Can't override the same CSS files multiple times

The root cause of this is that uasort() behavior has changed in PHP 7, which is called in drupal_get_css(). When 2 weights are equal, then we start to see different results in PHP 5 and PHP 7.

uasort() WTF: https://3v4l.org/mYVu1

I think the fix is the correct approach, because we want to make the weight unique in any case in druapl_add_css().

1. +++ b/includes/common.inc
   @@ -3025,6 +3025,7 @@ function drupal_add_html_head_link($attributes, $header = FALSE) {
      $css = &drupal_static(__FUNCTION__, array());
   +  $count = &drupal_static(__FUNCTION__ . '_count', 0);
   

   We need to reset this counter every time drupal_add_css is reset. Please search the code base where that happens, one instance is in CascadingStylesheetsTestCase for example.

2. +++ b/includes/common.inc
   @@ -3068,11 +3069,13 @@ function drupal_add_css($data = NULL, $options = NULL) {
            $css[] = $options;
   +        $count++;
            break;
          default:
            // Local and external files must keep their name as the associative key
            // so the same CSS file is not be added twice.
            $css[$data] = $options;
   +        $count++;
   

   no need to have this in 2 places, just add it right under the call where you use $count.

Better idea: instead requiring drupal_static_reset('drupal_add_css_count') everywhere in core and contrib we can reset the count whenever count($css) is 0. When there are zero CSS files added then we can also start from zero with the counter.

This is what I have in mind.

This is a bigger core bug in itself, the count($css) is totally wrong, so I am fine with the solution here.

Note: There is an issue to make uasort a nice helper that always works the same.

I checked that we did not have to fix this in Drupal 8 as _drupal_add_css is just called once in the end.

Committed to 7.x - thanks!

I'm not totally convinced this doesn't need to be fixed in Drupal 8... but at least it doesn't break tests there. And for Drupal 7 (and perhaps Drupal 8 too) I think we need a similar fix for drupal_add_js(). But since this is the only one that is causing test failures, committing it to make progress and leaving the rest to #2380655: Make CSS/JS order with the same weight deterministic (or another issue?) seems fine to me.

I changed the code comment on commit, to make it more like the comment for an existing static variable in drupal_add_js(), and to hopefully better explain why we need this:

diff --git a/includes/common.inc b/includes/common.inc
index 442e0f3..f7180f4 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -3036,7 +3036,8 @@ function drupal_add_css($data = NULL, $options = NULL) {
   $css = &drupal_static(__FUNCTION__, array());
   $count = &drupal_static(__FUNCTION__ . '_count', 0);
 
-  // Reset the order counter every time the number of entries in $css is 0.
+  // If the $css variable has been reset with drupal_static_reset(), there is
+  // no longer any CSS being tracked, so set the counter back to 0 also.
   if (count($css) === 0) {
     $count = 0;
   }