Fixed #18251 -- Removed a deadlock possibility in apploading

from django.utils.importlib import import_module

from django.utils.module_loading import module_has_submodule

import imp

import sys

import os

import threading

__all__ = ('get_apps', 'get_app', 'get_models', 'get_model', 'register_models',

        'load_app', 'app_cache_ready')

        handled = {},

        postponed = [],

        nesting_level = 0,

        write_lock = threading.RLock(),

        _get_models_cache = {},

    )

        """

        if self.loaded:

            return

        with self.write_lock:

        # Note that we want to use the import lock here - the app loading is

        # in many cases initiated implicitly by importing, and thus it is

        # possible to end up in deadlock when one thread initiates loading

        # without holding the importer lock and another thread then tries to

        # import something which also launches the app loading. For details of

        # this situation see #18251.

        imp.acquire_lock()

        try:

            if self.loaded:

                return

            for app_name in settings.INSTALLED_APPS:

                for app_name in self.postponed:

                    self.load_app(app_name)

                self.loaded = True

        finally:

            imp.release_lock()

    def _label_for(self, app_mod):

        """

        the app has no models in it and 'emptyOK' is True, returns None.

        """

        self._populate()

        with self.write_lock:

        imp.acquire_lock()

        try:

            for app_name in settings.INSTALLED_APPS:

                if app_label == app_name.split('.')[-1]:

                    mod = self.load_app(app_name, False)

                    else:

                        return mod

            raise ImproperlyConfigured("App with label %s could not be found" % app_label)

        finally:

            imp.release_lock()

    def get_app_errors(self):

        "Returns the map of known problems with the INSTALLED_APPS."