[1.10.x] Fixed #27658 -- Prevented collectstatic from overwriting newer files in remote storages.

Thanks revimi for the initial patch.

Backport of c85831e4b7 from master

django/contrib/staticfiles/management/commands/collectstatic.py

@@ -269,12 +269,24 @@ class Command(BaseCommand):
                     # The full path of the target file
                     if self.local:
                         full_path = self.storage.path(prefixed_path)
+                        # If it's --link mode and the path isn't a link (i.e.
+                        # the previous collectstatic wasn't with --link) or if
+                        # it's non-link mode and the path is a link (i.e. the
+                        # previous collectstatic was with --link), the old
+                        # links/files must be deleted so it's not safe to skip
+                        # unmodified files.
+                        can_skip_unmodified_files = not (self.symlink ^ os.path.islink(full_path))
                     else:
                         full_path = None
-                    # Skip the file if the source file is younger
+                        # In remote storages, skipping is only based on the
+                        # modified times since symlinks aren't relevant.
+                        can_skip_unmodified_files = True
                     # Avoid sub-second precision (see #14665, #19540)
-                    if (target_last_modified.replace(microsecond=0) >= source_last_modified.replace(microsecond=0) and
-                            full_path and not (self.symlink ^ os.path.islink(full_path))):
+                    file_is_unmodified = (
+                        target_last_modified.replace(microsecond=0) >=
+                        source_last_modified.replace(microsecond=0)
+                    )
+                    if file_is_unmodified and can_skip_unmodified_files:
                         if prefixed_path not in self.unmodified_files:
                             self.unmodified_files.append(prefixed_path)
                         self.log("Skipping '%s' (not modified)" % path)

docs/releases/1.10.5.txt

@@ -17,3 +17,6 @@ Bugfixes
 
 * Fixed a regression in the ``timesince`` and ``timeuntil`` filters that caused
   incorrect results for dates in a leap year (:ticket:`27637`).
+
+* Fixed a regression where ``collectstatic`` overwrote newer files in remote
+  storages (:ticket:`27658`).

tests/staticfiles_tests/cases.py

@@ -82,7 +82,8 @@ class CollectionTestCase(BaseStaticFilesMixin, SimpleTestCase):
         super(CollectionTestCase, self).tearDown()
 
     def run_collectstatic(self, **kwargs):
-        call_command('collectstatic', interactive=False, verbosity=0,
+        verbosity = kwargs.pop('verbosity', 0)
+        call_command('collectstatic', interactive=False, verbosity=verbosity,
                      ignore_patterns=['*.ignoreme'], **kwargs)
 
     def _get_file(self, filepath):

tests/staticfiles_tests/storage.py

@@ -1,6 +1,6 @@
 import errno
 import os
-from datetime import datetime
+from datetime import datetime, timedelta
 
 from django.conf import settings
 from django.contrib.staticfiles.storage import CachedStaticFilesStorage
@@ -59,6 +59,14 @@ class PathNotImplementedStorage(storage.Storage):
         raise NotImplementedError
 
 
+class NeverCopyRemoteStorage(PathNotImplementedStorage):
+    """
+    Return a future modified time for all files so that nothing is collected.
+    """
+    def get_modified_time(self, name):
+        return datetime.now() + timedelta(days=30)
+
+
 class SimpleCachedStaticFilesStorage(CachedStaticFilesStorage):
 
     def file_hash(self, name, content=None):

tests/staticfiles_tests/test_management.py

@@ -337,6 +337,23 @@ class TestCollectionNonLocalStorage(TestNoFilesCreated, CollectionTestCase):
     pass
 
 
+class TestCollectionNeverCopyStorage(CollectionTestCase):
+
+    @override_settings(STATICFILES_STORAGE='staticfiles_tests.storage.NeverCopyRemoteStorage')
+    def test_skips_newer_files_in_remote_storage(self):
+        """
+        collectstatic skips newer files in a remote storage.
+        run_collectstatic() in setUp() copies the static files, then files are
+        always skipped after NeverCopyRemoteStorage is activated since
+        NeverCopyRemoteStorage.get_modified_time() returns a datetime in the
+        future to simulate an unmodified file.
+        """
+        stdout = six.StringIO()
+        self.run_collectstatic(stdout=stdout, verbosity=2)
+        output = force_text(stdout.getvalue())
+        self.assertIn("Skipping 'test.txt' (not modified)", output)
+
+
 @unittest.skipUnless(symlinks_supported(), "Must be able to symlink to run this test.")
 class TestCollectionLinks(TestDefaults, CollectionTestCase):
     """