gecko-dev/browser/modules/ContentMetaHandler.jsm

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";
Cu.importGlobalProperties(["URL"]);
ChromeUtils.import("resource://gre/modules/Services.jsm");

// Debounce time in milliseconds - this should be long enough to account for
// sync script tags that could appear between desired meta tags
const TIMEOUT_DELAY = 1000;

const ACCEPTED_PROTOCOLS = ["http:", "https:"];

// Possible description tags, listed in order from least favourable to most favourable
const DESCRIPTION_RULES = [
  "twitter:description",
  "description",
  "og:description"
];

// Possible image tags, listed in order from least favourable to most favourable
const PREVIEW_IMAGE_RULES = [
  "thumbnail",
  "twitter:image",
  "og:image",
  "og:image:url",
  "og:image:secure_url"
];

/*
 * Checks if the incoming meta tag has a greater score than the current best
 * score by checking the index of the meta tag in the list of rules provided.
 *
 * @param {Array} aRules
 *          The list of rules for a given type of meta tag
 * @param {String} aTag
 *          The name or property of the incoming meta tag
 * @param {String} aEntry
 *          The current best entry for the given meta tag
 *
 * @returns {Boolean} true if the incoming meta tag is better than the current
 *                    best meta tag of that same kind, false otherwise
 */
function shouldExtractMetadata(aRules, aTag, aEntry) {
  return aRules.indexOf(aTag) > aEntry.currMaxScore;
}

/*
 * Ensure that the preview image URL is safe and valid before storing
 *
 * @param {URL} aURL
 *          A URL object that needs to be checked for valid principal and protocol
 *
 * @returns {Boolean} true if the preview URL is safe and can be stored, false otherwise
 */
function checkLoadURIStr(aURL) {
  if (!ACCEPTED_PROTOCOLS.includes(aURL.protocol)) {
    return false;
  }
  try {
    let ssm = Services.scriptSecurityManager;
    let principal = ssm.createNullPrincipal({});
    ssm.checkLoadURIStrWithPrincipal(principal, aURL.href, ssm.DISALLOW_INHERIT_PRINCIPAL);
  } catch (e) {
    return false;
  }
  return true;
}

var EXPORTED_SYMBOLS = [ "ContentMetaHandler" ];

/*
 * This listens to DOMMetaAdded events and collects relevant metadata about the
 * meta tag received. Then, it sends the metadata gathered from the meta tags
 * and the url of the page as it's payload to be inserted into moz_places.
 */

var ContentMetaHandler = {
  init(chromeGlobal) {
    // Store a locally-scoped (for this chromeGlobal) mapping of the best
    // description and preview image collected so far for a given URL
    const metaTags = new Map();
    chromeGlobal.addEventListener("DOMMetaAdded", event => {
      const metaTag = event.originalTarget;
      const window = metaTag.ownerGlobal;

      // If there's no meta tag, or we're in a sub-frame, ignore this
      if (!metaTag || !metaTag.ownerDocument || window != window.top) {
        return;
      }
      this.handleMetaTag(metaTag, chromeGlobal, metaTags);
    });
  },


  handleMetaTag(metaTag, chromeGlobal, metaTags) {
    const url = metaTag.ownerDocument.documentURI;

    let name = metaTag.name;
    let prop = metaTag.getAttributeNS(null, "property");
    if (!name && !prop) {
      return;
    }

    let tag = name || prop;

    const entry = metaTags.get(url) || {
      description: {value: null, currMaxScore: -1},
      image: {value: null, currMaxScore: -1},
      timeout: null
    };

    // Malformed meta tag - do not store it
    const content = metaTag.getAttributeNS(null, "content");
    if (!content) {
      return;
    }

    if (shouldExtractMetadata(DESCRIPTION_RULES, tag, entry.description)) {
      // Extract the description
      entry.description.value = content;
      entry.description.currMaxScore = DESCRIPTION_RULES.indexOf(tag);
    } else if (shouldExtractMetadata(PREVIEW_IMAGE_RULES, tag, entry.image)) {
      // Extract the preview image
      let value;
      try {
        value = new URL(content, url);
      } catch (e) {
        return;
      }
      if (value && checkLoadURIStr(value)) {
        entry.image.value = value.href;
        entry.image.currMaxScore = PREVIEW_IMAGE_RULES.indexOf(tag);
      }
    } else {
      // We don't care about other meta tags
      return;
    }

    if (!metaTags.has(url)) {
      metaTags.set(url, entry);
    }

    if (entry.timeout) {
      entry.timeout.delay = TIMEOUT_DELAY;
    } else {
      // We want to debounce incoming meta tags until we're certain we have the
      // best one for description and preview image, and only store that one
      entry.timeout = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
      entry.timeout.initWithCallback(() => {
        entry.timeout = null;

        // Save description and preview image to moz_places
        chromeGlobal.sendAsyncMessage("Meta:SetPageInfo", {
          url,
          description: entry.description.value,
          previewImageURL: entry.image.value
        });

        // Telemetry for recording the size of page metadata
        let metadataSize = entry.description.value ? entry.description.value.length : 0;
        metadataSize += entry.image.value ? entry.image.value.length : 0;
        Services.telemetry.getHistogramById("PAGE_METADATA_SIZE").add(metadataSize);
        metaTags.delete(url);
      }, TIMEOUT_DELAY, Ci.nsITimer.TYPE_ONE_SHOT);
    }
  }
};