Skip inlining CSS for content with over 3k tags

Email content with too many tags can cause performance issues during the
inlining process. Typically, emails are not expected to contain tags more
than 1k.

Author: sunnavy

lib/RT/Util.pm

@@ -294,6 +294,7 @@ For html CONTENT, convert CSS <style> blocks to inline styles.
 =cut
 
 our $INLINE_CSS_MAX_SIZE = 1024 * 1024;
+our $INLINE_CSS_MAX_TAGS = 3_000;
 
 sub InlineCSS {
     my $content = shift // return;
@@ -308,6 +309,16 @@ sub InlineCSS {
         return $content;
     }
 
+    if ($INLINE_CSS_MAX_TAGS) {
+        my $tags;
+        while ( $content =~ /<\w+/g ) {
+            if ( ++$tags > $INLINE_CSS_MAX_TAGS ) {
+                RT->Logger->debug("Content exceeds max tag limit ($INLINE_CSS_MAX_TAGS), skipping");
+                return $content;
+            }
+        }
+    }
+
     # HTML::Query generates a ton of warnings about unsupported
     # pseudoclasses. Suppress those since they don't help the person
     # running RT.