if necessary and possible */
-                r = transaction_verify_order(tr, &generation, e);
-                if (r >= 0)
-                        break;
-
-                if (r != -EAGAIN) {
-                        log_warning("Requested transaction contains an unfixable cyclic ordering dependency: %s", bus_error(e, r));
-                        return r;
-                }
-
-                /* Let's see if the resulting transaction ordering
-                 * graph is still cyclic... */
-        }
-
-        for (;;) {
-                /* Sixth step: let's drop unmergeable entries if
-                 * necessary and possible, merge entries we can
-                 * merge */
-                r = transaction_merge_jobs(tr, e);
-                if (r >= 0)
-                        break;
-
-                if (r != -EAGAIN) {
-                        log_warning("Requested transaction contains unmergeable jobs: %s", bus_error(e, r));
-                        return r;
-                }
-
-                /* Seventh step: an entry got dropped, let's garbage
-                 * collect its dependencies. */
-                if (mode != JOB_ISOLATE)
-                        transaction_collect_garbage(tr);
-
-                /* Let's see if the resulting transaction still has
-                 * unmergeable entries ... */
-        }
-
-        /* Eights step: Drop redundant jobs again, if the merging now allows us to drop more. */
-        transaction_drop_redundant(tr);
-
-        /* Ninth step: check whether we can actually apply this */
-        if (mode == JOB_FAIL) {
-                r = transaction_is_destructive(tr, e);
-                if (r < 0) {
-                        log_notice("Requested transaction contradicts existing jobs: %s", bus_error(e, r));
-                        return r;
-                }
-        }
-
-        /* Tenth step: apply changes */
-        r = transaction_apply(tr, m, mode);
-        if (r < 0) {
-                log_warning("Failed to apply transaction: %s", strerror(-r));
-                return r;
-        }
-
-        assert(hashmap_isempty(tr->jobs));
-        assert(!tr->anchor);
-
-        return 0;
-}
-
-static Job* transaction_add_one_job(Transaction *tr, JobType type, Unit *unit, bool override, bool *is_new) {
-        Job *j, *f;
-
-        assert(tr);
-        assert(unit);
-
-        /* Looks for an existing prospective job and returns that. If
-         * it doesn't exist it is created and added to the prospective
-         * jobs list. */
-
-        f = hashmap_get(tr->jobs, unit);
-
-        LIST_FOREACH(transaction, j, f) {
-                assert(j->unit == unit);
-
-                if (j->type == type) {
-                        if (is_new)
-                                *is_new = false;
-                        return j;
-                }
-        }
-
-        if (unit->job && unit->job->type == type)
-                j = unit->job;
-        else {
-                j = job_new(unit->manager, type, unit);
-                if (!j)
-                        return NULL;
-        }
-
-        j->generation = 0;
-        j->marker = NULL;
-        j->matters_to_anchor = false;
-        j->override = override;
-
-        LIST_PREPEND(Job, transaction, f, j);
-
-        if (hashmap_replace(tr->jobs, unit, f) < 0) {
-                LIST_REMOVE(Job, transaction, f, j);
-                job_free(j);
-                return NULL;
-        }
-
-        if (is_new)
-                *is_new = true;
-
-        /* log_debug("Added job %s/%s to transaction.", unit->id, job_type_to_string(type)); */
-
-        return j;
-}
-
-static void transaction_unlink_job(Transaction *tr, Job *j, bool delete_dependencies) {
-        assert(tr);
-        assert(j);
-
-        if (j->transaction_prev)
-                j->transaction_prev->transaction_next = j->transaction_next;
-        else if (j->transaction_next)
-                hashmap_replace(tr->jobs, j->unit, j->transaction_next);
-        else
-                hashmap_remove_value(tr->jobs, j->unit, j);
-
-        if (j->transaction_next)
-                j->transaction_next->transaction_prev = j->transaction_prev;
-
-        j->transaction_prev = j->transaction_next = NULL;
-
-        while (j->subject_list)
-                job_dependency_free(j->subject_list, tr);
-
-        while (j->object_list) {
-                Job *other = j->object_list->matters ? j->object_list->subject : NULL;
-
-                job_dependency_free(j->object_list, tr);
-
-                if (other && delete_dependencies) {
-                        log_debug("Deleting job %s/%s as dependency of job %s/%s",
-                                  other->unit->id, job_type_to_string(other->type),
-                                  j->unit->id, job_type_to_string(j->type));
-                        transaction_delete_job(tr, other, delete_dependencies);
-                }
-        }
-}
-
-static int transaction_add_job_and_dependencies(
-                Transaction *tr,
-                JobType type,
-                Unit *unit,
-                Job *by,
-                bool matters,
-                bool override,
-                bool conflicts,
-                bool ignore_requirements,
-                bool ignore_order,
-                DBusError *e,
-                Job **_ret) {
-        Job *ret;
-        Iterator i;
-        Unit *dep;
-        int r;
-        bool is_new;
-
-        assert(tr);
-        assert(type < _JOB_TYPE_MAX);
-        assert(unit);
-
-        /* log_debug("Pulling in %s/%s from %s/%s", */
-        /*           unit->id, job_type_to_string(type), */
-        /*           by ? by->unit->id : "NA", */
-        /*           by ? job_type_to_string(by->type) : "NA"); */
-
-        if (unit->load_state != UNIT_LOADED &&
-            unit->load_state != UNIT_ERROR &&
-            unit->load_state != UNIT_MASKED) {
-                dbus_set_error(e, BUS_ERROR_LOAD_FAILED, "Unit %s is not loaded properly.", unit->id);
-                return -EINVAL;
-        }
-
-        if (type != JOB_STOP && unit->load_state == UNIT_ERROR) {
-                dbus_set_error(e, BUS_ERROR_LOAD_FAILED,
-                               "Unit %s failed to load: %s. "
-                               "See system logs and 'systemctl status %s' for details.",
-                               unit->id,
-                               strerror(-unit->load_error),
-                               unit->id);
-                return -EINVAL;
-        }
-
-        if (type != JOB_STOP && unit->load_state == UNIT_MASKED) {
