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

sarahboyce · sarahboyce · commit 318c16d2b815 · 2025-04-07T16:17:50.000+02:00
Regression in 58cd490. Thanks Baptiste Mispelon for the report. Backport of 8ad3e80 from main.
diff --git a/django/core/files/move.py b/django/core/files/move.py
@@ -67,6 +67,7 @@ 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:
diff --git a/docs/releases/4.2.21.txt b/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`).
diff --git a/docs/releases/index.txt b/docs/releases/index.txt
@@ -26,6 +26,7 @@ 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
diff --git a/tests/files/tests.py b/tests/files/tests.py
@@ -475,6 +475,27 @@ def test_file_move_permissionerror(self):
             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):

Original file line number	Diff line number	Diff line change
`@@ -67,6 +67,7 @@ def file_move_safe(`
`67`	`67`	`\| os.O_CREAT`
`68`	`68`	`\| getattr(os, "O_BINARY", 0)`
`69`	`69`	`\| (os.O_EXCL if not allow_overwrite else 0)`
	`70`	`+ \| os.O_TRUNC`
`70`	`71`	`),`
`71`	`72`	`)`
`72`	`73`	`try:`