Log missing CSS assets in CSS Optimizer

This comment was added in #927406: Colorable themes fail when CSS aggregation is enabled due to improper stream wrapper usage → but then I looked at the issue and the error suppression was replacing a file_exists(), which was added in #901062: Regression: Themes can no longer remove stylesheets by overriding them in .info files → . I really wonder whether we even support that feature at all? Even if we do, we could deprecate it - stylesheets_remove exists.

Given the uncertainty about what's supported or not in #2, I think maybe we could add this behind an opt-in behavior to allow for a graceful path to deprecation?

I'm sure that a lot of sites out there have missing assets that's considered very, very normal.

Ideally when we know that a corrupted asset is going to be generated, we'd want to issue an HTTP/500 to prevent the asset from becoming cached by downstream infrastructure, but that definitely can't happen without breaking huge swathes of sites.

Here's something for us to run temporarily in the new year...

diff --git a/core/assets/scaffold/files/default.settings.php b/core/assets/scaffold/files/default.settings.php
index cd364bb00d..751a5de5ff 100644
--- a/core/assets/scaffold/files/default.settings.php
+++ b/core/assets/scaffold/files/default.settings.php
@@ -811,6 +811,14 @@
  */
 $settings['migrate_node_migrate_type_classic'] = FALSE;
 
+/**
+ * Aggregated asset error logging.
+ *
+ * This is used to inform site owners when CSS or JS assets fail to be included
+ * in an aggregated asset.
+ */
+$settings['log_aggregated_asset_errors'] = FALSE;
+
 /**
  * The default settings for migration sources.
  *
diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index 8f2d910250..61d00ea129 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -5,6 +5,7 @@
 use Drupal\Component\Utility\Unicode;
 use Drupal\Core\StreamWrapper\StreamWrapperManager;
 use Drupal\Core\File\FileUrlGeneratorInterface;
+use Drupal\Core\Site\Settings;
 
 /**
  * Optimizes a CSS asset.
@@ -145,7 +146,11 @@ public function loadFile($file, $optimize = NULL, $reset_base_path = TRUE) {
     // stylesheets in their .info.yml file that don't exist in the theme's path,
     // but are merely there to disable certain module CSS files.
     $content = '';
-    if ($contents = @file_get_contents($file)) {
+
+    // Temporary hack for debugging impossible to diagnose aggregation issues...
+    $contents = Settings::get('log_aggregated_asset_errors') ? file_get_contents($file) : @file_get_contents($file);
+
+    if ($contents !== FALSE) {
       // If a BOM is found, convert the file to UTF-8, then use substr() to
       // remove the BOM from the result.
       if ($encoding = (Unicode::encodingFromBOM($contents))) {

Unrelated - there should probably be a strict comparison against the file_get_contents result here -- otherwise some false positives can slip through depending on the file contents 😱!