The Tor Browser: uriloader/exthandler/nsHandlerService.js@6474c204b198 (annotated)

uriloader/exthandler/nsHandlerService.js@6474c204b198 (annotated)

uriloader/exthandler/nsHandlerService.js

Wed, 31 Dec 2014 06:09:35 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 06:09:35 +0100
changeset 0: 6474c204b198
permissions: -rw-r--r--

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

 /* 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/. */
 const Ci = Components.interfaces;
 const Cc = Components.classes;
 const Cu = Components.utils;
 const Cr = Components.results;
 const CLASS_MIMEINFO        = "mimetype";
 const CLASS_PROTOCOLINFO    = "scheme";
 // namespace prefix
 const NC_NS                 = "http://home.netscape.com/NC-rdf#";
 // the most recent default handlers that have been injected.  Note that
 // this is used to construct an RDF resource, which needs to have NC_NS
 // prepended, since that hasn't been done yet
 const DEFAULT_HANDLERS_VERSION = "defaultHandlersVersion";
 // type list properties
 const NC_MIME_TYPES         = NC_NS + "MIME-types";
 const NC_PROTOCOL_SCHEMES   = NC_NS + "Protocol-Schemes";
 // content type ("type") properties
 // nsIHandlerInfo::type
 const NC_VALUE              = NC_NS + "value";
 const NC_DESCRIPTION        = NC_NS + "description";
 // additional extensions
 const NC_FILE_EXTENSIONS    = NC_NS + "fileExtensions";
 // references nsIHandlerInfo record
 const NC_HANDLER_INFO       = NC_NS + "handlerProp";
 // handler info ("info") properties
 // nsIHandlerInfo::preferredAction
 const NC_SAVE_TO_DISK       = NC_NS + "saveToDisk";
 const NC_HANDLE_INTERNALLY  = NC_NS + "handleInternal";
 const NC_USE_SYSTEM_DEFAULT = NC_NS + "useSystemDefault";
 // nsIHandlerInfo::alwaysAskBeforeHandling
 const NC_ALWAYS_ASK         = NC_NS + "alwaysAsk";
 // references nsIHandlerApp records
 const NC_PREFERRED_APP      = NC_NS + "externalApplication";
 const NC_POSSIBLE_APP       = NC_NS + "possibleApplication";
 // handler app ("handler") properties
 // nsIHandlerApp::name
 const NC_PRETTY_NAME        = NC_NS + "prettyName";
 // nsILocalHandlerApp::executable
 const NC_PATH               = NC_NS + "path";
 // nsIWebHandlerApp::uriTemplate
 const NC_URI_TEMPLATE       = NC_NS + "uriTemplate";
 // nsIDBusHandlerApp::service
 const NC_SERVICE            = NC_NS + "service";
 // nsIDBusHandlerApp::method
 const NC_METHOD             = NC_NS + "method";
 // nsIDBusHandlerApp::objectPath
 const NC_OBJPATH            = NC_NS + "objectPath";
 // nsIDBusHandlerApp::dbusInterface
 const NC_INTERFACE            = NC_NS + "dBusInterface";
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 function HandlerService() {
   this._init();
 }
 const HandlerServiceFactory = {
   _instance: null,
   createInstance: function (outer, iid) {
     if (this._instance)
       return this._instance;
     let processType = Cc["@mozilla.org/xre/runtime;1"].
       getService(Ci.nsIXULRuntime).processType;
     if (processType != Ci.nsIXULRuntime.PROCESS_TYPE_DEFAULT)
       return Cr.NS_ERROR_NOT_IMPLEMENTED;
     return (this._instance = new HandlerService());
   }
 };
 HandlerService.prototype = {
   //**************************************************************************//
   // XPCOM Plumbing
   classID:          Components.ID("{32314cc8-22f7-4f7f-a645-1a45453ba6a6}"),
   QueryInterface:   XPCOMUtils.generateQI([Ci.nsIHandlerService]),
   _xpcom_factory: HandlerServiceFactory,
   //**************************************************************************//
   // Initialization & Destruction
   _init: function HS__init() {
     // Observe profile-before-change so we can switch to the datasource
     // in the new profile when the user changes profiles.
