add a check if app_name is wrong in rxconfig (#5410)

* add a check if manual renaming was done wrong

* refacto/condense

* handle testing env

* skip check in pytest

* fix everything

* better handling, don't exclude app_module_import

* remove comment

Author: Lendemor

reflex/utils/misc.py

@@ -1,7 +1,11 @@
 """Miscellaneous functions for the experimental package."""
 
 import asyncio
+import contextlib
+import sys
+import threading
 from collections.abc import Callable
+from pathlib import Path
 from typing import Any
 
 
@@ -23,3 +27,27 @@ async def run_in_thread(func: Callable) -> Any:
         msg = "func must be a non-async function"
         raise ValueError(msg)
     return await asyncio.get_event_loop().run_in_executor(None, func)
+
+
+# Global lock for thread-safe sys.path manipulation
+_sys_path_lock = threading.RLock()
+
+
+@contextlib.contextmanager
+def with_cwd_in_syspath():
+    """Temporarily add current working directory to sys.path in a thread-safe manner.
+
+    This context manager temporarily prepends the current working directory to sys.path,
+    ensuring that modules in the current directory can be imported. The original sys.path
+    is restored when exiting the context.
+
+    Yields:
+        None
+    """
+    with _sys_path_lock:
+        orig_sys_path = sys.path.copy()
+        sys.path.insert(0, str(Path.cwd()))
+        try:
+            yield
+        finally:
+            sys.path[:] = orig_sys_path

reflex/utils/prerequisites.py

@@ -334,13 +334,14 @@ def npm_escape_hatch() -> bool:
 
 
 def _check_app_name(config: Config):
-    """Check if the app name is set in the config.
+    """Check if the app name is valid and matches the folder structure.
 
     Args:
         config: The config object.
 
     Raises:
-        RuntimeError: If the app name is not set in the config.
+        RuntimeError: If the app name is not set, folder doesn't exist, or doesn't match config.
+        ModuleNotFoundError: If the app_name is not importable (i.e., not a valid Python package, folder structure being wrong).
     """
     if not config.app_name:
         msg = (
@@ -349,6 +350,21 @@ def _check_app_name(config: Config):
         )
         raise RuntimeError(msg)
 
+    from reflex.utils.misc import with_cwd_in_syspath
+
+    with with_cwd_in_syspath():
+        try:
+            mod_spec = importlib.util.find_spec(config.module)
+        except ModuleNotFoundError:
+            mod_spec = None
+        if mod_spec is None:
+            msg = f"Module {config.module} not found. "
+            if config.app_module_import is not None:
+                msg += f"Ensure app_module_import='{config.app_module_import}' in rxconfig.py matches your folder structure."
+            else:
+                msg += f"Ensure app_name='{config.app_name}' in rxconfig.py matches your folder structure."
+            raise ModuleNotFoundError(msg)
+
 
 def get_app(reload: bool = False) -> ModuleType:
     """Get the app module based on the default config.