-                dbus_set_error(e, BUS_ERROR_MASKED, "Unit %s is masked.", unit->id);
-                return -EINVAL;
-        }
-
-        if (!unit_job_is_applicable(unit, type)) {
-                dbus_set_error(e, BUS_ERROR_JOB_TYPE_NOT_APPLICABLE, "Job type %s is not applicable for unit %s.", job_type_to_string(type), unit->id);
-                return -EBADR;
-        }
-
-        /* First add the job. */
-        ret = transaction_add_one_job(tr, type, unit, override, &is_new);
-        if (!ret)
-                return -ENOMEM;
-
-        ret->ignore_order = ret->ignore_order || ignore_order;
-
-        /* Then, add a link to the job. */
-        if (!job_dependency_new(by, ret, matters, conflicts, tr))
-                return -ENOMEM;
-
-        if (is_new && !ignore_requirements) {
-                Set *following;
-
-                /* If we are following some other unit, make sure we
-                 * add all dependencies of everybody following. */
-                if (unit_following_set(ret->unit, &following) > 0) {
-                        SET_FOREACH(dep, following, i) {
-                                r = transaction_add_job_and_dependencies(tr, type, dep, ret, false, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        set_free(following);
-                }
-
-                /* Finally, recursively add in all dependencies. */
-                if (type == JOB_START || type == JOB_RELOAD_OR_START) {
-                        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, NULL);
-                                if (r < 0) {
-                                        if (r != -EBADR)
-                                                goto fail;
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_BIND_TO], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, true, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        if (r != -EBADR)
-                                                goto fail;
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUIRES_OVERRIDABLE], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, !override, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_WANTS], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, false, false, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUISITE], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_VERIFY_ACTIVE, dep, ret, true, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        if (r != -EBADR)
-                                                goto fail;
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUISITE_OVERRIDABLE], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_VERIFY_ACTIVE, dep, ret, !override, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_CONFLICTS], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_STOP, dep, ret, true, override, true, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        if (r != -EBADR)
-                                                goto fail;
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_CONFLICTED_BY], i) {
-                                r = transaction_add_job_and_dependencies(tr, JOB_STOP, dep, ret, false, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                }
-
-                if (type == JOB_STOP || type == JOB_RESTART || type == JOB_TRY_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, NULL);
-                                if (r < 0) {
-                                        if (r != -EBADR)
-                                                goto fail;
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-
-                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_BOUND_BY], i) {
-                                r = transaction_add_job_and_dependencies(tr, type, dep, ret, true, override, false, false, ignore_order, e, NULL);
-                                if (r < 0) {
-                                        if (r != -EBADR)
-                                                goto fail;
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-                }
-
-                if (type == JOB_RELOAD || type == JOB_RELOAD_OR_START) {
-
-                        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, NULL);
-                                if (r < 0) {
-                                        log_warning("Cannot add dependency reload job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
-
-                                        if (e)
-                                                dbus_error_free(e);
-                                }
-                        }
-                }
-
-                /* JOB_VERIFY_STARTED, JOB_RELOAD require no dependency handling */
-        }
-
-        if (_ret)
-                *_ret = ret;
-
-        return 0;
-
-fail:
-        return r;
-}
-
-static int transaction_add_isolate_jobs(Transaction *tr, Manager *m) {
-        Iterator i;
-        Unit *u;
-        char *k;
-        int r;
-
-        assert(tr);
-        assert(m);
-
-        HASHMAP_FOREACH_KEY(u, k, m->units, i) {
-
-                /* ignore aliases */
-                if (u->id != k)
-                        continue;
-
-                if (u->ignore_on_isolate)
-                        continue;
-
-                /* No need to stop inactive jobs */
-                if (UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(u)) && !u->job)
-                        continue;
-
-                /* Is there already something listed for this? */
-                if (hashmap_get(tr->jobs, u))
-                        continue;
-
-                r = transaction_add_job_and_dependencies(tr, JOB_STOP, u, NULL, true, false, false, false, false, NULL, NULL);
-                if (r < 0)
-                        log_warning("Cannot add isolate job for unit %s, ignoring: %s", u->id, strerror(-r));
-        }
-
-        return 0;
-}
-
-static Transaction *transaction_new(void) {
-        Transaction *tr;
-
-        tr = new0(Transaction, 1);
-        if (!tr)
-                return NULL;
-
-        tr->jobs = hashmap_new(trivial_hash_func, trivial_compare_func);
-        if (!tr->jobs) {
-                free(tr);
-                return NULL;
-        }
-
-        return tr;
-}
-
-static void transaction_free(Transaction *tr) {
-        assert(hashmap_isempty(tr->jobs));
-        hashmap_free(tr->jobs);
-        free(tr);
-}
-
 int manager_add_job(Manager *m, JobType type, Unit *unit, JobMode mode, bool override, DBusError *e, Job **_ret) {
         int r;
         Job *ret;
diff --git a/src/core/manager.h b/src/core/manager.h
index 2bf7b7ae5e..20b743ab35 100644
--- a/src/core/manager.h
