Prevent serving JS/CSS files if they have a filesize of zero

Wouldn't it be better to add proper error handling to file_unmanaged_save_data(), i.e. if it fails to save the file, it should remove the empty file and return FALSE?

-  return file_unmanaged_move($temp_name, $destination, $replace);
+  $path = file_unmanaged_move($temp_name, $destination, $replace);
+
+  // Make sure everything was moved correctly
+  if (strlen($data) > 0 && @filesize($path) == 0) {

If writing the file fails, file_unmanaged_move() is supposed to return FALSE. Shouldn't you just check for that?

So, this still seems to be in the same state in 8.x

Patch no longer applies.

The patch in #9 doesn't just affect serving JS/CSS files, it is a blanket modification to file_unmanaged_save_data(). It's unclear from the issue summary and subsequent comments whether the intention is to do this or just handle JS/CSS - if the former, we should be checking the size of $data before doing anything else and bail early if appropriate.

This patch needs tests.

The referenced issue #818818: Race Condition when using file_save_data FILE_EXISTS_REPLACE is closed as a duplicate #1377740: file_unmanaged_move() should issue rename() where possible instead of copy() & unlink(), which doesn't seem to be handling zero size files.

Review bonus #2094585: [policy, no patch] Core review bonus for #1898420: image.module - Convert theme_ functions to Twig.

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Per #755586-46: Fallback for CSS/JS aggregation for non-writable directories, moving back to Drupal 7.