Transports: better handling of empty list

* BaseWorker: accept an empty list in the command setter. It's its
  default value, there is no point in forbidding a client to set it
  to the same value.
* ClusterShellWorker: return immediately if there are no target hosts.

Bug: T174911
Change-Id: I0d2bec28dd8381104fd56f22194dbeffb8b30570

cumin/tests/unit/transports/test_clustershell.py

@@ -95,9 +95,10 @@ def test_execute_custom_handler(self):
         assert kwargs['handler'] == self.worker._handler_instance
 
     def test_execute_no_commands(self):
-        """Calling execute() without commands should raise WorkerError."""
-        with pytest.raises(WorkerError, match=r'commands must be a non-empty list'):
-            self.worker.commands = []
+        """Calling execute() without commands should return immediately."""
+        self.worker.handler = ConcreteBaseEventHandler
+        self.worker.commands = None
+        assert self.worker.execute() is None
         assert not self.worker.task.shell.called
 
     def test_execute_one_command_no_mode(self):
@@ -106,6 +107,13 @@ def test_execute_one_command_no_mode(self):
         with pytest.raises(RuntimeError, match=r'An EventHandler is mandatory\.'):
             self.worker.execute()
 
+    def test_execute_no_target_hosts(self):
+        """Calling execute() with a target without hosts should return immediately."""
+        self.target.hosts = []
+        self.worker.handler = ConcreteBaseEventHandler
+        assert self.worker.execute() is None
+        assert not self.worker.task.shell.called
+
     def test_execute_wrong_mode(self):
         """Calling execute() without setting the mode with multiple commands should raise RuntimeError."""
         with pytest.raises(RuntimeError, match=r'An EventHandler is mandatory\.'):

cumin/tests/unit/transports/test_init.py

@@ -427,6 +427,8 @@ def test_commands_setter(self):
         assert self.worker._commands == self.commands
         self.worker.commands = None
         assert self.worker._commands is None
+        self.worker.commands = []
+        assert self.worker._commands == []
         self.worker.commands = ['command1', 'command2']
         assert self.worker._commands == self.commands
 

cumin/transports/__init__.py

@@ -347,7 +347,7 @@ def commands(self, value):
             self._commands = value
             return
 
-        validate_list('commands', value)
+        validate_list('commands', value, allow_empty=True)
         commands = []
         for command in value:
             if isinstance(command, Command):

cumin/transports/clustershell.py

@@ -38,6 +38,10 @@ def execute(self):
             self.logger.warning('No commands provided')
             return
 
+        if not self.target.hosts:
+            self.logger.warning('No target hosts provided')
+            return
+
         if self.handler is None:
             raise RuntimeError('An EventHandler is mandatory.')