+++ b/src/core/manager.h
@@ -33,7 +33,6 @@
 #define MANAGER_MAX_NAMES 131072 /* 128K */
 
 typedef struct Manager Manager;
-typedef struct Transaction Transaction;
 typedef enum WatchType WatchType;
 typedef struct Watch Watch;
 
@@ -92,12 +91,6 @@ struct Watch {
 #include "dbus.h"
 #include "path-lookup.h"
 
-struct Transaction {
-        /* Jobs to be added */
-        Hashmap *jobs;      /* Unit object => Job object list 1:1 */
-        JobDependency *anchor;
-};
-
 struct Manager {
         /* Note that the set of units we know of is allowed to be
          * inconsistent. However the subset of it that is loaded may
diff --git a/src/core/transaction.c b/src/core/transaction.c
new file mode 100644
index 0000000000..8fa89a7be3
--- /dev/null
+++ b/src/core/transaction.c
@@ -0,0 +1,1101 @@
+#include "transaction.h"
+#include "bus-errors.h"
+
+static void transaction_unlink_job(Transaction *tr, Job *j, bool delete_dependencies);
+
+static void transaction_delete_job(Transaction *tr, Job *j, bool delete_dependencies) {
+        assert(tr);
+        assert(j);
+
+        /* Deletes one job from the transaction */
+
+        transaction_unlink_job(tr, j, delete_dependencies);
+
+        if (!j->installed)
+                job_free(j);
+}
+
+static void transaction_delete_unit(Transaction *tr, Unit *u) {
+        Job *j;
+
+        /* Deletes all jobs associated with a certain unit from the
+         * transaction */
+
+        while ((j = hashmap_get(tr->jobs, u)))
+                transaction_delete_job(tr, j, true);
+}
+
+void transaction_abort(Transaction *tr) {
+        Job *j;
+
+        assert(tr);
+
+        while ((j = hashmap_first(tr->jobs)))
+                transaction_delete_job(tr, j, true);
+
+        assert(hashmap_isempty(tr->jobs));
+
+        assert(!tr->anchor);
+}
+
+static void transaction_find_jobs_that_matter_to_anchor(Transaction *tr, Job *j, unsigned generation) {
+        JobDependency *l;
+
+        assert(tr);
+
+        /* A recursive sweep through the graph that marks all units
+         * that matter to the anchor job, i.e. are directly or
+         * indirectly a dependency of the anchor job via paths that
+         * are fully marked as mattering. */
+
+        if (j)
+                l = j->subject_list;
+        else
+                l = tr->anchor;
+
+        LIST_FOREACH(subject, l, l) {
+
+                /* This link does not matter */
+                if (!l->matters)
+                        continue;
+
+                /* This unit has already been marked */
+                if (l->object->generation == generation)
+                        continue;
+
+                l->object->matters_to_anchor = true;
+                l->object->generation = generation;
+
+                transaction_find_jobs_that_matter_to_anchor(tr, l->object, generation);
+        }
+}
+
+static void transaction_merge_and_delete_job(Transaction *tr, Job *j, Job *other, JobType t) {
+        JobDependency *l, *last;
+
+        assert(j);
+        assert(other);
+        assert(j->unit == other->unit);
+        assert(!j->installed);
+
+        /* Merges 'other' into 'j' and then deletes 'other'. */
+
+        j->type = t;
+        j->state = JOB_WAITING;
+        j->override = j->override || other->override;
+
+        j->matters_to_anchor = j->matters_to_anchor || other->matters_to_anchor;
+
+        /* Patch us in as new owner of the JobDependency objects */
+        last = NULL;
+        LIST_FOREACH(subject, l, other->subject_list) {
+                assert(l->subject == other);
+                l->subject = j;
+                last = l;
+        }
+
+        /* Merge both lists */
+        if (last) {
+                last->subject_next = j->subject_list;
+                if (j->subject_list)
+                        j->subject_list->subject_prev = last;
+                j->subject_list = other->subject_list;
+        }
+
+        /* Patch us in as new owner of the JobDependency objects */
+        last = NULL;
+        LIST_FOREACH(object, l, other->object_list) {
+                assert(l->object == other);
+                l->object = j;
+                last = l;
+        }
+
+        /* Merge both lists */
+        if (last) {
+                last->object_next = j->object_list;
+                if (j->object_list)
+                        j->object_list->object_prev = last;
+                j->object_list = other->object_list;
+        }
+
+        /* Kill the other job */
+        other->subject_list = NULL;
+        other->object_list = NULL;
+        transaction_delete_job(tr, other, true);
+}
+
+static bool job_is_conflicted_by(Job *j) {
+        JobDependency *l;
+
+        assert(j);
+
+        /* Returns true if this job is pulled in by a least one
+         * ConflictedBy dependency. */
+
+        LIST_FOREACH(object, l, j->object_list)
+                if (l->conflicts)
+                        return true;
+
+        return false;
+}
+
+static int delete_one_unmergeable_job(Transaction *tr, Job *j) {
+        Job *k;
+
+        assert(j);
+
+        /* Tries to delete one item in the linked list
+         * j->transaction_next->transaction_next->... that conflicts
+         * with another one, in an attempt to make an inconsistent
+         * transaction work. */
+
+        /* We rely here on the fact that if a merged with b does not
+         * merge with c, either a or b merge with c neither */
+        LIST_FOREACH(transaction, j, j)
+                LIST_FOREACH(transaction, k, j->transaction_next) {
+                        Job *d;
+
+                        /* Is this one mergeable? Then skip it */
+                        if (job_type_is_mergeable(j->type, k->type))
+                                continue;
+
+                        /* Ok, we found two that conflict, let's see if we can
+                         * drop one of them */
+                        if (!j->matters_to_anchor && !k->matters_to_anchor) {
+
+                                /* Both jobs don't matter, so let's
+                                 * find the one that is smarter to
+                                 * remove. Let's think positive and
+                                 * rather remove stops then starts --
+                                 * except if something is being
+                                 * stopped because it is conflicted by
+                                 * another unit in which case we
+                                 * rather remove the start. */
+
+                                log_debug("Looking at job %s/%s conflicted_by=%s", j->unit->id, job_type_to_string(j->type), yes_no(j->type == JOB_STOP && job_is_conflicted_by(j)));
