ENG-7314: Only check for valid app name once per config instance

After the backend comes up, we check the app name once and validate that it's
good. There is no need to check it again in the same process.

Fixes the spurious `app.app module not found` errors that occur when deleting
and replacing the main app module while the backend is running, but before
triggering a reload.

Author: masenf

reflex/config.py

@@ -284,6 +284,9 @@ class Config(BaseConfig):
     See the [configuration](https://reflex.dev/docs/getting-started/configuration/) docs for more info.
     """
 
+    # Track whether the app name has already been validated for this Config instance.
+    _app_name_is_valid: bool = dataclasses.field(default=False)
+
     def _post_init(self, **kwargs):
         """Post-initialization method to set up the config.
 

reflex/utils/prerequisites.py

@@ -165,6 +165,7 @@ def _check_app_name(config: Config):
             else:
                 msg += f"Ensure app_name='{config.app_name}' in rxconfig.py matches your folder structure."
             raise ModuleNotFoundError(msg)
+    config._app_name_is_valid = True
 
 
 def get_app(reload: bool = False) -> ModuleType:
@@ -184,7 +185,9 @@ def get_app(reload: bool = False) -> ModuleType:
     try:
         config = get_config()
 
-        _check_app_name(config)
+        # Avoid hitting disk when the app name has already been validated in this process.
+        if not config._app_name_is_valid:
+            _check_app_name(config)
 
         module = config.module
         sys.path.insert(0, str(Path.cwd()))