Enable var-name rule to detect read-only variables (#3462)

examples/playbooks/vars/rule_var_naming_fail.yml

@@ -9,3 +9,6 @@ CamelCaseButErrorIgnored: true # noqa: var-naming
 assert: true # invalid due to being Python reserved keyword
 é: true # invalid due to non-ascii character
 hosts: true # invalid as being Ansible reserved name
+role_name: boo # invalid as being Ansible special magic variable
+ansible_facts: {} # special variable that we allow to be written
+ansible_python_interpreter: python3 # special variable that we allow to be written

src/ansiblelint/rules/var_naming.md

@@ -9,6 +9,11 @@ alphabetic or underscore `_` character.
 For more information see the [creating valid variable names][var-names] topic in
 Ansible documentation and [Naming things (Good Practices for Ansible)][cop].
 
+You should also be fully aware of [special variables][magic-vars], also known as
+magic variables, especially as most of them can only be read. While Ansible will
+just ignore any attempt to set them, the linter will notify the user, so they
+would not be confused about a line that does not effectively do anything.
+
 Possible errors messages:
 
 - `var-naming[non-string]`: Variables names must be strings.
@@ -19,6 +24,7 @@ Possible errors messages:
 - `var-naming[no-role-prefix]`: Variables names from within roles should use
   `role_name_` as a prefix.
 - `var-naming[no-reserved]`: Variables names must not be Ansible reserved names.
+- `var-naming[read-only]`: This special variable is read-only.
 
 ## Settings
 
@@ -40,6 +46,7 @@ var_naming_pattern: "^[a-z_][a-z0-9_]*$"
     ALL_CAPS: bar # <- Contains only uppercase characters.
     v@r!able: baz # <- Contains special characters.
     hosts: [] # <- hosts is an Ansible reserved name
+    role_name: boo # <-- invalid as being Ansible special magic variable
 ```
 
 ## Correct Code
@@ -53,8 +60,11 @@ var_naming_pattern: "^[a-z_][a-z0-9_]*$"
     no_caps: bar # <- Does not contains uppercase characters.
     variable: baz # <- Does not contain special characters.
     my_hosts: [] # <- Does not use a reserved names.
+    my_role_name: boo
 ```
 
 [cop]: https://redhat-cop.github.io/automation-good-practices/#_naming_things
 [var-names]:
   https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_variables.html#creating-valid-variable-names
+[magic-vars]:
+  https://docs.ansible.com/ansible/latest/reference_appendices/special_variables.html

src/ansiblelint/rules/var_naming.py

@@ -33,6 +33,64 @@ class VariableNamingRule(AnsibleLintRule):
     re_pattern_str = options.var_naming_pattern or "^[a-z_][a-z0-9_]*$"
     re_pattern = re.compile(re_pattern_str)
     reserved_names = get_reserved_names()
+    # List of special variables that should be treated as read-only. This list
+    # does not include connection variables, which we expect users to tune in
+    # specific cases.
+    # https://docs.ansible.com/ansible/latest/reference_appendices/special_variables.html
+    read_only_names = {
+        "ansible_check_mode",
+        "ansible_collection_name",
+        "ansible_config_file",
+        "ansible_dependent_role_names",
+        "ansible_diff_mode",
+        "ansible_forks",
+        "ansible_index_var",
+        "ansible_inventory_sources",
+        "ansible_limit",
+        "ansible_local",  # special fact
+        "ansible_loop",
+        "ansible_loop_var",
+        "ansible_parent_role_names",
+        "ansible_parent_role_paths",
+        "ansible_play_batch",
+        "ansible_play_hosts",
+        "ansible_play_hosts_all",
+        "ansible_play_name",
+        "ansible_play_role_names",
+        "ansible_playbook_python",
+        "ansible_role_name",
+        "ansible_role_names",
+        "ansible_run_tags",
+        "ansible_search_path",
+        "ansible_skip_tags",
+        "ansible_verbosity",
+        "ansible_version",
+        "group_names",
+        "groups",
+        "hostvars",
+        "inventory_dir",
+        "inventory_file",
+        "inventory_hostname",
+        "inventory_hostname_short",
+        "omit",
+        "play_hosts",
+        "playbook_dir",
+        "role_name",
+        "role_names",
+        "role_path",
+    }
+
+    # These special variables are used by Ansible but we allow users to set
+    # them as they might need it in certain cases.
+    allowed_special_names = {
+        "ansible_facts",
+        "ansible_become_user",
+        "ansible_connection",
+        "ansible_host",
+        "ansible_python_interpreter",
+        "ansible_user",
+        "ansible_remote_tmp",  # no included in docs
+    }
 
     # pylint: disable=too-many-return-statements
     def get_var_naming_matcherror(
@@ -49,7 +107,7 @@ def get_var_naming_matcherror(
                 rule=self,
             )
 
-        if ident in ANNOTATION_KEYS:
+        if ident in ANNOTATION_KEYS or ident in self.allowed_special_names:
             return None
 
         try:
@@ -75,6 +133,13 @@ def get_var_naming_matcherror(
                 rule=self,
             )
 
+        if ident in self.read_only_names:
+            return MatchError(
+                tag="var-naming[read-only]",
+                message=f"This special variable is read-only. ({ident})",
+                rule=self,
+            )
+
         # We want to allow use of jinja2 templating for variable names
         if "{{" in ident:
             return MatchError(
@@ -251,6 +316,7 @@ def test_invalid_var_name_varsfile(
             ("var-naming[no-keyword]", 9),
             ("var-naming[non-ascii]", 10),
             ("var-naming[no-reserved]", 11),
+            ("var-naming[read-only]", 12),
         )
         assert len(results) == len(expected_errors)
         for idx, result in enumerate(results):