From 0d1dd6bba0c18b7feb6caa5cbd8df80fbac54afd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ga=C3=ABl=20Utard?= <gael.utard@eedf.fr>
Date: Sat, 15 Feb 2025 15:55:33 +0100
Subject: [PATCH] Fixed #36191 -- Truncated the overwritten file content in
 FileSystemStorage.

---
 AUTHORS                                 |  1 +
 django/core/files/storage/filesystem.py |  2 +-
 docs/releases/5.1.7.txt                 |  4 ++++
 tests/file_storage/tests.py             | 12 ++++++++++++
 4 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/AUTHORS b/AUTHORS
index e3389edbf5..b816c305e5 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -369,6 +369,7 @@ answer newbie questions, and generally made Django that much better:
     Fraser Nevett <mail@nevett.org>
     Gabriel Grant <g@briel.ca>
     Gabriel Hurley <gabriel@strikeawe.com>
+    Gaël Utard
     gandalf@owca.info
     Garry Lawrence
     Garry Polley <garrympolley@gmail.com>
diff --git a/django/core/files/storage/filesystem.py b/django/core/files/storage/filesystem.py
index b8de9b0a58..54c31e536a 100644
--- a/django/core/files/storage/filesystem.py
+++ b/django/core/files/storage/filesystem.py
@@ -113,7 +113,7 @@ class FileSystemStorage(Storage, StorageSettingsMixin):
                         | getattr(os, "O_BINARY", 0)
                     )
                     if self._allow_overwrite:
-                        open_flags = open_flags & ~os.O_EXCL
+                        open_flags = open_flags & ~os.O_EXCL | os.O_TRUNC
                     fd = os.open(full_path, open_flags, 0o666)
                     _file = None
                     try:
diff --git a/docs/releases/5.1.7.txt b/docs/releases/5.1.7.txt
index e184da6aca..deda4f2f92 100644
--- a/docs/releases/5.1.7.txt
+++ b/docs/releases/5.1.7.txt
@@ -12,3 +12,7 @@ Bugfixes
 * Fixed a bug in Django 5.1 where the ``{% querystring %}`` template tag
   returned an empty string rather than ``"?"`` when all parameters had been
   removed from the query string (:ticket:`36182`).
+
+* Fixed a bug in Django 5.1 where ``FileSystemStorage``, with
+  ``allow_overwrite`` set to ``True``, did not truncate the overwritten file
+  content (:ticket:`36191`).
diff --git a/tests/file_storage/tests.py b/tests/file_storage/tests.py
index c048b8f071..70c7a4f15f 100644
--- a/tests/file_storage/tests.py
+++ b/tests/file_storage/tests.py
@@ -634,6 +634,18 @@ class OverwritingStorageTests(FileStorageTests):
         finally:
             self.storage.delete(name)
 
+    def test_save_overwrite_behavior_truncate(self):
+        name = "test.file"
+        original_content = b"content extra extra extra"
+        new_smaller_content = b"content"
+        self.storage.save(name, ContentFile(original_content))
+        try:
+            self.storage.save(name, ContentFile(new_smaller_content))
+            with self.storage.open(name) as fp:
+                self.assertEqual(fp.read(), new_smaller_content)
+        finally:
+            self.storage.delete(name)
+
     def test_save_overwrite_behavior_temp_file(self):
         """Saving to same file name twice overwrites the first file."""
         name = "test.file"