Avoid crash with None tasks

As ansible allows tasks to be None, we should also avoid crashing when
encountering them.

Includes regression prevention by assuring we can parse files
with empty tasks.

Fixes: #468

examples/example.yml

@@ -47,3 +47,6 @@
     - name: always run
       debug: msg="always_run is deprecated"
       always_run: true
+
+    # empty task is currently accepted by ansible as valid code:
+    -

lib/ansiblelint/rules/BecomeUserWithoutBecomeRule.py

@@ -34,14 +34,15 @@ def _get_subtasks(data):
             'always',
             'rescue']
     for name in block_names:
-        if name in data:
+        if data and name in data:
             result += (data[name] or [])
     return result
 
 
 def _nested_search(term, data):
-    return ((term in data) or
-            reduce((lambda x, y: x or _nested_search(term, y)), _get_subtasks(data), False))
+    if data and term in data:
+        return True
+    return reduce((lambda x, y: x or _nested_search(term, y)), _get_subtasks(data), False)
 
 
 def _become_user_without_become(becomeuserabove, data):

lib/ansiblelint/rules/IncludeMissingFileRule.py

@@ -27,8 +27,9 @@ def matchplay(self, file, data):
         # avoid failing with a playbook having tasks: null
         for task in (data.get('tasks', []) or []):
 
-            # check if the id of the current rule is not in list of skipped rules for this play
-            if self.id in task.get('skipped_rules', ()):
+            # ignore None tasks or
+            # if the id of the current rule is not in list of skipped rules for this play
+            if not task or self.id in task.get('skipped_rules', ()):
                 continue
 
             # collect information which file was referenced for include / import

lib/ansiblelint/skip_utils.py

@@ -110,6 +110,11 @@ def _append_skipped_rules(pyyaml_data: Sequence, file_text: str, file_type: File
 
     # append skipped_rules for each task
     for ruamel_task, pyyaml_task in zip(ruamel_tasks, pyyaml_tasks):
+
+        # ignore empty tasks
+        if not pyyaml_task and not ruamel_task:
+            continue
+
         if pyyaml_task.get('name') != ruamel_task.get('name'):
             raise RuntimeError('Error in matching skip comment to a task')
         pyyaml_task['skipped_rules'] = _get_rule_skips_from_yaml(ruamel_task)
@@ -147,7 +152,7 @@ def _get_tasks_from_blocks(task_blocks: Sequence) -> Generator:
     def get_nested_tasks(task: Any) -> Generator[Any, None, None]:
         return (
             subtask
-            for k in NESTED_TASK_KEYS if k in task
+            for k in NESTED_TASK_KEYS if task and k in task
             for subtask in task[k]
         )
 

lib/ansiblelint/utils.py

@@ -235,6 +235,11 @@ def _include_children(basedir, k, v, parent_type):
 def _taskshandlers_children(basedir, k, v, parent_type: FileType) -> List:
     results = []
     for th in v:
+
+        # ignore empty tasks, `-`
+        if not th:
+            continue
+
         with contextlib.suppress(LookupError):
             children = _get_task_handler_children_for_tasks_or_playbooks(
                 th, basedir, k, parent_type,
@@ -267,9 +272,16 @@ def _get_task_handler_children_for_tasks_or_playbooks(
 ) -> dict:
     """Try to get children of taskhandler for include/import tasks/playbooks."""
     child_type = k if parent_type == 'playbook' else parent_type
+
     task_include_keys = 'include', 'include_tasks', 'import_playbook', 'import_tasks'
     for task_handler_key in task_include_keys:
+
         with contextlib.suppress(KeyError):
+
+            # ignore empty tasks
+            if not task_handler:
+                continue
+
             return {
                 'path': path_dwim(basedir, task_handler[task_handler_key]),
                 'type': child_type,
@@ -546,6 +558,9 @@ def extract_from_list(blocks, candidates):
 def add_action_type(actions, action_type):
     results = list()
     for action in actions:
+        # ignore empty task
+        if not action:
+            continue
         action['__ansible_action_type__'] = BLOCK_NAME_TO_ACTION_TYPE_MAP[action_type]
         results.append(action)
     return results