[1.6.x] Reworked the detection of local storages for the collectstatic command.

Before 4befb30 the detection was broken because we used isinstance
against a LazyObject rather than against a Storage class. That commit
fixed it by looking directly at the object wrapped by LazyObject.
This could however be a problem to anyone who subclasses the
collectstatic management Command and directly supplies a Storage class.

Refs #21581.

Backport of 7e27885c6e from master.

django/contrib/staticfiles/management/commands/collectstatic.py

@@ -8,6 +8,7 @@ from django.core.files.storage import FileSystemStorage
 from django.core.management.base import CommandError, NoArgsCommand
 from django.utils.encoding import smart_text
 from django.utils.datastructures import SortedDict
+from django.utils.functional import LazyObject
 from django.utils.six.moves import input
 
 from django.contrib.staticfiles import finders, storage
@@ -149,8 +150,7 @@ class Command(NoArgsCommand):
             'location as specified in your settings'
         )
 
-        if (isinstance(self.storage._wrapped, FileSystemStorage) and
-                self.storage.location):
+        if self.is_local_storage() and self.storage.location:
             destination_path = self.storage.location
             message.append(':\n\n    %s\n\n' % destination_path)
         else:
@@ -197,6 +197,13 @@ class Command(NoArgsCommand):
         if self.verbosity >= level:
             self.stdout.write(msg)
 
+    def is_local_storage(self):
+        if issubclass(self.storage.__class__, LazyObject):
+            storage = self.storage._wrapped
+        else:
+            storage = self.storage
+        return isinstance(storage, FileSystemStorage)
+
     def clear_dir(self, path):
         """
         Deletes the given relative path using the destination storage backend.

tests/staticfiles_tests/tests.py

@@ -22,6 +22,9 @@ from django.utils import six
 
 from django.contrib.staticfiles import finders, storage
 
+from .storage import DummyStorage
+
+
 TEST_ROOT = os.path.dirname(upath(__file__))
 TEST_SETTINGS = {
     'DEBUG': True,
@@ -233,6 +236,29 @@ class TestConfiguration(StaticFilesTestCase):
                         'without having set the STATIC_ROOT setting to a filesystem path'):
                     call_command('collectstatic', interactive=False, verbosity=0, stderr=err)
 
+    def test_local_storage_detection_helper(self):
+        staticfiles_storage = storage.staticfiles_storage
+        try:
+            storage.staticfiles_storage._wrapped = empty
+            with override_settings(STATICFILES_STORAGE='django.contrib.staticfiles.storage.StaticFilesStorage'):
+                command = collectstatic.Command()
+                self.assertTrue(command.is_local_storage())
+
+            storage.staticfiles_storage._wrapped = empty
+            with override_settings(STATICFILES_STORAGE='staticfiles_tests.storage.DummyStorage'):
+                command = collectstatic.Command()
+                self.assertFalse(command.is_local_storage())
+
+            storage.staticfiles_storage = storage.FileSystemStorage()
+            command = collectstatic.Command()
+            self.assertTrue(command.is_local_storage())
+
+            storage.staticfiles_storage = DummyStorage()
+            command = collectstatic.Command()
+            self.assertFalse(command.is_local_storage())
+        finally:
+            storage.staticfiles_storage = staticfiles_storage
+
 
 class TestCollection(CollectionTestCase, TestDefaults):
     """