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diff --git a/background/ResponseProcessor.mjs b/background/ResponseProcessor.mjs
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+++ b/background/ResponseProcessor.mjs
@@ -0,0 +1,145 @@
+/**
+* GNU LibreJS - A browser add-on to block nonfree nontrivial JavaScript.
+*
+* Copyright (C) 2018 Giorgio Maone <giorgio@maone.net>
+*
+* This file is part of GNU LibreJS.
+*
+* GNU LibreJS is free software: you can redistribute it and/or modify
+* it under the terms of the GNU General Public License as published by
+* the Free Software Foundation, either version 3 of the License, or
+* (at your option) any later version.
+*
+* GNU LibreJS is distributed in the hope that it will be useful,
+* but WITHOUT ANY WARRANTY; without even the implied warranty of
+* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+* GNU General Public License for more details.
+*
+* You should have received a copy of the GNU General Public License
+* along with GNU LibreJS.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+/**
+  An abstraction layer over the StreamFilter API, allowing its clients to process
+  only the "interesting" HTML and script requests and leaving the other alone
+*/
+
+import ResponseMetaData from './ResponseMetaData.mjs';
+import browser from '/common/browser.mjs';
+
+let listeners = new WeakMap();
+let webRequestEvent = browser.webRequest.onHeadersReceived;
+
+class ResponseProcessor {
+
+  static install(handler, types = ["main_frame", "sub_frame"]) {
+    if (listeners.has(handler)) return false;
+    let listener =
+      async request => await new ResponseTextFilter(request).process(handler);
+    listeners.set(handler, listener);
+    webRequestEvent.addListener(
+  		listener,
+  		{urls: ["<all_urls>"], types},
+  		["blocking", "responseHeaders"]
+  	);
+    return true;
+  }
+
+  static uninstall(handler) {
+    let listener = listeners.get(handler);
+    if (listener) {
+      webRequestEvent.removeListener(listener);
+    }
+  }
+}
+
+Object.assign(ResponseProcessor, {
+  // control flow values to be returned by handler.pre() callbacks
+	ACCEPT: {},
+	REJECT: {cancel: true},
+	CONTINUE: null
+});
+
+class ResponseTextFilter {
+  constructor(request) {
+    this.request = request;
+    let {type, statusCode} = request;
+    let md = this.metaData = new ResponseMetaData(request);
+    this.canProcess = // we want to process html documents and scripts only
+      (statusCode < 300 || statusCode >= 400) && // skip redirections
+      !md.disposition && // skip forced downloads
+      (type === "script" || /\bhtml\b/i.test(md.contentType));
+  }
+
+  async process(handler) {
+    if (!this.canProcess) return ResponseProcessor.ACCEPT;
+    let {metaData, request} = this;
+    let response = {request, metaData}; // we keep it around allowing callbacks to store state
+    if (typeof handler.pre === "function") {
+      let res = await handler.pre(response);
+      if (res) return res;
+      if (handler.post) handler = handler.post;
+      if (typeof handler !== "function") return ResponseProcessor.ACCEPT;
+    }
+
+    return ResponseProcessor.ACCEPT;  
+
+    let {requestId, responseHeaders} = request;
+    let filter = browser.webRequest.filterResponseData(requestId);
+    let buffer = [];
+
+    filter.ondata = event => {
+      buffer.push(event.data);
+    };
+
+    filter.onstop = async event => {
+      // concatenate chunks
+      let size = buffer.reduce((sum, chunk, n) => sum + chunk.byteLength, 0)
+      let allBytes = new Uint8Array(size);
+      let pos = 0;
+      for (let chunk of buffer) {
+        allBytes.set(new Uint8Array(chunk), pos);
+        pos += chunk.byteLength;
+      }
+      buffer = null; // allow garbage collection
+      if (allBytes.indexOf(0) !== -1) {
+        console.debug("Warning: zeroes in bytestream, probable cached encoding mismatch.", request);
+        if (request.type === "script") {
+          console.debug("It's a script, trying to refetch it.");
+          response.text = await (await fetch(request.url, {cache: "reload", credentials: "include"})).text();
+        } else {
+          console.debug("It's a %s, trying to decode it as UTF-16.", request.type);
+          response.text = new TextDecoder("utf-16be").decode(allBytes, {stream: true});
+        }
+      } else {
+        response.text = metaData.decode(allBytes);
+      }
+      let editedText = null;
+      try {
+        editedText = await handler(response);
+      } catch(e) {
+        console.error(e);
+      }
+      if (editedText !== null) {
+        // we changed the content, let's re-encode
+        let encoded = new TextEncoder().encode(editedText);
+        // pre-pending the UTF-8 BOM will force the charset per HTML 5 specs
+        allBytes = new Uint8Array(encoded.byteLength + 3);
+        allBytes.set(ResponseMetaData.UTF8BOM, 0); // UTF-8 BOM
+        allBytes.set(encoded, 3);
+      }
+      filter.write(allBytes);
+      filter.close();
+    }
+
+    return ResponseProcessor.ACCEPT;
+  }
+}
+
+/* The following was originally in Storage.js */
+function url_item(url) {
+    let queryPos = url.indexOf("?");
+    return queryPos === -1 ? url : url.substring(0, queryPos);
+}
+
+export {ResponseProcessor, url_item};