[6.0.x] Fixed 36622 -- Prevented LazyObject FileField storages from evaluating at boot time.

Co-authored-by: Fabien MICHEL <fmichel@adista.fr>

Backport of 6862d46dd96d71d80d6d2fa9873a93d811b39562 from main.

django/db/models/fields/files.py

@@ -248,7 +248,7 @@ class FileField(Field):
     ):
         self._primary_key_set_explicitly = "primary_key" in kwargs
 
-        self.storage = storage or default_storage
+        self.storage = storage if storage is not None else default_storage
         if callable(self.storage):
             # Hold a reference to the callable for deconstruct().
             self._storage_callable = self.storage

tests/file_storage/models.py

@@ -11,6 +11,7 @@ from pathlib import Path
 
 from django.core.files.storage import FileSystemStorage, default_storage
 from django.db import models
+from django.utils.functional import LazyObject
 
 
 class CustomValidNameStorage(FileSystemStorage):
@@ -37,6 +38,11 @@ class CallableStorage(FileSystemStorage):
         return self
 
 
+class LazyTempStorage(LazyObject):
+    def _setup(self):
+        self._wrapped = temp_storage
+
+
 class Storage(models.Model):
     def custom_upload_to(self, filename):
         return "foo"
@@ -82,3 +88,4 @@ class Storage(models.Model):
     extended_length = models.FileField(
         storage=temp_storage, upload_to="tests", max_length=1024
     )
+    lazy_storage = models.FileField(storage=LazyTempStorage(), upload_to="tests")

tests/file_storage/tests.py

@@ -29,6 +29,7 @@ from django.test.utils import requires_tz_support
 from django.urls import NoReverseMatch, reverse_lazy
 from django.utils import timezone
 from django.utils._os import symlinks_supported
+from django.utils.functional import empty
 
 from .models import (
     Storage,
@@ -1267,3 +1268,11 @@ class StorageHandlerTests(SimpleTestCase):
         )
         with self.assertRaisesMessage(InvalidStorageError, msg):
             test_storages["invalid_backend"]
+
+
+class StorageLazyObjectTests(SimpleTestCase):
+    def test_lazy_object_is_not_evaluated_before_manual_access(self):
+        obj = Storage()
+        self.assertIs(obj.lazy_storage.storage._wrapped, empty)
+        # assertEqual triggers resolution.
+        self.assertEqual(obj.lazy_storage.storage, temp_storage)