+                                log_debug("Looking at job %s/%s conflicted_by=%s", k->unit->id, job_type_to_string(k->type), yes_no(k->type == JOB_STOP && job_is_conflicted_by(k)));
+
+                                if (j->type == JOB_STOP) {
+
+                                        if (job_is_conflicted_by(j))
+                                                d = k;
+                                        else
+                                                d = j;
+
+                                } else if (k->type == JOB_STOP) {
+
+                                        if (job_is_conflicted_by(k))
+                                                d = j;
+                                        else
+                                                d = k;
+                                } else
+                                        d = j;
+
+                        } else if (!j->matters_to_anchor)
+                                d = j;
+                        else if (!k->matters_to_anchor)
+                                d = k;
+                        else
+                                return -ENOEXEC;
+
+                        /* Ok, we can drop one, so let's do so. */
+                        log_debug("Fixing conflicting jobs by deleting job %s/%s", d->unit->id, job_type_to_string(d->type));
+                        transaction_delete_job(tr, d, true);
+                        return 0;
+                }
+
+        return -EINVAL;
+}
+
+static int transaction_merge_jobs(Transaction *tr, DBusError *e) {
+        Job *j;
+        Iterator i;
+        int r;
+
+        assert(tr);
+
+        /* First step, check whether any of the jobs for one specific
+         * task conflict. If so, try to drop one of them. */
+        HASHMAP_FOREACH(j, tr->jobs, i) {
+                JobType t;
+                Job *k;
+
+                t = j->type;
+                LIST_FOREACH(transaction, k, j->transaction_next) {
+                        if (job_type_merge(&t, k->type) >= 0)
+                                continue;
+
+                        /* OK, we could not merge all jobs for this
+                         * action. Let's see if we can get rid of one
+                         * of them */
+
+                        r = delete_one_unmergeable_job(tr, j);
+                        if (r >= 0)
+                                /* Ok, we managed to drop one, now
+                                 * let's ask our callers to call us
+                                 * again after garbage collecting */
+                                return -EAGAIN;
+
+                        /* We couldn't merge anything. Failure */
+                        dbus_set_error(e, BUS_ERROR_TRANSACTION_JOBS_CONFLICTING, "Transaction contains conflicting jobs '%s' and '%s' for %s. Probably contradicting requirement dependencies configured.",
+                                       job_type_to_string(t), job_type_to_string(k->type), k->unit->id);
+                        return r;
+                }
+        }
+
+        /* Second step, merge the jobs. */
+        HASHMAP_FOREACH(j, tr->jobs, i) {
+                JobType t = j->type;
+                Job *k;
+
+                /* Merge all transactions */
+                LIST_FOREACH(transaction, k, j->transaction_next)
+                        assert_se(job_type_merge(&t, k->type) == 0);
+
+                /* If an active job is mergeable, merge it too */
+                if (j->unit->job)
+                        job_type_merge(&t, j->unit->job->type); /* Might fail. Which is OK */
+
+                while ((k = j->transaction_next)) {
+                        if (j->installed) {
+                                transaction_merge_and_delete_job(tr, k, j, t);
+                                j = k;
+                        } else
+                                transaction_merge_and_delete_job(tr, j, k, t);
+                }
+
+                if (j->unit->job && !j->installed)
+                        transaction_merge_and_delete_job(tr, j, j->unit->job, t);
+
+                assert(!j->transaction_next);
+                assert(!j->transaction_prev);
+        }
+
+        return 0;
+}
+
+static void transaction_drop_redundant(Transaction *tr) {
+        bool again;
+
+        assert(tr);
+
+        /* Goes through the transaction and removes all jobs that are
+         * a noop */
+
+        do {
+                Job *j;
+                Iterator i;
+
+                again = false;
+
+                HASHMAP_FOREACH(j, tr->jobs, i) {
+                        bool changes_something = false;
+                        Job *k;
+
+                        LIST_FOREACH(transaction, k, j) {
+
+                                if (!job_is_anchor(k) &&
+                                    (k->installed || job_type_is_redundant(k->type, unit_active_state(k->unit))) &&
+                                    (!k->unit->job || !job_type_is_conflicting(k->type, k->unit->job->type)))
+                                        continue;
+
+                                changes_something = true;
+                                break;
+                        }
+
+                        if (changes_something)
+                                continue;
+
+                        /* log_debug("Found redundant job %s/%s, dropping.", j->unit->id, job_type_to_string(j->type)); */
+                        transaction_delete_job(tr, j, false);
+                        again = true;
+                        break;
+                }
+
+        } while (again);
+}
+
+static bool unit_matters_to_anchor(Unit *u, Job *j) {
+        assert(u);
+        assert(!j->transaction_prev);
+
+        /* Checks whether at least one of the jobs for this unit
+         * matters to the anchor. */
+
+        LIST_FOREACH(transaction, j, j)
+                if (j->matters_to_anchor)
+                        return true;
+
+        return false;
+}
+
+static int transaction_verify_order_one(Transaction *tr, Job *j, Job *from, unsigned generation, DBusError *e) {
+        Iterator i;
+        Unit *u;
+        int r;
+
+        assert(tr);
+        assert(j);
+        assert(!j->transaction_prev);
+
+        /* Does a recursive sweep through the ordering graph, looking
+         * for a cycle. If we find cycle we try to break it. */
+
+        /* Have we seen this before? */
+        if (j->generation == generation) {
+                Job *k, *delete;
+
+                /* If the marker is NULL we have been here already and
