mirror: Do not clear .cancelled

Clearing .cancelled before leaving the main loop when the job has been
soft-cancelled is no longer necessary since job_is_cancelled() only
returns true for jobs that have been force-cancelled.

Therefore, this only makes a differences in places that call
job_cancel_requested().  In block/mirror.c, this is done only before
.cancelled was cleared.

In job.c, there are two callers:
- job_completed_txn_abort() asserts that .cancelled is true, so keeping
  it true will not affect this place.

- job_complete() refuses to let a job complete that has .cancelled set.
  It is correct to refuse to let the user invoke job-complete on mirror
  jobs that have already been soft-cancelled.

With this change, there are no places that reset .cancelled to false and
so we can be sure that .force_cancel can only be true if .cancelled is
true as well.  Assert this in job_is_cancelled().

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211006151940.214590-13-hreitz@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

block/mirror.c

@@ -946,7 +946,6 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
         while (!job_cancel_requested(&s->common.job) && !s->should_complete) {
             job_yield(&s->common.job);
         }
-        s->common.job.cancelled = false;
         goto immediate_exit;
     }
 
@@ -1085,7 +1084,6 @@ static int coroutine_fn mirror_run(Job *job, Error **errp)
              * completion.
              */
             assert(QLIST_EMPTY(&bs->tracked_requests));
-            s->common.job.cancelled = false;
             need_drain = false;
             break;
         }

job.c

@@ -217,7 +217,9 @@ const char *job_type_str(const Job *job)
 
 bool job_is_cancelled(Job *job)
 {
-    return job->cancelled && job->force_cancel;
+    /* force_cancel may be true only if cancelled is true, too */
+    assert(job->cancelled || !job->force_cancel);
+    return job->force_cancel;
 }
 
 bool job_cancel_requested(Job *job)