Add ability for scriptlets to share local data

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/1741

As a result of the new capability, usage of RegExp API in `aost`
scriptlet has been shielded from the webpage tampering with the
API.

assets/resources/scriptlets.js

@@ -22,6 +22,9 @@
     web page context.
 */
 
+// Externally added to the private namespace in which scriptlets execute.
+/* global sriptletGlobals */
+
 'use strict';
 
 export const builtinScriptlets = [];
@@ -34,6 +37,25 @@ export const builtinScriptlets = [];
 
 *******************************************************************************/
 
+builtinScriptlets.push({
+    name: 'safe-self.fn',
+    fn: safeSelf,
+});
+function safeSelf() {
+    if ( sriptletGlobals.has('safeSelf') ) {
+        return sriptletGlobals.get('safeSelf');
+    }
+    const safe = {
+        'RegExp': self.RegExp,
+        'RegExp_test': self.RegExp.prototype.test,
+        'RegExp_exec': self.RegExp.prototype.exec,
+    };
+    sriptletGlobals.set('safeSelf', safe);
+    return safe;
+}
+
+/******************************************************************************/
+
 builtinScriptlets.push({
     name: 'pattern-to-regex.fn',
     fn: patternToRegex,
@@ -257,6 +279,7 @@ builtinScriptlets.push({
     aliases: [ 'aost.js' ],
     fn: abortOnStackTrace,
     dependencies: [
+        'safe-self.fn',
         'pattern-to-regex.fn',
         'get-exception-token.fn',
     ],
@@ -268,6 +291,7 @@ function abortOnStackTrace(
     logLevel = ''
 ) {
     if ( typeof chain !== 'string' ) { return; }
+    const safe = safeSelf();
     const reNeedle = patternToRegex(needle);
     const exceptionToken = getExceptionToken();
     const log = console.log.bind(console);
@@ -279,11 +303,12 @@ function abortOnStackTrace(
             docURL = docURL.slice(0, pos);
         }
         // Normalize stack trace
+        const reLine = /(.*?@)?(\S+)(:\d+):\d+\)?$/;
         const lines = [];
         for ( let line of err.stack.split(/[\n\r]+/) ) {
             if ( line.includes(exceptionToken) ) { continue; }
             line = line.trim();
-            let match = /(.*?@)?(\S+)(:\d+):\d+\)?$/.exec(line);
+            let match = safe.RegExp_exec.call(reLine, line);
             if ( match === null ) { continue; }
             let url = match[2];
             if ( url.startsWith('(') ) { url = url.slice(1); }
@@ -301,7 +326,7 @@ function abortOnStackTrace(
         }
         lines[0] = `stackDepth:${lines.length-1}`;
         const stack = lines.join('\t');
-        const r = reNeedle.test(stack);
+        const r = safe.RegExp_test.call(reNeedle, stack);
         if (
             logLevel === '1' ||
             logLevel === '2' && r ||

src/js/scriptlet-filtering.js

@@ -379,22 +379,20 @@ scriptletFilteringEngine.retrieve = function(request, options = {}) {
 
     if ( cacheDetails.code === '' ) { return; }
 
-    const out = [ cacheDetails.code ];
-
-    if ( µb.hiddenSettings.debugScriptlets ) {
-        out.unshift('debugger;');
-    }
-
-    out.unshift(
+    const out = [
         '(function() {',
         '// >>>> start of private namespace',
-        ''
-    );
-    out.push(
+        '',
+        µb.hiddenSettings.debugScriptlets ? 'debugger;' : ';',
+        '',
+        // For use by scriptlets to share local data among themselves
+        'const sriptletGlobals = new Map();',
+        '',
+        cacheDetails.code,
         '',
         '// <<<< end of private namespace',
-        '})();'
-    );
+        '})();',
+    ];
 
     return out.join('\n');
 };