iotests/030: Unthrottle parallel jobs in reverse

See the comment for why this is necessary.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20211111120829.81329-11-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/030

@@ -251,7 +251,16 @@ class TestParallelOps(iotests.QMPTestCase):
                                  speed=1024)
             self.assert_qmp(result, 'return', {})
 
-        for job in pending_jobs:
+        # Do this in reverse: After unthrottling them, some jobs may finish
+        # before we have unthrottled all of them.  This will drain their
+        # subgraph, and this will make jobs above them advance (despite those
+        # jobs on top being throttled).  In the worst case, all jobs below the
+        # top one are finished before we can unthrottle it, and this makes it
+        # advance so far that it completes before we can unthrottle it - which
+        # results in an error.
+        # Starting from the top (i.e. in reverse) does not have this problem:
+        # When a job finishes, the ones below it are not advanced.
+        for job in reversed(pending_jobs):
             result = self.vm.qmp('block-job-set-speed', device=job, speed=0)
             self.assert_qmp(result, 'return', {})