+                 * decided the job was loop-free from here. Hence
+                 * shortcut things and return right-away. */
+                if (!j->marker)
+                        return 0;
+
+                /* So, the marker is not NULL and we already have been
+                 * here. We have a cycle. Let's try to break it. We go
+                 * backwards in our path and try to find a suitable
+                 * job to remove. We use the marker to find our way
+                 * back, since smart how we are we stored our way back
+                 * in there. */
+                log_warning("Found ordering cycle on %s/%s", j->unit->id, job_type_to_string(j->type));
+
+                delete = NULL;
+                for (k = from; k; k = ((k->generation == generation && k->marker != k) ? k->marker : NULL)) {
+
+                        log_info("Walked on cycle path to %s/%s", k->unit->id, job_type_to_string(k->type));
+
+                        if (!delete &&
+                            !k->installed &&
+                            !unit_matters_to_anchor(k->unit, k)) {
+                                /* Ok, we can drop this one, so let's
+                                 * do so. */
+                                delete = k;
+                        }
+
+                        /* Check if this in fact was the beginning of
+                         * the cycle */
+                        if (k == j)
+                                break;
+                }
+
+
+                if (delete) {
+                        log_warning("Breaking ordering cycle by deleting job %s/%s", delete->unit->id, job_type_to_string(delete->type));
+                        transaction_delete_unit(tr, delete->unit);
+                        return -EAGAIN;
+                }
+
+                log_error("Unable to break cycle");
+
+                dbus_set_error(e, BUS_ERROR_TRANSACTION_ORDER_IS_CYCLIC, "Transaction order is cyclic. See system logs for details.");
+                return -ENOEXEC;
+        }
+
+        /* Make the marker point to where we come from, so that we can
+         * find our way backwards if we want to break a cycle. We use
+         * a special marker for the beginning: we point to
+         * ourselves. */
+        j->marker = from ? from : j;
+        j->generation = generation;
+
+        /* We assume that the the dependencies are bidirectional, and
+         * hence can ignore UNIT_AFTER */
+        SET_FOREACH(u, j->unit->dependencies[UNIT_BEFORE], i) {
+                Job *o;
+
+                /* Is there a job for this unit? */
+                o = hashmap_get(tr->jobs, u);
+                if (!o) {
+                        /* Ok, there is no job for this in the
+                         * transaction, but maybe there is already one
+                         * running? */
+                        o = u->job;
+                        if (!o)
+                                continue;
+                }
+
+                r = transaction_verify_order_one(tr, o, j, generation, e);
+                if (r < 0)
+                        return r;
+        }
+
+        /* Ok, let's backtrack, and remember that this entry is not on
+         * our path anymore. */
+        j->marker = NULL;
+
+        return 0;
+}
+
+static int transaction_verify_order(Transaction *tr, unsigned *generation, DBusError *e) {
+        Job *j;
+        int r;
+        Iterator i;
+        unsigned g;
+
+        assert(tr);
+        assert(generation);
+
+        /* Check if the ordering graph is cyclic. If it is, try to fix
+         * that up by dropping one of the jobs. */
+
+        g = (*generation)++;
+
+        HASHMAP_FOREACH(j, tr->jobs, i)
+                if ((r = transaction_verify_order_one(tr, j, NULL, g, e)) < 0)
+                        return r;
+
+        return 0;
+}
+
+static void transaction_collect_garbage(Transaction *tr) {
+        bool again;
+
+        assert(tr);
+
+        /* Drop jobs that are not required by any other job */
+
+        do {
+                Iterator i;
+                Job *j;
+
+                again = false;
+
+                HASHMAP_FOREACH(j, tr->jobs, i) {
+                        if (j->object_list) {
+                                /* log_debug("Keeping job %s/%s because of %s/%s", */
+                                /*           j->unit->id, job_type_to_string(j->type), */
+                                /*           j->object_list->subject ? j->object_list->subject->unit->id : "root", */
+                                /*           j->object_list->subject ? job_type_to_string(j->object_list->subject->type) : "root"); */
+                                continue;
+                        }
+
+                        /* log_debug("Garbage collecting job %s/%s", j->unit->id, job_type_to_string(j->type)); */
+                        transaction_delete_job(tr, j, true);
+                        again = true;
+                        break;
+                }
+
+        } while (again);
+}
+
+static int transaction_is_destructive(Transaction *tr, DBusError *e) {
+        Iterator i;
+        Job *j;
+
+        assert(tr);
+
+        /* Checks whether applying this transaction means that
+         * existing jobs would be replaced */
+
+        HASHMAP_FOREACH(j, tr->jobs, i) {
+
+                /* Assume merged */
+                assert(!j->transaction_prev);
+                assert(!j->transaction_next);
+
+                if (j->unit->job &&
+                    j->unit->job != j &&
+                    !job_type_is_superset(j->type, j->unit->job->type)) {
+
+                        dbus_set_error(e, BUS_ERROR_TRANSACTION_IS_DESTRUCTIVE, "Transaction is destructive.");
+                        return -EEXIST;
+                }
+        }
+
+        return 0;
+}
+
+static void transaction_minimize_impact(Transaction *tr) {
+        bool again;
+        assert(tr);
+
+        /* Drops all unnecessary jobs that reverse already active jobs
+         * or that stop a running service. */
+
+        do {
+                Job *j;
+                Iterator i;
+
+                again = false;
+
+                HASHMAP_FOREACH(j, tr->jobs, i) {
+                        LIST_FOREACH(transaction, j, j) {
+                                bool stops_running_service, changes_existing_job;
+
+                                /* If it matters, we shouldn't drop it */
+                                if (j->matters_to_anchor)
+                                        continue;
+
+                                /* Would this stop a running service?
