[5.1.x] Fixed #36298 -- Truncated the overwritten file content in file_move_safe().

Regression in 58cd4902a7.

Thanks Baptiste Mispelon for the report.

Backport of 8ad3e80e88 from main.

django/core/files/move.py

@@ -55,6 +55,7 @@ def file_move_safe(
                 | os.O_CREAT
                 | getattr(os, "O_BINARY", 0)
                 | (os.O_EXCL if not allow_overwrite else 0)
+                | os.O_TRUNC
             ),
         )
         try:

docs/releases/4.2.21.txt

@@ -0,0 +1,15 @@
+===========================
+Django 4.2.21 release notes
+===========================
+
+*Expected May 7, 2025*
+
+Django 4.2.21 fixes a data loss bug in 4.2.20.
+
+Bugfixes
+========
+
+* Fixed a data corruption possibility in ``file_move_safe()`` when
+  ``allow_overwrite=True``, where leftover content from a previously larger
+  file could remain after overwriting with a smaller one due to lack of
+  truncation (:ticket:`36298`).

docs/releases/5.1.9.txt

@@ -0,0 +1,15 @@
+==========================
+Django 5.1.9 release notes
+==========================
+
+*Expected May 7, 2025*
+
+Django 5.1.9 fixes a data loss bug in 5.1.8.
+
+Bugfixes
+========
+
+* Fixed a data corruption possibility in ``file_move_safe()`` when
+  ``allow_overwrite=True``, where leftover content from a previously larger
+  file could remain after overwriting with a smaller one due to lack of
+  truncation (:ticket:`36298`).

docs/releases/index.txt

@@ -25,6 +25,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   5.1.9
    5.1.8
    5.1.7
    5.1.6
@@ -62,6 +63,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   4.2.21
    4.2.20
    4.2.19
    4.2.18

tests/files/tests.py

@@ -496,6 +496,27 @@ class FileMoveSafeTests(unittest.TestCase):
             os.close(handle_b)
             os.close(handle_c)
 
+    def test_file_move_ensure_truncation(self):
+        with tempfile.NamedTemporaryFile(delete=False) as src:
+            src.write(b"content")
+            src_name = src.name
+        self.addCleanup(
+            lambda: os.remove(src_name) if os.path.exists(src_name) else None
+        )
+
+        with tempfile.NamedTemporaryFile(delete=False) as dest:
+            dest.write(b"This is a longer content.")
+            dest_name = dest.name
+        self.addCleanup(os.remove, dest_name)
+
+        with mock.patch("django.core.files.move.os.rename", side_effect=OSError()):
+            file_move_safe(src_name, dest_name, allow_overwrite=True)
+
+        with open(dest_name, "rb") as f:
+            content = f.read()
+
+        self.assertEqual(content, b"content")
+
 
 class SpooledTempTests(unittest.TestCase):
     def test_in_memory_spooled_temp(self):