atomicc.rb: Don't assume we may chown/chmod a file.

Previously this code just assumed it is capable of changing the file
ownership, both user and group. This will fail in a lot of scenario's
unless:
* The process is run as a superuser (root);
* The owning user and group are already set to the user and group we're
  trying to chown to;
* The user chown'ing only changes the group to another group it is a
  member of.
If either of those conditions are not met the filesystem will simply
deny the operation throwing an error.

It is also not always possible to do a chmod, there might be a SELinux
policy or another limitation preventing the user to change the file
mode. To this end the chmod call has also been added to the rescue
block.

I've also added a little comment above the chmod command that doing a
chmod on a file which has an ACL set will cause the ACL to be
recalculated / modified.

1 files changed, 7 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/core_ext/file/atomic.rb b/activesupport/lib/active_support/core_ext/file/atomic.rb
index 38ea7f8fb3..c3e6124a57 100644
--- a/activesupport/lib/active_support/core_ext/file/atomic.rb
+++ b/activesupport/lib/active_support/core_ext/file/atomic.rb
@@ -36,8 +36,13 @@ class File
     FileUtils.mv(temp_file.path, file_name)
 
     # Set correct permissions on new file
-    chown(old_stat.uid, old_stat.gid, file_name)
-    chmod(old_stat.mode, file_name)
+    begin
+      chown(old_stat.uid, old_stat.gid, file_name)
+      # This operation will affect filesystem ACL's
+      chmod(old_stat.mode, file_name)
+    rescue Errno::EPERM
+      # Changing file ownership failed, moving on.
+    end
   end
 
   # Private utility method.