fix(parser) Fix freezing issue with illegal 0 width matches (#2524)

* fix[parser] add edge case handle for illegal 0 width matches
* add last ditch catch all that tries to detect other uncaught freezes

CHANGES.md

@@ -37,10 +37,9 @@ Brower build:
 - [Issue](https://github.com/highlightjs/highlight.js/issues/2505) (bug) Fix: Version 10 fails to load as CommonJS module. (#2511) [Josh Goebel][]
 - [Issue](https://github.com/highlightjs/highlight.js/issues/2505) (removal) AMD module loading support has been removed. (#2511) [Josh Goebel][]
 
-
 Parser Engine Changes:
 
-- ...
+- [Issue](https://github.com/highlightjs/highlight.js/issues/2522) fix(parser) Fix freez issue with illegal 0 width matches (#2524) [Josh Goebel][]
 
 
 [Josh Goebel]: https://github.com/yyyc514

src/highlight.js

@@ -365,6 +365,23 @@ const HLJS = function(hljs) {
         }
       }
 
+      // edge case for when illegal matches $ (end of line) which is technically
+      // a 0 width match but not a begin/end match so it's not caught by the
+      // first handler (when ignoreIllegals is true)
+      if (match.type === "illegal" && lexeme === "") {
+        // advance so we aren't stuck in an infinite loop
+        return 1;
+      }
+
+      // infinite loops are BAD, this is a last ditch catch all. if we have a
+      // decent number of iterations yet our index (cursor position in our
+      // parsing) still 3x behind our index then something is very wrong
+      // so we bail
+      if (iterations > 100000 && iterations > match.index * 3) {
+        const err = new Error('potential infinite loop, way more iterations than matches');
+        throw err;
+      }
+
       /*
       Why might be find ourselves here?  Only one occasion now.  An end match that was
       triggered but could not be completed.  When might this happen?  When an `endSameasBegin`
@@ -396,12 +413,14 @@ const HLJS = function(hljs) {
     var mode_buffer = '';
     var relevance = 0;
     var index = 0;
+    var iterations = 0;
     var continueScanAtSamePosition = false;
 
     try {
       top.matcher.considerAll();
 
       for (;;) {
+        iterations++;
         if (continueScanAtSamePosition) {
           continueScanAtSamePosition = false;
           // only regexes not matched previously will now be