+                                 * Would this change an existing job?
+                                 * If so, let's drop this entry */
+
+                                stops_running_service =
+                                        j->type == JOB_STOP && UNIT_IS_ACTIVE_OR_ACTIVATING(unit_active_state(j->unit));
+
+                                changes_existing_job =
+                                        j->unit->job &&
+                                        job_type_is_conflicting(j->type, j->unit->job->type);
+
+                                if (!stops_running_service && !changes_existing_job)
+                                        continue;
+
+                                if (stops_running_service)
+                                        log_debug("%s/%s would stop a running service.", j->unit->id, job_type_to_string(j->type));
+
+                                if (changes_existing_job)
+                                        log_debug("%s/%s would change existing job.", j->unit->id, job_type_to_string(j->type));
+
+                                /* Ok, let's get rid of this */
+                                log_debug("Deleting %s/%s to minimize impact.", j->unit->id, job_type_to_string(j->type));
+
+                                transaction_delete_job(tr, j, true);
+                                again = true;
+                                break;
+                        }
+
+                        if (again)
+                                break;
+                }
+
+        } while (again);
+}
+
+static int transaction_apply(Transaction *tr, Manager *m, JobMode mode) {
+        Iterator i;
+        Job *j;
+        int r;
+
+        /* Moves the transaction jobs to the set of active jobs */
+
+        if (mode == JOB_ISOLATE) {
+
+                /* When isolating first kill all installed jobs which
+                 * aren't part of the new transaction */
+        rescan:
+                HASHMAP_FOREACH(j, m->jobs, i) {
+                        assert(j->installed);
+
+                        if (hashmap_get(tr->jobs, j->unit))
+                                continue;
+
+                        /* 'j' itself is safe to remove, but if other jobs
+                           are invalidated recursively, our iterator may become
+                           invalid and we need to start over. */
+                        if (job_finish_and_invalidate(j, JOB_CANCELED) > 0)
+                                goto rescan;
+                }
+        }
+
+        HASHMAP_FOREACH(j, tr->jobs, i) {
+                /* Assume merged */
+                assert(!j->transaction_prev);
+                assert(!j->transaction_next);
+
+                if (j->installed)
+                        continue;
+
+                r = hashmap_put(m->jobs, UINT32_TO_PTR(j->id), j);
+                if (r < 0)
+                        goto rollback;
+        }
+
+        while ((j = hashmap_steal_first(tr->jobs))) {
+                Job *uj;
+                if (j->installed) {
+                        /* log_debug("Skipping already installed job %s/%s as %u", j->unit->id, job_type_to_string(j->type), (unsigned) j->id); */
+                        continue;
+                }
+
+                uj = j->unit->job;
+                if (uj) {
+                        job_uninstall(uj);
+                        job_free(uj);
+                }
+
+                j->unit->job = j;
+                j->installed = true;
+                m->n_installed_jobs ++;
+
+                /* We're fully installed. Now let's free data we don't
+                 * need anymore. */
+
+                assert(!j->transaction_next);
+                assert(!j->transaction_prev);
+
+                /* Clean the job dependencies */
+                transaction_unlink_job(tr, j, false);
+
+                job_add_to_run_queue(j);
+                job_add_to_dbus_queue(j);
+                job_start_timer(j);
+
+                log_debug("Installed new job %s/%s as %u", j->unit->id, job_type_to_string(j->type), (unsigned) j->id);
+        }
+
+        assert(!tr->anchor);
+
+        return 0;
+
+rollback:
+
+        HASHMAP_FOREACH(j, tr->jobs, i) {
+                if (j->installed)
+                        continue;
+
+                hashmap_remove(m->jobs, UINT32_TO_PTR(j->id));
+        }
+
+        return r;
+}
+
+int transaction_activate(Transaction *tr, Manager *m, JobMode mode, DBusError *e) {
+        int r;
+        unsigned generation = 1;
+
+        assert(tr);
+
+        /* This applies the changes recorded in tr->jobs to
+         * the actual list of jobs, if possible. */
+
+        /* First step: figure out which jobs matter */
+        transaction_find_jobs_that_matter_to_anchor(tr, NULL, generation++);
+
+        /* Second step: Try not to stop any running services if
+         * we don't have to. Don't try to reverse running
+         * jobs if we don't have to. */
+        if (mode == JOB_FAIL)
+                transaction_minimize_impact(tr);
+
+        /* Third step: Drop redundant jobs */
+        transaction_drop_redundant(tr);
+
+        for (;;) {
+                /* Fourth step: Let's remove unneeded jobs that might
+                 * be lurking. */
+                if (mode != JOB_ISOLATE)
+                        transaction_collect_garbage(tr);
+
+                /* Fifth step: verify order makes sense and correct
+                 * cycles if necessary and possible */
+                r = transaction_verify_order(tr, &generation, e);
+                if (r >= 0)
+                        break;
+
+                if (r != -EAGAIN) {
+                        log_warning("Requested transaction contains an unfixable cyclic ordering dependency: %s", bus_error(e, r));
+                        return r;
+                }
+
+                /* Let's see if the resulting transaction ordering
+                 * graph is still cyclic... */
+        }
+
+        for (;;) {
+                /* Sixth step: let's drop unmergeable entries if
+                 * necessary and possible, merge entries we can
+                 * merge */
+                r = transaction_merge_jobs(tr, e);
+                if (r >= 0)
+                        break;
+
+                if (r != -EAGAIN) {
+                        log_warning("Requested transaction contains unmergeable jobs: %s", bus_error(e, r));
+                        return r;
+                }
+
+                /* Seventh step: an entry got dropped, let's garbage
+                 * collect its dependencies. */