     this._observerSvc.addObserver(this, "profile-before-change", false);
     // Observe xpcom-shutdown so we can remove these observers
     // when the application shuts down.
     this._observerSvc.addObserver(this, "xpcom-shutdown", false);
     // Observe profile-do-change so that non-default profiles get upgraded too
     this._observerSvc.addObserver(this, "profile-do-change", false);
     // do any necessary updating of the datastore
     this._updateDB();
   },
   _updateDB: function HS__updateDB() {
     try {
       var defaultHandlersVersion = this._datastoreDefaultHandlersVersion;
     } catch(ex) {
       // accessing the datastore failed, we can't update anything
       return;
     }
     try {
       // if we don't have the current version of the default prefs for
       // this locale, inject any new default handers into the datastore
       if (defaultHandlersVersion < this._prefsDefaultHandlersVersion) {
         // set the new version first so that if we recurse we don't
         // call _injectNewDefaults several times
         this._datastoreDefaultHandlersVersion =
           this._prefsDefaultHandlersVersion;
         this._injectNewDefaults();
       }
     } catch (ex) {
       // if injecting the defaults failed, set the version back to the
       // previous value
       this._datastoreDefaultHandlersVersion = defaultHandlersVersion;
     }
   },
   get _currentLocale() {
     var chromeRegistry = Cc["@mozilla.org/chrome/chrome-registry;1"].
                          getService(Ci.nsIXULChromeRegistry);
     var currentLocale = chromeRegistry.getSelectedLocale("global");
     return currentLocale;
   },
   _destroy: function HS__destroy() {
     this._observerSvc.removeObserver(this, "profile-before-change");
     this._observerSvc.removeObserver(this, "xpcom-shutdown");
     this._observerSvc.removeObserver(this, "profile-do-change");
     // XXX Should we also null references to all the services that get stored
     // by our memoizing getters in the Convenience Getters section?
   },
   _onProfileChange: function HS__onProfileChange() {
     // Lose our reference to the datasource so we reacquire it
     // from the new profile the next time we need it.
     this.__ds = null;
   },
   _isInHandlerArray: function HS__isInHandlerArray(aArray, aHandler) {
     var enumerator = aArray.enumerate();
     while (enumerator.hasMoreElements()) {
       let handler = enumerator.getNext();
       handler.QueryInterface(Ci.nsIHandlerApp);
       if (handler.equals(aHandler))
         return true;
     }
     return false;
   },
   // note that this applies to the current locale only
   get _datastoreDefaultHandlersVersion() {
     var version = this._getValue("urn:root", NC_NS + this._currentLocale +
                                              "_" + DEFAULT_HANDLERS_VERSION);
     return version ? version : -1;
   },
   set _datastoreDefaultHandlersVersion(aNewVersion) {
     return this._setLiteral("urn:root", NC_NS + this._currentLocale + "_" +
                             DEFAULT_HANDLERS_VERSION, aNewVersion);
   },
   get _prefsDefaultHandlersVersion() {
     // get handler service pref branch
     var prefSvc = Cc["@mozilla.org/preferences-service;1"].
                   getService(Ci.nsIPrefService);
     var handlerSvcBranch = prefSvc.getBranch("gecko.handlerService.");
     // get the version of the preferences for this locale
     return Number(handlerSvcBranch.
                   getComplexValue("defaultHandlersVersion",
                                   Ci.nsIPrefLocalizedString).data);
   },
   _injectNewDefaults: function HS__injectNewDefaults() {
     // get handler service pref branch
     var prefSvc = Cc["@mozilla.org/preferences-service;1"].
                   getService(Ci.nsIPrefService);
     let schemesPrefBranch = prefSvc.getBranch("gecko.handlerService.schemes.");
     let schemePrefList = schemesPrefBranch.getChildList("");
     var schemes = {};
     // read all the scheme prefs into a hash
     for each (var schemePrefName in schemePrefList) {
       let [scheme, handlerNumber, attribute] = schemePrefName.split(".");
       try {
         var attrData =
           schemesPrefBranch.getComplexValue(schemePrefName,
                                             Ci.nsIPrefLocalizedString).data;
         if (!(scheme in schemes))
           schemes[scheme] = {};
         if (!(handlerNumber in schemes[scheme]))
           schemes[scheme][handlerNumber] = {};
         schemes[scheme][handlerNumber][attribute] = attrData;
       } catch (ex) {}
     }
     let protoSvc = Cc["@mozilla.org/uriloader/external-protocol-service;1"].
                    getService(Ci.nsIExternalProtocolService);
     for (var scheme in schemes) {
       // This clause is essentially a reimplementation of
       // nsIExternalProtocolHandlerService.getProtocolHandlerInfo().
