Terminated AppCache._populate() with extreme prejudice.

It was called _populate() before I renamed it to populate(). Since it
has been superseded by populate_models() there's no reason to keep it.

Removed the can_postpone argument of load_app() as it was only used by
populate(). It's a private API and there's no replacement. Simplified
load_app() accordingly. Then new version behaves exactly like the old
one even though it's much shorter.

9 files changed, 24 insertions, 84 deletions

diff --git a/django/contrib/admin/validation.py b/django/contrib/admin/validation.py
index a15967e024..ee945f73e5 100644
--- a/django/contrib/admin/validation.py
+++ b/django/contrib/admin/validation.py
@@ -18,7 +18,7 @@ class BaseValidator(object):
     def __init__(self):
         # Before we can introspect models, they need the app cache to be fully
         # loaded so that inter-relations are set up correctly.
-        app_cache.populate()
+        app_cache.populate_models()
 
     def validate(self, cls, model):
         for m in dir(self):
diff --git a/django/core/apps/cache.py b/django/core/apps/cache.py
index cf2489185e..e9a4215440 100644
--- a/django/core/apps/cache.py
+++ b/django/core/apps/cache.py
@@ -2,17 +2,16 @@
 
 from collections import defaultdict, OrderedDict
 from contextlib import contextmanager
-from importlib import import_module
 import os
 import sys
 import warnings
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
-from django.utils.module_loading import import_lock, module_has_submodule
+from django.utils.module_loading import import_lock
 from django.utils._os import upath
 
-from .base import AppConfig, MODELS_MODULE_NAME
+from .base import AppConfig
 
 
 class UnavailableApp(Exception):
@@ -51,15 +50,11 @@ class AppCache(object):
         # Used by TransactionTestCase.available_apps for performance reasons.
         self.available_apps = None
 
-        # Internal flags used when populating the cache.
-        self._apps_loaded = False
-        self._models_loaded = False
+        # Internal flags used when populating the master cache.
+        self._apps_loaded = not self.master
+        self._models_loaded = not self.master
 
-        # -- Everything below here is only used when populating the cache --
-        self.loaded = False
-        self.handled = set()
-        self.postponed = []
-        self.nesting_level = 0
+        # Cache for get_models.
         self._get_models_cache = {}
 
     def populate_apps(self):
@@ -138,71 +133,15 @@ class AppCache(object):
 
                 self._models_loaded = True
 
-    def populate(self):
-        """
-        Fill in all the cache information. This method is threadsafe, in the
-        sense that every caller will see the same state upon return, and if the
-        cache is already initialised, it does no work.
-        """
-        if self.loaded:
-            return
-        if not self.master:
-            self.loaded = True
-            return
-        # 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.
-        with import_lock():
-            if self.loaded:
-                return
-            for app_name in settings.INSTALLED_APPS:
-                if app_name in self.handled:
-                    continue
-                self.load_app(app_name, can_postpone=True)
-            if not self.nesting_level:
-                for app_name in self.postponed:
-                    self.load_app(app_name)
-                self.loaded = True
-
-    def load_app(self, app_name, can_postpone=False):
+    def load_app(self, app_name):
         """
         Loads the app with the provided fully qualified name, and returns the
         model module.
         """
-        app_module = import_module(app_name)
-        self.handled.add(app_name)
-        self.nesting_level += 1
-        try:
-            models_module = import_module('%s.%s' % (app_name, MODELS_MODULE_NAME))
-        except ImportError:
-            # If the app doesn't have a models module, we can just swallow the
-            # ImportError and return no models for this app.
-            if not module_has_submodule(app_module, MODELS_MODULE_NAME):
-                models_module = None
-            # But if the app does have a models module, we need to figure out
-            # whether to suppress or propagate the error. If can_postpone is
-            # True then it may be that the package is still being imported by
-            # Python and the models module isn't available yet. So we add the
-            # app to the postponed list and we'll try it again after all the
-            # recursion has finished (in populate). If can_postpone is False
-            # then it's time to raise the ImportError.
-            else:
-                if can_postpone:
-                    self.postponed.append(app_name)
-                    return
-                else:
-                    raise
-        finally:
-            self.nesting_level -= 1
-
         app_config = AppConfig(app_name)
         app_config.import_models(self.all_models[app_config.label])
         self.app_configs[app_config.label] = app_config
-
-        return models_module
+        return app_config.models_module
 
     def app_cache_ready(self):
         """
@@ -211,7 +150,7 @@ class AppCache(object):
         Useful for code that wants to cache the results of get_models() for
         themselves once it is safe to do so.
         """
-        return self.loaded
+        return self._models_loaded              # implies self._apps_loaded.
 