+                if (mode != JOB_ISOLATE)
+                        transaction_collect_garbage(tr);
+
+                /* Let's see if the resulting transaction still has
+                 * unmergeable entries ... */
+        }
+
+        /* Eights step: Drop redundant jobs again, if the merging now allows us to drop more. */
+        transaction_drop_redundant(tr);
+
+        /* Ninth step: check whether we can actually apply this */
+        if (mode == JOB_FAIL) {
+                r = transaction_is_destructive(tr, e);
+                if (r < 0) {
+                        log_notice("Requested transaction contradicts existing jobs: %s", bus_error(e, r));
+                        return r;
+                }
+        }
+
+        /* Tenth step: apply changes */
+        r = transaction_apply(tr, m, mode);
+        if (r < 0) {
+                log_warning("Failed to apply transaction: %s", strerror(-r));
+                return r;
+        }
+
+        assert(hashmap_isempty(tr->jobs));
+        assert(!tr->anchor);
+
+        return 0;
+}
+
+static Job* transaction_add_one_job(Transaction *tr, JobType type, Unit *unit, bool override, bool *is_new) {
+        Job *j, *f;
+
+        assert(tr);
+        assert(unit);
+
+        /* Looks for an existing prospective job and returns that. If
+         * it doesn't exist it is created and added to the prospective
+         * jobs list. */
+
+        f = hashmap_get(tr->jobs, unit);
+
+        LIST_FOREACH(transaction, j, f) {
+                assert(j->unit == unit);
+
+                if (j->type == type) {
+                        if (is_new)
+                                *is_new = false;
+                        return j;
+                }
+        }
+
+        if (unit->job && unit->job->type == type)
+                j = unit->job;
+        else {
+                j = job_new(unit->manager, type, unit);
+                if (!j)
+                        return NULL;
+        }
+
+        j->generation = 0;
+        j->marker = NULL;
+        j->matters_to_anchor = false;
+        j->override = override;
+
+        LIST_PREPEND(Job, transaction, f, j);
+
+        if (hashmap_replace(tr->jobs, unit, f) < 0) {
+                LIST_REMOVE(Job, transaction, f, j);
+                job_free(j);
+                return NULL;
+        }
+
+        if (is_new)
+                *is_new = true;
+
+        /* log_debug("Added job %s/%s to transaction.", unit->id, job_type_to_string(type)); */
+
+        return j;
+}
+
+static void transaction_unlink_job(Transaction *tr, Job *j, bool delete_dependencies) {
+        assert(tr);
+        assert(j);
+
+        if (j->transaction_prev)
+                j->transaction_prev->transaction_next = j->transaction_next;
+        else if (j->transaction_next)
+                hashmap_replace(tr->jobs, j->unit, j->transaction_next);
+        else
+                hashmap_remove_value(tr->jobs, j->unit, j);
+
+        if (j->transaction_next)
+                j->transaction_next->transaction_prev = j->transaction_prev;
+
+        j->transaction_prev = j->transaction_next = NULL;
+
+        while (j->subject_list)
+                job_dependency_free(j->subject_list, tr);
+
+        while (j->object_list) {
+                Job *other = j->object_list->matters ? j->object_list->subject : NULL;
+
+                job_dependency_free(j->object_list, tr);
+
+                if (other && delete_dependencies) {
+                        log_debug("Deleting job %s/%s as dependency of job %s/%s",
+                                  other->unit->id, job_type_to_string(other->type),
+                                  j->unit->id, job_type_to_string(j->type));
+                        transaction_delete_job(tr, other, delete_dependencies);
+                }
+        }
+}
+
+int transaction_add_job_and_dependencies(
+                Transaction *tr,
+                JobType type,
+                Unit *unit,
+                Job *by,
+                bool matters,
+                bool override,
+                bool conflicts,
+                bool ignore_requirements,
+                bool ignore_order,
+                DBusError *e,
+                Job **_ret) {
+        Job *ret;
+        Iterator i;
+        Unit *dep;
+        int r;
+        bool is_new;
+
+        assert(tr);
+        assert(type < _JOB_TYPE_MAX);
+        assert(unit);
+
+        /* log_debug("Pulling in %s/%s from %s/%s", */
+        /*           unit->id, job_type_to_string(type), */
+        /*           by ? by->unit->id : "NA", */
+        /*           by ? job_type_to_string(by->type) : "NA"); */
+
+        if (unit->load_state != UNIT_LOADED &&
+            unit->load_state != UNIT_ERROR &&
+            unit->load_state != UNIT_MASKED) {
+                dbus_set_error(e, BUS_ERROR_LOAD_FAILED, "Unit %s is not loaded properly.", unit->id);
+                return -EINVAL;
+        }
+
+        if (type != JOB_STOP && unit->load_state == UNIT_ERROR) {
+                dbus_set_error(e, BUS_ERROR_LOAD_FAILED,
+                               "Unit %s failed to load: %s. "
+                               "See system logs and 'systemctl status %s' for details.",
+                               unit->id,
+                               strerror(-unit->load_error),
+                               unit->id);
+                return -EINVAL;
+        }
+
+        if (type != JOB_STOP && unit->load_state == UNIT_MASKED) {
+                dbus_set_error(e, BUS_ERROR_MASKED, "Unit %s is masked.", unit->id);
+                return -EINVAL;
+        }
+
+        if (!unit_job_is_applicable(unit, type)) {
+                dbus_set_error(e, BUS_ERROR_JOB_TYPE_NOT_APPLICABLE, "Job type %s is not applicable for unit %s.", job_type_to_string(type), unit->id);
+                return -EBADR;
+        }
+
+        /* First add the job. */
+        ret = transaction_add_one_job(tr, type, unit, override, &is_new);
+        if (!ret)
+                return -ENOMEM;
+
+        ret->ignore_order = ret->ignore_order || ignore_order;
+
+        /* Then, add a link to the job. */
+        if (!job_dependency_new(by, ret, matters, conflicts, tr))
+                return -ENOMEM;
+
+        if (is_new && !ignore_requirements) {
+                Set *following;
+
+                /* If we are following some other unit, make sure we
+                 * add all dependencies of everybody following. */
+                if (unit_following_set(ret->unit, &following) > 0) {
+                        SET_FOREACH(dep, following, i) {
