Validate mkdocs.yml config on values that we manipulate

Make sure that the values we will manipulate from the mkdocs.yml file
the correct type we need them to be. If we found a problem with them,
we report a proper message to user to debug it by themselves.

Author: humitos

readthedocs/doc_builder/backends/mkdocs.py

@@ -84,7 +84,7 @@ def load_yaml_config(self):
         """
         Load a YAML config.
 
-        Raise BuildEnvironmentError if failed due to syntax errors.
+        :raises: ``MkDocsYAMLParseError`` if failed due to syntax errors.
         """
         try:
             return yaml.safe_load(
@@ -105,20 +105,40 @@ def load_yaml_config(self):
             )
 
     def append_conf(self, **__):
-        """Set mkdocs config values."""
+        """
+        Set mkdocs config values.
+
+        :raises: ``MkDocsYAMLParseError`` if failed due to known type errors
+                 (i.e. expecting a list and a string is found).
+        """
         if not self.yaml_file:
             self.yaml_file = os.path.join(self.root_path, 'mkdocs.yml')
 
         user_config = self.load_yaml_config()
 
         # Handle custom docs dirs
         user_docs_dir = user_config.get('docs_dir')
+        if not isinstance(user_docs_dir, str):
+            raise MkDocsYAMLParseError(
+                MkDocsYAMLParseError.INVALID_DOCS_DIR_CONFIG,
+            )
+
         docs_dir = self.docs_dir(docs_dir=user_docs_dir)
         self.create_index(extension='md')
         user_config['docs_dir'] = docs_dir
 
         # Set mkdocs config values
         static_url = get_absolute_static_url()
+
+        for config in ('extra_css', 'extra_javascript'):
+            user_value = user_config.get(config)
+            if not isinstance(user_value, (type(None), list)):
+                raise MkDocsYAMLParseError(
+                    MkDocsYAMLParseError.INVALID_EXTRA_CONFIG.format(
+                        config=config,
+                    ),
+                )
+
         user_config.setdefault('extra_javascript', []).extend([
             'readthedocs-data.js',
             '%score/js/readthedocs-doc-embed.js' % static_url,

readthedocs/doc_builder/exceptions.py

@@ -59,3 +59,13 @@ class MkDocsYAMLParseError(BuildEnvironmentError):
     GENERIC_WITH_PARSE_EXCEPTION = ugettext_noop(
         'Problem parsing MkDocs YAML configuration. {exception}',
     )
+
+    INVALID_DOCS_DIR_CONFIG = ugettext_noop(
+        'The "docs_dir" config from your MkDocs YAML config file has to be a '
+        'string with relative or absolute path.',
+    )
+
+    INVALID_EXTRA_CONFIG = ugettext_noop(
+        'The "{config}" config from your MkDocs YAML config file has to be a '
+        'a list of relative paths.',
+    )