       // Necessary because calling that from here would make XPConnect barf
       // when getService tried to re-enter the constructor for this
       // service.
       let osDefaultHandlerFound = {};
       let protoInfo = protoSvc.getProtocolHandlerInfoFromOS(scheme,
                                osDefaultHandlerFound);
       if (this.exists(protoInfo))
         this.fillHandlerInfo(protoInfo, null);
       else
         protoSvc.setProtocolHandlerDefaults(protoInfo,
                                             osDefaultHandlerFound.value);
       // cache the possible handlers to avoid extra xpconnect traversals.
       let possibleHandlers = protoInfo.possibleApplicationHandlers;
       for each (var handlerPrefs in schemes[scheme]) {
         let handlerApp = Cc["@mozilla.org/uriloader/web-handler-app;1"].
                          createInstance(Ci.nsIWebHandlerApp);
         handlerApp.uriTemplate = handlerPrefs.uriTemplate;
         handlerApp.name = handlerPrefs.name;
         if (!this._isInHandlerArray(possibleHandlers, handlerApp)) {
           possibleHandlers.appendElement(handlerApp, false);
         }
       }
       this.store(protoInfo);
     }
   },
   //**************************************************************************//
   // nsIObserver
   observe: function HS__observe(subject, topic, data) {
     switch(topic) {
       case "profile-before-change":
         this._onProfileChange();
         break;
       case "xpcom-shutdown":
         this._destroy();
         break;
       case "profile-do-change":
         this._updateDB();
         break;
     }
   },
   //**************************************************************************//
   // nsIHandlerService
   enumerate: function HS_enumerate() {
     var handlers = Cc["@mozilla.org/array;1"].
                    createInstance(Ci.nsIMutableArray);
     this._appendHandlers(handlers, CLASS_MIMEINFO);
     this._appendHandlers(handlers, CLASS_PROTOCOLINFO);
     return handlers.enumerate();
   },
   fillHandlerInfo: function HS_fillHandlerInfo(aHandlerInfo, aOverrideType) {
     var type = aOverrideType || aHandlerInfo.type;
     var typeID = this._getTypeID(this._getClass(aHandlerInfo), type);
     // Determine whether or not information about this handler is available
     // in the datastore by looking for its "value" property, which stores its
     // type and should always be present.
     if (!this._hasValue(typeID, NC_VALUE))
       throw Cr.NS_ERROR_NOT_AVAILABLE;
     // Retrieve the human-readable description of the type.
     if (this._hasValue(typeID, NC_DESCRIPTION))
       aHandlerInfo.description = this._getValue(typeID, NC_DESCRIPTION);
     // Note: for historical reasons, we don't actually check that the type
     // record has a "handlerProp" property referencing the info record.  It's
     // unclear whether or not we should start doing this check; perhaps some
     // legacy datasources don't have such references.
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), type);
     aHandlerInfo.preferredAction = this._retrievePreferredAction(infoID);
     var preferredHandlerID =
       this._getPreferredHandlerID(this._getClass(aHandlerInfo), type);
     // Retrieve the preferred handler.
