ansible-galaxy - fix traceback error for invalid req file (#81917)

Provide a better error message when encountering a YAML requirements file that is not a dictionary or list.

Fixes: #81901

changelogs/fragments/81901-galaxy-requirements-format.yml

@@ -0,0 +1,2 @@
+bugfixes:
+- ansible-galaxy - Provide a better error message when using a requirements file with an invalid format - https://github.com/ansible/ansible/issues/81901

lib/ansible/cli/galaxy.py

@@ -834,7 +834,7 @@ class GalaxyCLI(CLI):
             for role_req in file_requirements:
                 requirements['roles'] += parse_role_req(role_req)
 
-        else:
+        elif isinstance(file_requirements, dict):
             # Newer format with a collections and/or roles key
             extra_keys = set(file_requirements.keys()).difference(set(['roles', 'collections']))
             if extra_keys:
@@ -853,6 +853,9 @@ class GalaxyCLI(CLI):
                 for collection_req in file_requirements.get('collections') or []
             ]
 
+        else:
+            raise AnsibleError(f"Expecting requirements yaml to be a list or dictionary but got {type(file_requirements).__name__}")
+
         return requirements
 
     def _init_coll_req_dict(self, coll_req):

test/units/cli/test_galaxy.py

@@ -769,6 +769,20 @@ def test_collection_install_with_names(collection_install):
     assert mock_install.call_args[0][6] is False  # force_deps
 
 
+def test_collection_install_with_invalid_requirements_format(collection_install):
+    output_dir = collection_install[2]
+
+    requirements_file = os.path.join(output_dir, 'requirements.yml')
+    with open(requirements_file, 'wb') as req_obj:
+        req_obj.write(b'"invalid"')
+
+    galaxy_args = ['ansible-galaxy', 'collection', 'install', '--requirements-file', requirements_file,
+                   '--collections-path', output_dir]
+
+    with pytest.raises(AnsibleError, match="Expecting requirements yaml to be a list or dictionary but got str"):
+        GalaxyCLI(args=galaxy_args).run()
+
+
 def test_collection_install_with_requirements_file(collection_install):
     mock_install, mock_warning, output_dir = collection_install