cmdlib: Fix tasklets handling if no tasklets are added

If no tasklets are added, self.tasklets evaluates to None. The LU base
class will throw an exception because it thinks the derived class doesn't
implement the right methods.
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

lib/cmdlib.py

@@ -94,7 +94,7 @@ class LogicalUnit(object):
     self.dry_run_result = None
 
     # Tasklets
-    self.tasklets = []
+    self.tasklets = None
 
     for attr_name in self._OP_REQP:
       attr_val = getattr(op, attr_name, None)
@@ -213,7 +213,7 @@ class LogicalUnit(object):
     their canonical form if it hasn't been done by ExpandNames before.
 
     """
-    if self.tasklets:
+    if self.tasklets is not None:
       for tl in self.tasklets:
         tl.CheckPrereq()
     else:
@@ -227,7 +227,7 @@ class LogicalUnit(object):
     code, or expected.
 
     """
-    if self.tasklets:
+    if self.tasklets is not None:
       for tl in self.tasklets:
         tl.Exec(feedback_fn)
     else:
@@ -3918,7 +3918,7 @@ class LUMigrateInstance(LogicalUnit):
 
     self._migrater = TLMigrateInstance(self, self.op.instance_name,
                                        self.op.live, self.op.cleanup)
-    self.tasklets.append(self._migrater)
+    self.tasklets = [self._migrater]
 
   def DeclareLocks(self, level):
     if level == locking.LEVEL_NODE:
@@ -5249,7 +5249,7 @@ class LUReplaceDisks(LogicalUnit):
                                    self.op.iallocator, self.op.remote_node,
                                    self.op.disks)
 
-    self.tasklets.append(self.replacer)
+    self.tasklets = [self.replacer]
 
   def DeclareLocks(self, level):
     # If we're not already locking all nodes in the set we have to declare the
@@ -5331,6 +5331,7 @@ class LUEvacuateNode(LogicalUnit):
     # Create tasklets for replacing disks for all secondary instances on this
     # node
     names = []
+    tasklets = []
 
     for inst in _GetNodeSecondaryInstances(self.cfg, self.op.node_name):
       logging.debug("Replacing disks for instance %s", inst.name)
@@ -5338,8 +5339,9 @@ class LUEvacuateNode(LogicalUnit):
 
       replacer = TLReplaceDisks(self, inst.name, constants.REPLACE_DISK_CHG,
                                 self.op.iallocator, self.op.remote_node, [])
-      self.tasklets.append(replacer)
+      tasklets.append(replacer)
 
+    self.tasklets = tasklets
     self.instance_names = names
 
     # Declare instance locks