Avoid exception with YAML files which are just strings

Fixes: #1082

.flake8

@@ -55,8 +55,9 @@ per-file-ignores =
   lib/ansiblelint/__main__.py: C901
   lib/ansiblelint/cli.py: D101 D102 D103
   lib/ansiblelint/formatters/__init__.py: D101 D102
-  lib/ansiblelint/utils.py: D103
+  lib/ansiblelint/utils.py: D103 C901
   lib/ansiblelint/rules/*.py: D100 D101 D102
+  lib/ansiblelint/rules/__init__.py: D100 D101 D102 C901
 
   # FIXME: drop these once they're fixed
   # Ref: https://github.com/ansible-community/ansible-lint/issues/725

a.yml

@@ -0,0 +1 @@
+foo

examples/plain_string.yml

@@ -0,0 +1,3 @@
+foo
+# This file is valid YAML but from our point of view is an error, as is
+# neither a Sequence or a Mapping.

lib/ansiblelint/_internal/rules.py

@@ -62,3 +62,14 @@ class AnsibleParserErrorRule(BaseRule):
     severity = 'VERY_HIGH'
     tags = ['core']
     version_added = 'v5.0.0'
+
+
+class LoadingFailureRule(BaseRule):
+    """File loading failure."""
+
+    id = '901'
+    shortdesc = 'Failed to load or parse file'
+    description = 'Linter failed to process a YAML file, possible not an Ansible file.'
+    severity = 'VERY_HIGH'
+    tags = ['core']
+    version_added = 'v4.3.0'

lib/ansiblelint/rules/__init__.py

@@ -10,7 +10,7 @@
 from typing import List
 
 import ansiblelint.utils
-from ansiblelint._internal.rules import AnsibleParserErrorRule
+from ansiblelint._internal.rules import AnsibleParserErrorRule, LoadingFailureRule
 from ansiblelint.errors import BaseRule, MatchError, RuntimeErrorRule
 from ansiblelint.skip_utils import append_skipped_rules, get_rule_skips_from_line
 
@@ -129,6 +129,14 @@ def matchyaml(self, file: dict, text: str) -> List[MatchError]:
             return matches
 
         yaml = ansiblelint.utils.parse_yaml_linenumbers(text, file['path'])
+        # yaml returned can be an AnsibleUnicode (a string) when the yaml
+        # file contains a single string. YAML spec allows this but we consider
+        # this an fatal error.
+        if 'get' not in dir(yaml):
+            return [MatchError(
+                filename=file['path'],
+                rule=LoadingFailureRule()
+            )]
         if not yaml:
             return matches
 
@@ -190,7 +198,7 @@ def __init__(self, rulesdirs=None) -> None:
         # internal rules included in order to expose them for docs as they are
         # not directly loaded by our rule loader.
         self.rules.extend(
-            [RuntimeErrorRule(), AnsibleParserErrorRule()])
+            [RuntimeErrorRule(), AnsibleParserErrorRule(), LoadingFailureRule()])
         for rulesdir in self.rulesdirs:
             _logger.debug("Loading rules from %s", rulesdir)
             self.extend(load_plugins(rulesdir))

lib/ansiblelint/runner.py

@@ -7,8 +7,8 @@
 import ansiblelint.file_utils
 import ansiblelint.skip_utils
 import ansiblelint.utils
+from ansiblelint._internal.rules import LoadingFailureRule
 from ansiblelint.errors import MatchError
-from ansiblelint.rules.LoadingFailureRule import LoadingFailureRule
 
 if TYPE_CHECKING:
     from ansiblelint.rules import RulesCollection

lib/ansiblelint/utils.py

@@ -56,7 +56,7 @@
 from yaml.composer import Composer
 from yaml.representer import RepresenterError
 
-from ansiblelint._internal.rules import AnsibleParserErrorRule
+from ansiblelint._internal.rules import AnsibleParserErrorRule, LoadingFailureRule
 from ansiblelint.constants import CUSTOM_RULESDIR_ENVVAR, DEFAULT_RULESDIR, FileType
 from ansiblelint.errors import MatchError
 from ansiblelint.file_utils import normpath
@@ -187,6 +187,11 @@ def find_children(playbook: Tuple[str, str], playbook_dir: str) -> List:
             raise SystemExit(str(e))
     results = []
     basedir = os.path.dirname(playbook[0])
+    # playbook_ds can be an AnsibleUnicode string, which we consider invalid
+    if "get" not in playbook_ds:
+        raise MatchError(
+            filename=playbook[0],
+            rule=LoadingFailureRule)
     items = _playbook_items(playbook_ds)
     for item in items:
         for child in _rebind_match_filename(playbook[0], play_children)(

test/TestExamples.py

@@ -6,3 +6,10 @@ def test_example(default_rules_collection):
     """example.yml is expected to have 15 match errors inside."""
     result = Runner(default_rules_collection, 'examples/example.yml', [], [], []).run()
     assert len(result) == 15
+
+
+def test_example_plain_string(default_rules_collection):
+    """Validates that loading valid YAML string produce error."""
+    result = Runner(default_rules_collection, 'examples/plain_string.yml', [], [], []).run()
+    assert len(result) == 1
+    assert "Failed to load or parse file" in result[0]