Ensure CSSTree does not hold onto last parsed string

When done compiling, force CSSTree to parse an empty string, so
as to ensure it doesn't keep a reference to that string.

Typically, the string passed to CSSTree is a small slice of a
larger string which is a whole filter list. This means that
holding a reference to the sliced string causes the JS engine
to hold in memory to the whole filter list last parsed.

src/js/static-filtering-parser.js

@@ -908,6 +908,10 @@ export class AstFilterParser {
  this.scriptletArgListParser = new ArgListParser(',');
  }
 
+ finish() {
+ this.selectorCompiler.finish();
+ }
+
  parse(raw) {
  this.raw = raw;
  this.rawEnd = raw.length;
@@ -3229,6 +3233,14 @@ class ExtSelectorCompiler {
  this.error = undefined;
  }
 
+ // CSSTree library holds onto last string parsed, and this is problematic
+ // when the string is a slice of a huge parent string (typically a whole
+ // filter lists), it causes the huge parent string to stay in memory.
+ // Asking CSSTree to parse an empty string resolves this issue.
+ finish() {
+ cssTree.parse('');
+ }
+
  compile(raw, out, compileOptions = {}) {
  this.asProcedural = compileOptions.asProcedural === true;
 

src/js/storage.js

@@ -1102,6 +1102,7 @@ import {
  }
 
  compiler.finish(writer);
+ parser.finish();
 
  // https://github.com/uBlockOrigin/uBlock-issues/issues/1365
  // Embed version into compiled list itself: it is encoded in as the