Disable default syntax highlighting for large files in DocumentEngine

Summary: Ref T13105. See also T7895. When users render very large files as source via DocumentEngine, skip highlighting.

Test Plan: Fiddled with the limit, viewed files, saw highlighting degrade.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19306

src/applications/files/document/PhabricatorSourceDocumentEngine.php

@@ -24,18 +24,31 @@ final class PhabricatorSourceDocumentEngine
   protected function newDocumentContent(PhabricatorDocumentRef $ref) {
     $content = $this->loadTextData($ref);
 
+    $messages = array();
+
     $highlighting = $this->getHighlightingConfiguration();
     if ($highlighting !== null) {
       $content = PhabricatorSyntaxHighlighter::highlightWithLanguage(
         $highlighting,
         $content);
+    } else {
+      $highlight_limit = DifferentialChangesetParser::HIGHLIGHT_BYTE_LIMIT;
+      if (strlen($content) > $highlight_limit) {
+        $messages[] = $this->newMessage(
+          pht(
+            'This file is larger than %s, so syntax highlighting was skipped.',
+            phutil_format_bytes($highlight_limit)));
       } else {
         $content = PhabricatorSyntaxHighlighter::highlightWithFilename(
           $ref->getName(),
           $content);
       }
+    }
 
-    return $this->newTextDocumentContent($content);
+    return array(
+      $messages,
+      $this->newTextDocumentContent($content),
+    );
   }
 
 }