orchestrator: Check that service's SpecVersion exists before derefere…

…ncing

IsTaskDirty assumed that if a task had a SpecVersion, the corresponding
service must also have a SpecVersion, since that is where the task's
field is copied from. However, there are mixed-version scenarios where a
service could get updated by a manager running an older version of
swarmkit than the manager that created the task. Add a check to avoid a
crash in this case.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

manager/orchestrator/task.go

@@ -63,7 +63,7 @@ func IsTaskDirty(s *api.Service, t *api.Task) bool {
 	// If the spec version matches, we know the task is not dirty. However,
 	// if it does not match, that doesn't mean the task is dirty, since
 	// only a portion of the spec is included in the comparison.
-	if t.SpecVersion != nil && *s.SpecVersion == *t.SpecVersion {
+	if t.SpecVersion != nil && s.SpecVersion != nil && *s.SpecVersion == *t.SpecVersion {
 		return false
 	}