+                                r = transaction_add_job_and_dependencies(tr, type, dep, ret, false, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        set_free(following);
+                }
+
+                /* Finally, recursively add in all dependencies. */
+                if (type == JOB_START || type == JOB_RELOAD_OR_START) {
+                        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, NULL);
+                                if (r < 0) {
+                                        if (r != -EBADR)
+                                                goto fail;
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_BIND_TO], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, true, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        if (r != -EBADR)
+                                                goto fail;
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUIRES_OVERRIDABLE], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, !override, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_WANTS], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_START, dep, ret, false, false, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUISITE], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_VERIFY_ACTIVE, dep, ret, true, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        if (r != -EBADR)
+                                                goto fail;
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_REQUISITE_OVERRIDABLE], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_VERIFY_ACTIVE, dep, ret, !override, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_CONFLICTS], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_STOP, dep, ret, true, override, true, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        if (r != -EBADR)
+                                                goto fail;
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_CONFLICTED_BY], i) {
+                                r = transaction_add_job_and_dependencies(tr, JOB_STOP, dep, ret, false, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        log_warning("Cannot add dependency job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                }
+
+                if (type == JOB_STOP || type == JOB_RESTART || type == JOB_TRY_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, NULL);
+                                if (r < 0) {
+                                        if (r != -EBADR)
+                                                goto fail;
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+
+                        SET_FOREACH(dep, ret->unit->dependencies[UNIT_BOUND_BY], i) {
+                                r = transaction_add_job_and_dependencies(tr, type, dep, ret, true, override, false, false, ignore_order, e, NULL);
+                                if (r < 0) {
+                                        if (r != -EBADR)
+                                                goto fail;
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+                }
+
+                if (type == JOB_RELOAD || type == JOB_RELOAD_OR_START) {
+
+                        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, NULL);
+                                if (r < 0) {
+                                        log_warning("Cannot add dependency reload job for unit %s, ignoring: %s", dep->id, bus_error(e, r));
+
+                                        if (e)
+                                                dbus_error_free(e);
+                                }
+                        }
+                }
+
+                /* JOB_VERIFY_STARTED, JOB_RELOAD require no dependency handling */
+        }
+
+        if (_ret)
+                *_ret = ret;
+
+        return 0;
+
+fail:
+        return r;
+}
+
+int transaction_add_isolate_jobs(Transaction *tr, Manager *m) {
+        Iterator i;
+        Unit *u;
+        char *k;
+        int r;
+
+        assert(tr);
+        assert(m);
+
+        HASHMAP_FOREACH_KEY(u, k, m->units, i) {
+
+                /* ignore aliases */
+                if (u->id != k)
+                        continue;
+
+                if (u->ignore_on_isolate)
+                        continue;
+
+                /* No need to stop inactive jobs */
+                if (UNIT_IS_INACTIVE_OR_FAILED(unit_active_state(u)) && !u->job)
+                        continue;
+
+                /* Is there already something listed for this? */
+                if (hashmap_get(tr->jobs, u))
+                        continue;
+
+                r = transaction_add_job_and_dependencies(tr, JOB_STOP, u, NULL, true, false, false, false, false, NULL, NULL);
+                if (r < 0)
+                        log_warning("Cannot add isolate job for unit %s, ignoring: %s", u->id, strerror(-r));
+        }
+
+        return 0;
+}
+
+Transaction *transaction_new(void) {
+        Transaction *tr;
+
+        tr = new0(Transaction, 1);
+        if (!tr)
+                return NULL;
+
+        tr->jobs = hashmap_new(trivial_hash_func, trivial_compare_func);
+        if (!tr->jobs) {
+                free(tr);
+                return NULL;
+        }
+
+        return tr;
+}
+
+void transaction_free(Transaction *tr) {
+        assert(hashmap_isempty(tr->jobs));
+        hashmap_free(tr->jobs);
+        free(tr);
+}
diff --git a/src/core/transaction.h b/src/core/transaction.h
new file mode 100644
index 0000000000..d519ffb1f5
--- /dev/null
+++ b/src/core/transaction.h
@@ -0,0 +1,36 @@
+#ifndef footransactionhfoo
+#define footransactionhfoo
+
+typedef struct Transaction Transaction;
+
+#include "unit.h"
+#include "manager.h"
+#include "job.h"
+#include "hashmap.h"
+
+struct Transaction {
+        /* Jobs to be added */
+        Hashmap *jobs;      /* Unit object => Job object list 1:1 */
+        JobDependency *anchor;
+};
+
+Transaction *transaction_new(void);
+void transaction_free(Transaction *tr);
+
+int transaction_add_job_and_dependencies(
+                Transaction *tr,
+                JobType type,
+                Unit *unit,
+                Job *by,
+                bool matters,
+                bool override,
+                bool conflicts,
+                bool ignore_requirements,
+                bool ignore_order,
+                DBusError *e,
+                Job **_ret);
+int transaction_activate(Transaction *tr, Manager *m, JobMode mode, DBusError *e);
+int transaction_add_isolate_jobs(Transaction *tr, Manager *m);
+void transaction_abort(Transaction *tr);
+
+#endif