Improve prevent-fetch scriptlet

Related issue:
uBlockOrigin/uBlock-issues#2526

Improvements:

Support fulfilling the response with the content of a
`web_accessible_resources` resource, using the syntax already
supported by `prevent-xhr`: `war:[name of resource]`

Support fulfilling the response with randomized text with length
specified using `length:min[-max]` directive.

assets/resources/scriptlets.js

@@ -52,6 +52,8 @@ function safeSelf() {
  'Function_toStringFn': self.Function.prototype.toString,
  'Function_toString': thisArg => safe.Function_toStringFn.call(thisArg),
  'Math_floor': Math.floor,
+ 'Math_max': Math.max,
+ 'Math_min': Math.min,
  'Math_random': Math.random,
  'Object_defineProperty': Object.defineProperty.bind(Object),
  'RegExp': self.RegExp,
@@ -242,6 +244,64 @@ function runAtHtmlElementFn(fn) {
 
 /******************************************************************************/
 
+// Reference:
+// https://github.com/AdguardTeam/Scriptlets/blob/master/wiki/about-scriptlets.md#prevent-xhr
+
+builtinScriptlets.push({
+ name: 'generate-content.fn',
+ fn: generateContentFn,
+ dependencies: [
+ 'safe-self.fn',
+ ],
+});
+function generateContentFn(directive) {
+ const safe = safeSelf();
+ const randomize = len => {
+ const chunks = [];
+ let textSize = 0;
+ do {
+ const s = safe.Math_random().toString(36).slice(2);
+ chunks.push(s);
+ textSize += s.length;
+ }
+ while ( textSize < len );
+ return chunks.join(' ').slice(0, len);
+ };
+ if ( directive === 'true' ) {
+ return Promise.resolve(randomize(10));
+ }
+ if ( directive.startsWith('length:') ) {
+ const match = /^length:(\d+)(?:-(\d+))?$/.exec(directive);
+ if ( match ) {
+ const min = parseInt(match[1], 10);
+ const extent = safe.Math_max(parseInt(match[2], 10) || 0, min) - min;
+ const len = safe.Math_min(min + extent * safe.Math_random(), 500000);
+ return Promise.resolve(randomize(len | 0));
+ }
+ }
+ if ( directive.startsWith('war:') && scriptletGlobals.has('warOrigin') ) {
+ return new Promise(resolve => {
+ const warOrigin = scriptletGlobals.get('warOrigin');
+ const warName = directive.slice(4);
+ const fullpath = [ warOrigin, '/', warName ];
+ const warSecret = scriptletGlobals.get('warSecret');
+ if ( warSecret !== undefined ) {
+ fullpath.push('?secret=', warSecret);
+ }
+ const warXHR = new safe.XMLHttpRequest();
+ warXHR.responseType = 'text';
+ warXHR.onloadend = ev => {
+ resolve(ev.target.responseText || '');
+ };
+ warXHR.open('GET', fullpath.join(''));
+ warXHR.send();
+ });
+ }
+ return Promise.resolve('');
+}
+
+/******************************************************************************/
+
 builtinScriptlets.push({
  name: 'abort-current-script-core.fn',
  fn: abortCurrentScriptCore,
@@ -1757,16 +1817,18 @@ builtinScriptlets.push({
  ],
  fn: noFetchIf,
  dependencies: [
+ 'generate-content.fn',
  'safe-self.fn',
  ],
 });
 function noFetchIf(
- arg1 = '',
+ propsToMatch = '',
+ directive = ''
 ) {
- if ( typeof arg1 !== 'string' ) { return; }
+ if ( typeof propsToMatch !== 'string' ) { return; }
  const safe = safeSelf();
  const needles = [];
- for ( const condition of arg1.split(/\s+/) ) {
+ for ( const condition of propsToMatch.split(/\s+/) ) {
  if ( condition === '' ) { continue; }
  const pos = condition.indexOf(':');
  let key, value;
@@ -1782,14 +1844,11 @@ function noFetchIf(
  const log = needles.length === 0 ? console.log.bind(console) : undefined;
  self.fetch = new Proxy(self.fetch, {
  apply: function(target, thisArg, args) {
+ const details = args[0] instanceof self.Request
+ ? args[0]
+ : Object.assign({ url: args[0] }, args[1]);
  let proceed = true;
  try {
- let details;
- if ( args[0] instanceof self.Request ) {
- details = args[0];
- } else {
- details = Object.assign({ url: args[0] }, args[1]);
- }
  const props = new Map();
  for ( const prop in details ) {
  let v = details[prop];
@@ -1818,9 +1877,21 @@ function noFetchIf(
  }
  } catch(ex) {
  }
- return proceed
- ? Reflect.apply(target, thisArg, args)
- : Promise.resolve(new Response());
+ if ( proceed ) {
+ return Reflect.apply(target, thisArg, args);
+ }
+ return generateContentFn(directive).then(text => {
+ const response = new Response(text, {
+ statusText: 'OK',
+ headers: {
+ 'Content-Length': text.length,
+ }
+ });
+ Object.defineProperty(response, 'url', {
+ value: details.url
+ });
+ return response;
+ });
  }
  });
 }
@@ -2259,6 +2330,7 @@ builtinScriptlets.push({
  ],
  fn: noXhrIf,
  dependencies: [
+ 'generate-content.fn',
  'match-object-properties.fn',
  'parse-properties-to-match.fn',
  'safe-self.fn',
@@ -2269,41 +2341,10 @@ function noXhrIf(
  directive = ''
 ) {
  if ( typeof propsToMatch !== 'string' ) { return; }
- const safe = safeSelf();
  const xhrInstances = new WeakMap();
  const propNeedles = parsePropertiesToMatch(propsToMatch, 'url');
  const log = propNeedles.size === 0 ? console.log.bind(console) : undefined;
  const warOrigin = scriptletGlobals.get('warOrigin');
- const generateRandomString = len => {
- let s = '';
- do { s += safe.Math_random().toString(36).slice(2); }
- while ( s.length < 10 );
- return s.slice(0, len);
- };
- const generateContent = async directive => {
- if ( directive === 'true' ) {
- return generateRandomString(10);
- }
- if ( directive.startsWith('war:') ) {
- if ( warOrigin === undefined ) { return ''; }
- return new Promise(resolve => {
- const warName = directive.slice(4);
- const fullpath = [ warOrigin, '/', warName ];
- const warSecret = scriptletGlobals.get('warSecret');
- if ( warSecret !== undefined ) {
- fullpath.push('?secret=', warSecret);
- }
- const warXHR = new safe.XMLHttpRequest();
- warXHR.responseType = 'text';
- warXHR.onloadend = ev => {
- resolve(ev.target.responseText || '');
- };
- warXHR.open('GET', fullpath.join(''));
- warXHR.send();
- });
- }
- return '';
- };
  self.XMLHttpRequest = class extends self.XMLHttpRequest {
  open(method, url, ...args) {
  if ( log !== undefined ) {
@@ -2370,7 +2411,7 @@ function noXhrIf(
  default:
  if ( directive === '' ) { break; }
  promise = promise.then(details => {
- return generateContent(details.directive).then(text => {
+ return generateContentFn(details.directive).then(text => {
  details.props.response.value = text;
  details.props.responseText.value = text;
  return details;