transaction: rework merging with installed jobs

Previously transactions could reference installed jobs. It made some issues
difficult to fix.

This sets new rules for jobs:
A job cannot be both a member of a transaction and installed. When jobs are
created, they are linked to a transaction. The whole transaction is constructed
(with merging of jobs within, etc.). When it's complete, all the jobs are
unlinked from it one by one and let to install themselves. It is during the
installation when merging with previously installed jobs (from older
transactions) is contemplated.

Merging with installed jobs has different rules than merging within a
transaction:
 - An installed conflicting job gets cancelled. It cannot be simply deleted,
   because someone might be waiting for its completion on DBus.
 - An installed, but still waiting, job can be safely merged into.
 - An installed and running job can be tricky. For some job types it is safe to
   just merge. For the other types we merge anyway, but put the job back into
   JOB_WAITING to allow it to run again. This may be suboptimal, but it is not
   currently possible to have more than one installed job for a unit.

Note this also fixes a bug where the anchor job could be deleted during merging
within the transaction.

1 files changed, 55 insertions, 3 deletions

diff --git a/src/core/job.c b/src/core/job.c
index 0b50888184..3454ffd1db 100644
--- a/src/core/job.c
+++ b/src/core/job.c
@@ -96,18 +96,70 @@ void job_uninstall(Job *j) {
         j->installed = false;
 }
 
-void job_install(Job *j) {
+static bool job_type_allows_late_merge(JobType t) {
+        /* Tells whether it is OK to merge a job of type 't' with an already
+         * running job.
+         * Reloads cannot be merged this way. Think of the sequence:
+         * 1. Reload of a daemon is in progress; the daemon has already loaded
+         *    its config file, but hasn't completed the reload operation yet.
+         * 2. Edit foo's config file.
+         * 3. Trigger another reload to have the daemon use the new config.
+         * Should the second reload job be merged into the first one, the daemon
+         * would not know about the new config.
+         * JOB_RESTART jobs on the other hand can be merged, because they get
+         * 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);
+}
+
+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);
+
+        j->override = j->override || other->override;
+}
+
+Job* job_install(Job *j) {
         Job *uj = j->unit->job;
 
+        assert(!j->installed);
+
         if (uj) {
-                job_uninstall(uj);
-                job_free(uj);
+                if (job_type_is_conflicting(uj->type, j->type))
+                        job_finish_and_invalidate(uj, JOB_CANCELED);
+                else {
+                        /* not conflicting, i.e. mergeable */
+
+                        if (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",
+                                          uj->unit->id, job_type_to_string(uj->type), (unsigned) uj->id);
+                                return uj;
+                        } else {
+                                /* already running and not safe to merge into */
+                                /* Patch uj to become a merged job and re-run it. */
+                                /* XXX It should be safer to queue j to run after uj finishes, but it is
+                                 * not currently possible to have more than one installed job per unit. */
+                                job_merge_into_installed(uj, j);
+                                log_debug("Merged into running job, re-running: %s/%s as %u",
+                                          uj->unit->id, job_type_to_string(uj->type), (unsigned) uj->id);
+                                uj->state = JOB_WAITING;
+                                return uj;
+                        }
+                }
         }
 
+        /* Install the job */
         j->unit->job = 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;
 }
 
 JobDependency* job_dependency_new(Job *subject, Job *object, bool matters, bool conflicts) {