add automatic tagging for roles

ansible.cfg

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+callback_plugins = plugins/callbacks
+callback_whitelist = auto_tags

plugins/callbacks/auto_tags.py

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+"""
+This module implements an Ansible plugin that is triggered at the start of a playbook.
+
+The plugin dynamically generates a tag for each role. Each tag has the same name as its role.
+The advantage of this is that it saves you some boilerplate, because you don't have to wrap
+all tasks of a role in an additional block and assign a tag to that.
+Additionally, it works automatically when you add new roles to your playbook.
+
+Usage is exactly the same as without this plugin:
+
+ansible-playbook --tags=some_tag provision.yml
+
+Here, the "some_tag" tag was generated dynamically (assuming there is a "some_tag" role).
+
+Installation:
+1. Place this file in `plugins/callbacks/auto_tags.py` (relative to your playbook root)
+2. Add the following two lines to your `ansible.cfg` file:
+
+callback_plugins = plugins/callbacks
+callback_whitelist = auto_tags
+"""
+from __future__ import print_function
+from ansible.plugins.callback import CallbackBase
+
+
+class CallbackModule(CallbackBase):
+    """
+    Ansible supports several types of plugins. We are using the *callback* type here, since
+    it seemed the best choice for our use case, because it allows you to hook into the start
+    of a playbook.
+    """
+    def v2_playbook_on_start(self, playbook):
+        """
+        Dynamically add a tag of the same name to each role.
+        Note: Plays, roles, task_blocks and tasks can have tags.
+        """
+        plays = playbook.get_plays()
+
+        # Note: Although identical roles are shared between plays we cannot deduplicate them,
+        # since Ansible treats them as different objects internally
+        roles = [role for play in plays for role in play.get_roles()]
+
+        # Note: Tags for roles are set dynamically in `_load_role_data` instead of in __init__
+        # I don't know why they do that.
+        for role in roles:
+            role_name = role._role_name
+            if role_name not in role.tags:
+                role.tags += [role_name]