Fixed #20486 -- Ensure that file_move_safe raises an error if the destination already exists.

Thanks to kux for the report, and Russ Webber for the patch.

AUTHORS

@@ -608,6 +608,7 @@ answer newbie questions, and generally made Django that much better:
     Filip Wasilewski <filip.wasilewski@gmail.com>
     Dan Watson <http://danwatson.net/>
     Joel Watts <joel@joelwatts.com>
+    Russ Webber
     Lakin Wecker <lakin@structuredabstraction.com>
     Chris Wesseling <Chris.Wesseling@cwi.nl>
     Benjamin Wohlwend <piquadrat@gmail.com>

django/core/files/move.py

@@ -51,6 +51,10 @@ def file_move_safe(old_file_name, new_file_name, chunk_size = 1024*64, allow_ove
         return
 
     try:
+        # If the destination file exists and allow_overwrite is False then raise an IOError
+        if not allow_overwrite and os.access(new_file_name, os.F_OK):
+            raise IOError("Destination file %s exists and allow_overwrite is False" % new_file_name)
+
         os.rename(old_file_name, new_file_name)
         return
     except OSError:

tests/files/tests.py

@@ -1,11 +1,13 @@
 from __future__ import absolute_import
 
+import os
 import gzip
 import shutil
 import tempfile
 
 from django.core.cache import cache
 from django.core.files import File
+from django.core.files.move import file_move_safe
 from django.core.files.base import ContentFile
 from django.core.files.uploadedfile import SimpleUploadedFile
 from django.test import TestCase
@@ -146,3 +148,15 @@ class FileTests(unittest.TestCase):
         file = SimpleUploadedFile("mode_test.txt", b"content")
         self.assertFalse(hasattr(file, 'mode'))
         g = gzip.GzipFile(fileobj=file)
+
+
+class FileMoveSafeTests(unittest.TestCase):
+    def test_file_move_overwrite(self):
+        handle_a, self.file_a = tempfile.mkstemp(dir=os.environ['DJANGO_TEST_TEMP_DIR'])
+        handle_b, self.file_b = tempfile.mkstemp(dir=os.environ['DJANGO_TEST_TEMP_DIR'])
+
+        # file_move_safe should raise an IOError exception if destination file exists and allow_overwrite is False
+        self.assertRaises(IOError, lambda: file_move_safe(self.file_a, self.file_b, allow_overwrite=False))
+
+        # should allow it and continue on if allow_overwrite is True
+        self.assertIsNone(file_move_safe(self.file_a, self.file_b, allow_overwrite=True))