Protect _CVar's storage with a thread lock

asgiref/local.py

@@ -9,33 +9,37 @@ class _CVar:
     """Storage utility for Local."""
 
     def __init__(self) -> None:
+        self._thread_lock = threading.RLock()
         self._data: dict[str, contextvars.ContextVar[Any]] = {}
 
     def __getattr__(self, key: str) -> Any:
-        try:
-            var = self._data[key]
-        except KeyError:
-            raise AttributeError(f"{self!r} object has no attribute {key!r}")
+        with self._thread_lock:
+            try:
+                var = self._data[key]
+            except KeyError:
+                raise AttributeError(f"{self!r} object has no attribute {key!r}")
 
         try:
             return var.get()
         except LookupError:
             raise AttributeError(f"{self!r} object has no attribute {key!r}")
 
     def __setattr__(self, key: str, value: Any) -> None:
-        if key == "_data":
+        if key in ("_data", "_thread_lock"):
             return super().__setattr__(key, value)
 
-        var = self._data.get(key)
-        if var is None:
-            self._data[key] = var = contextvars.ContextVar(key)
+        with self._thread_lock:
+            var = self._data.get(key)
+            if var is None:
+                self._data[key] = var = contextvars.ContextVar(key)
         var.set(value)
 
     def __delattr__(self, key: str) -> None:
-        if key in self._data:
-            del self._data[key]
-        else:
-            raise AttributeError(f"{self!r} object has no attribute {key!r}")
+        with self._thread_lock:
+            if key in self._data:
+                del self._data[key]
+            else:
+                raise AttributeError(f"{self!r} object has no attribute {key!r}")
 
 
 def Local(thread_critical: bool = False) -> threading.local | _CVar: