[s3] fix file reopening (#1321)

storages/backends/s3.py

@@ -130,18 +130,32 @@ def __init__(self, name, mode, storage, buffer_size=None):
                 self._storage.get_object_parameters(self.name)
             )
             self.obj.load(**params)
-        self._is_dirty = False
-        self._raw_bytes_written = 0
         self._closed = False
         self._file = None
-        self._multipart = None
         self._parts = None
         # 5 MB is the minimum part size (if there is more than one part).
         # Amazon allows up to 10,000 parts.  The default supports uploads
         # up to roughly 50 GB.  Increase the part size to accommodate
         # for files larger than this.
         self.buffer_size = buffer_size or setting("AWS_S3_FILE_BUFFER_SIZE", 5242880)
+        self._reset_file_properties()
+
+    def _reset_file_properties(self):
+        self._multipart = None
+        self._raw_bytes_written = 0
         self._write_counter = 0
+        self._is_dirty = False
+
+    def open(self, mode=None):
+        if self._file is not None and not self.closed:
+            self.seek(0)  # Mirror Django's behavior
+        elif mode and mode != self._mode:
+            raise ValueError("Cannot reopen file with a new mode.")
+
+        # Accessing the file will functionally re-open it
+        self.file  # noqa: B018
+
+        return self
 
     @property
     def size(self):
@@ -264,6 +278,8 @@ def close(self):
         if self._file is not None:
             self._file.close()
             self._file = None
+
+        self._reset_file_properties()
         self._closed = True
 
 

tests/test_s3.py

@@ -979,6 +979,8 @@ def test_save(self):
 
 
 class S3FileTests(TestCase):
+    # Remove the override_settings after Python3.7 is dropped
+    @override_settings(AWS_S3_OBJECT_PARAMETERS={"ContentType": "text/html"})
     def setUp(self) -> None:
         self.storage = s3.S3Storage()
         self.storage._connections.connection = mock.MagicMock()
@@ -1015,6 +1017,21 @@ def test_closed(self):
             f.file
             self.assertFalse(f.closed)
 
+    def test_reopening(self):
+        f = s3.S3File("test", "wb", self.storage)
+
+        with f.open() as fp:
+            fp.write(b"xyz")
+
+        with f.open() as fp:
+            fp.write(b"xyz")
+
+        # Properties are reset
+        self.assertEqual(f._write_counter, 0)
+        self.assertEqual(f._raw_bytes_written, 0)
+        self.assertFalse(f._is_dirty)
+        self.assertIsNone(f._multipart)
+
 
 @mock_s3
 class S3StorageTestsWithMoto(TestCase):