     def get_app_configs(self, only_with_models_module=False):
         """
@@ -220,7 +159,7 @@ class AppCache(object):
         If only_with_models_module in True (non-default), only applications
         containing a models module are considered.
         """
-        self.populate()
+        self.populate_models()
         for app_config in self.app_configs.values():
             if only_with_models_module and app_config.models_module is None:
                 continue
@@ -240,7 +179,7 @@ class AppCache(object):
         If only_with_models_module in True (non-default), only applications
         containing a models module are considered.
         """
-        self.populate()
+        self.populate_models()
         app_config = self.app_configs.get(app_label)
         if app_config is None:
             raise LookupError("No installed app with label %r." % app_label)
@@ -288,7 +227,7 @@ class AppCache(object):
             return model_list
         except KeyError:
             pass
-        self.populate()
+        self.populate_models()
         if app_mod:
             app_label = app_mod.__name__.split('.')[-2]
             if only_installed:
@@ -331,7 +270,7 @@ class AppCache(object):
         """
         if not self.master:
             only_installed = False
-        self.populate()
+        self.populate_models()
         if only_installed:
             app_config = self.app_configs.get(app_label)
             if app_config is None:
@@ -496,7 +435,7 @@ class AppCache(object):
             "[a.path for a in get_app_configs()] supersedes get_app_paths().",
             PendingDeprecationWarning, stacklevel=2)
 
-        self.populate()
+        self.populate_models()
 
         app_paths = []
         for app in self.get_apps():
diff --git a/django/core/serializers/base.py b/django/core/serializers/base.py
index db4b79a020..b0bc8138ab 100644
--- a/django/core/serializers/base.py
+++ b/django/core/serializers/base.py
@@ -139,7 +139,7 @@ class Deserializer(six.Iterator):
             self.stream = stream_or_string
         # Make sure the app cache is loaded before deserialization starts
         # (otherwise subclass calls to get_model() and friends might fail...)
-        app_cache.populate()
+        app_cache.populate_models()
 
     def __iter__(self):
         return self
diff --git a/django/core/serializers/python.py b/django/core/serializers/python.py
index 13d6f01a4e..1e4c37e2ae 100644
--- a/django/core/serializers/python.py
+++ b/django/core/serializers/python.py
@@ -88,7 +88,7 @@ def Deserializer(object_list, **options):
     db = options.pop('using', DEFAULT_DB_ALIAS)
     ignore = options.pop('ignorenonexistent', False)
 
-    app_cache.populate()
+    app_cache.populate_models()
 
     for d in object_list:
         # Look up the model and starting build a dict of data for it.
diff --git a/django/db/models/base.py b/django/db/models/base.py
index a694350b97..d1e7323d17 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -6,7 +6,7 @@ from functools import update_wrapper
 from django.utils.six.moves import zip
 
 from django.core.apps import app_cache
-from django.core.apps.cache import MODELS_MODULE_NAME
+from django.core.apps.base import MODELS_MODULE_NAME
 import django.db.models.manager  # NOQA: Imported to register signal handler.
 from django.conf import settings
 from django.core.exceptions import (ObjectDoesNotExist,
diff --git a/django/db/models/loading.py b/django/db/models/loading.py
index 5745dbed4d..00fae36769 100644
--- a/django/db/models/loading.py
+++ b/django/db/models/loading.py
@@ -30,6 +30,6 @@ def get_app_errors():
     try:
         return app_cache.app_errors
     except AttributeError:
-        app_cache.populate()
+        app_cache.populate_models()
         app_cache.app_errors = {}
         return app_cache.app_errors
diff --git a/django/db/models/options.py b/django/db/models/options.py
index f7be089539..fec9aa4fc4 100644
--- a/django/db/models/options.py
+++ b/django/db/models/options.py
@@ -93,7 +93,7 @@ class Options(object):
 
     @property
     def app_config(self):
-        # Don't go through get_app_config to avoid triggering populate().
+        # Don't go through get_app_config to avoid triggering imports.
         return self.app_cache.app_configs.get(self.app_label)
 
     @property
diff --git a/tests/app_loading/tests.py b/tests/app_loading/tests.py
index e282306ec0..45f6cd6c31 100644
--- a/tests/app_loading/tests.py
+++ b/tests/app_loading/tests.py
@@ -77,8 +77,9 @@ class EggLoadingTest(TestCase):
         error. Refs #17667.
         """
         app_cache = AppCache()
-        # Pretend we're the master app cache to test populate().
-        app_cache.master = True
+        # Pretend we're the master app cache to test the population process.
+        app_cache._apps_loaded = False
+        app_cache._models_loaded = False
         with override_settings(INSTALLED_APPS=('notexists',)):
             with self.assertRaises(ImportError):
                 app_cache.get_model('notexists', 'nomodel')
diff --git a/tests/runtests.py b/tests/runtests.py
index badbd6fbec..77cf005267 100755
--- a/tests/runtests.py
+++ b/tests/runtests.py
@@ -128,7 +128,7 @@ def setup(verbosity, test_labels):
     # Load all the ALWAYS_INSTALLED_APPS.
     with warnings.catch_warnings():
         warnings.filterwarnings('ignore', 'django.contrib.comments is deprecated and will be removed before Django 1.8.', DeprecationWarning)
-        app_cache.populate()
+        app_cache.populate_models()
 
     # Load all the test model apps.
     test_modules = get_test_modules()