Do not check for S3 key before attempting download (#19504)

S3Hook.download_file first checks object existence then downloads.
This resolves creds 2 times.  We don't need to check existence.
Just ask for the key and if it's not there you'll know from the error.
And if it is there, you'll only have resolved creds once.

Co-authored-by: Kaxil Naik <kaxilnaik@gmail.com>

airflow/providers/amazon/aws/hooks/s3.py

@@ -806,10 +806,15 @@ def download_file(
         """
         self.log.info('Downloading source S3 file from Bucket %s with path %s', bucket_name, key)
 
-        if not self.check_for_key(key, bucket_name):
-            raise AirflowException(f'The source file in Bucket {bucket_name} with path {key} does not exist')
-
-        s3_obj = self.get_key(key, bucket_name)
+        try:
+            s3_obj = self.get_key(key, bucket_name)
+        except ClientError as e:
+            if e.response.get('Error', {}).get('Code') == 404:
+                raise AirflowException(
+                    f'The source file in Bucket {bucket_name} with path {key} does not exist'
+                )
+            else:
+                raise e
 
         with NamedTemporaryFile(dir=local_path, prefix='airflow_tmp_', delete=False) as local_tmp_file:
             s3_obj.download_fileobj(local_tmp_file)

tests/providers/amazon/aws/hooks/test_s3.py

@@ -444,7 +444,6 @@ def test_download_file(self, mock_temp_file):
 
         s3_hook.download_file(key=key, bucket_name=bucket)
 
-        s3_hook.check_for_key.assert_called_once_with(key, bucket)
         s3_hook.get_key.assert_called_once_with(key, bucket)
         s3_obj.download_fileobj.assert_called_once_with(mock_temp_file)