[s3] fix disabling cloudfront signing with cloudfront_signer=None

This change fixes disabling cloudfront signing by specifying "None" (or
another falsy value). This allows multiple storage configs that are all
distributed via cloudfront, but only some of the configs need to be
signed.

storages/backends/s3.py

@@ -299,7 +299,8 @@ class S3Storage(CompressStorageMixin, BaseStorage):
     config = None
 
     def __init__(self, **settings):
-        self.cloudfront_signer = settings.pop("cloudfront_signer", None)
+        omitted = object()
+        self.cloudfront_signer = settings.pop("cloudfront_signer", omitted)
 
         super().__init__(**settings)
 
@@ -333,7 +334,7 @@ def __init__(self, **settings):
         if self.transfer_config is None:
             self.transfer_config = TransferConfig(use_threads=self.use_threads)
 
-        if not self.cloudfront_signer:
+        if self.cloudfront_signer is omitted:
             if self.cloudfront_key_id and self.cloudfront_key:
                 self.cloudfront_signer = self.get_cloudfront_signer(
                     self.cloudfront_key_id, self.cloudfront_key
@@ -343,6 +344,8 @@ def __init__(self, **settings):
                     "Both AWS_CLOUDFRONT_KEY_ID/cloudfront_key_id and "
                     "AWS_CLOUDFRONT_KEY/cloudfront_key must be provided together."
                 )
+            else:
+                self.cloudfront_signer = None
 
     def get_cloudfront_signer(self, key_id, key):
         return _cloud_front_signer_from_pem(key_id, key)

tests/test_s3.py

@@ -890,6 +890,10 @@ def test_cloudfront_config(self):
             storage = s3.S3Storage()
             self.assertIsNotNone(storage.cloudfront_signer)
 
+            # allow disabling cloudfront signing
+            storage = s3.S3Storage(cloudfront_signer=None)
+            self.assertIsNone(storage.cloudfront_signer)
+
         storage = s3.S3Storage(cloudfront_key_id=key_id, cloudfront_key=pem)
         self.assertIsNotNone(storage.cloudfront_signer)