Revert "Cleanup: delete yaml_load_safely (#11285)" (#11297)

This reverts commit 791643f.

readthedocs/config/tests/test_yaml_loader.py

@@ -0,0 +1,28 @@
+from readthedocs.doc_builder.backends.mkdocs import ProxyPythonName, yaml_load_safely
+
+content = """
+int: 3
+float: !!float 3
+function: !!python/name:python_function
+other_function: !!python/name:module.other.function
+unknown: !!python/module:python_module
+"""
+
+
+def test_yaml_load_safely():
+    expected = {
+        "int": 3,
+        "float": 3.0,
+        "function": ProxyPythonName("python_function"),
+        "other_function": ProxyPythonName("module.other.function"),
+        "unknown": None,
+    }
+    data = yaml_load_safely(content)
+
+    assert data == expected
+    assert type(data["int"]) is int
+    assert type(data["float"]) is float
+    assert type(data["function"]) is ProxyPythonName
+    assert type(data["other_function"]) is ProxyPythonName
+    assert data["function"].value == "python_function"
+    assert data["other_function"].value == "module.other.function"

readthedocs/doc_builder/backends/mkdocs.py

@@ -63,8 +63,7 @@ def get_final_doctype(self):
             allow_symlinks=True,
             base_path=self.project_path,
         ) as fh:
-            # Use ``.safe_load()`` since ``mkdocs.yml`` is an untrusted source.
-            config = yaml.safe_load(fh)
+            config = yaml_load_safely(fh)
             use_directory_urls = config.get("use_directory_urls", True)
             return MKDOCS if use_directory_urls else MKDOCS_HTML
 
@@ -120,3 +119,62 @@ def build(self):
 class MkdocsHTML(BaseMkdocs):
     builder = "build"
     build_dir = "_readthedocs/html"
+
+
+class ProxyPythonName(yaml.YAMLObject):
+    def __init__(self, value):
+        self.value = value
+
+    def __eq__(self, other):
+        return self.value == other.value
+
+
+class SafeLoader(yaml.SafeLoader):  # pylint: disable=too-many-ancestors
+
+    """
+    Safe YAML loader.
+
+    This loader parses special ``!!python/name:`` tags without actually
+    importing or executing code. Every other special tag is ignored.
+
+    Borrowed from https://stackoverflow.com/a/57121993
+    Issue https://github.com/readthedocs/readthedocs.org/issues/7461
+    """
+
+    def ignore_unknown(self, node):  # pylint: disable=unused-argument
+        return None
+
+    def construct_python_name(self, suffix, node):  # pylint: disable=unused-argument
+        return ProxyPythonName(suffix)
+
+
+class SafeDumper(yaml.SafeDumper):
+
+    """
+    Safe YAML dumper.
+
+    This dumper allows to avoid losing values of special tags that
+    were parsed by our safe loader.
+    """
+
+    def represent_name(self, data):
+        return self.represent_scalar("tag:yaml.org,2002:python/name:" + data.value, "")
+
+
+SafeLoader.add_multi_constructor(
+    "tag:yaml.org,2002:python/name:", SafeLoader.construct_python_name
+)
+SafeLoader.add_constructor(None, SafeLoader.ignore_unknown)
+SafeDumper.add_representer(ProxyPythonName, SafeDumper.represent_name)
+
+
+def yaml_load_safely(content):
+    """
+    Uses ``SafeLoader`` loader to skip unknown tags.
+
+    When a YAML contains ``!!python/name:int`` it will store the ``int``
+    suffix temporarily to be able to re-dump it later. We need this to avoid
+    executing random code, but still support these YAML files without
+    information loss.
+    """
+    return yaml.load(content, Loader=SafeLoader)