     // Note: for historical reasons, we don't actually check that the info
     // record has an "externalApplication" property referencing the preferred
     // handler record.  It's unclear whether or not we should start doing
     // this check; perhaps some legacy datasources don't have such references.
     aHandlerInfo.preferredApplicationHandler =
       this._retrieveHandlerApp(preferredHandlerID);
     // Fill the array of possible handlers with the ones in the datastore.
     this._fillPossibleHandlers(infoID,
                                aHandlerInfo.possibleApplicationHandlers,
                                aHandlerInfo.preferredApplicationHandler);
     // If we have an "always ask" flag stored in the RDF, always use its
     // value. Otherwise, use the default value stored in the pref service.
     var alwaysAsk;
     if (this._hasValue(infoID, NC_ALWAYS_ASK)) {
       alwaysAsk = (this._getValue(infoID, NC_ALWAYS_ASK) != "false");
     } else {
       var prefSvc = Cc["@mozilla.org/preferences-service;1"].
                     getService(Ci.nsIPrefService);
       var prefBranch = prefSvc.getBranch("network.protocol-handler.");
       try {
         alwaysAsk = prefBranch.getBoolPref("warn-external." + type);
       } catch (e) {
         // will throw if pref didn't exist.
         try {
           alwaysAsk = prefBranch.getBoolPref("warn-external-default");
         } catch (e) {
           // Nothing to tell us what to do, so be paranoid and prompt.
           alwaysAsk = true;
         }
       }
     }
     aHandlerInfo.alwaysAskBeforeHandling = alwaysAsk;
     // If the object represents a MIME type handler, then also retrieve
     // any file extensions.
     if (aHandlerInfo instanceof Ci.nsIMIMEInfo)
       for each (let fileExtension in this._retrieveFileExtensions(typeID))
         aHandlerInfo.appendExtension(fileExtension);
   },
   store: function HS_store(aHandlerInfo) {
     // FIXME: when we switch from RDF to something with transactions (like
     // SQLite), enclose the following changes in a transaction so they all
     // get rolled back if any of them fail and we don't leave the datastore
     // in an inconsistent state.
     this._ensureRecordsForType(aHandlerInfo);
     this._storePreferredAction(aHandlerInfo);
     this._storePreferredHandler(aHandlerInfo);
     this._storePossibleHandlers(aHandlerInfo);
     this._storeAlwaysAsk(aHandlerInfo);
     // Write the changes to the database immediately so we don't lose them
     // if the application crashes.
     if (this._ds instanceof Ci.nsIRDFRemoteDataSource)
       this._ds.Flush();
   },
   exists: function HS_exists(aHandlerInfo) {
     var found;
     try {
       var typeID = this._getTypeID(this._getClass(aHandlerInfo), aHandlerInfo.type);
       found = this._hasLiteralAssertion(typeID, NC_VALUE, aHandlerInfo.type);
     } catch (e) {
       // If the RDF threw (eg, corrupt file), treat as nonexistent.
       found = false;
     }
     return found;
   },
   remove: function HS_remove(aHandlerInfo) {
     var preferredHandlerID =
       this._getPreferredHandlerID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     this._removeAssertions(preferredHandlerID);
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     // Get a list of possible handlers.  After we have removed the info record,
     // we'll check if any other info records reference these handlers, and we'll
     // remove the handler records that aren't referenced by other info records.
     var possibleHandlerIDs = [];
     var possibleHandlerTargets = this._getTargets(infoID, NC_POSSIBLE_APP);
     while (possibleHandlerTargets.hasMoreElements()) {
       let possibleHandlerTarget = possibleHandlerTargets.getNext();
       // Note: possibleHandlerTarget should always be an nsIRDFResource.
       // The conditional is just here in case of a corrupt RDF datasource.
       if (possibleHandlerTarget instanceof Ci.nsIRDFResource)
         possibleHandlerIDs.push(possibleHandlerTarget.ValueUTF8);
     }
     // Remove the info record.
     this._removeAssertions(infoID);
     // Now that we've removed the info record, remove any possible handlers
     // that aren't referenced by other info records.
     for each (let possibleHandlerID in possibleHandlerIDs)
       if (!this._existsResourceTarget(NC_POSSIBLE_APP, possibleHandlerID))
         this._removeAssertions(possibleHandlerID);
     var typeID = this._getTypeID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     this._removeAssertions(typeID);
     // Now that there's no longer a handler for this type, remove the type
     // from the list of types for which there are known handlers.
     var typeList = this._ensureAndGetTypeList(this._getClass(aHandlerInfo));
     var type = this._rdf.GetResource(typeID);
     var typeIndex = typeList.IndexOf(type);
     if (typeIndex != -1)
       typeList.RemoveElementAt(typeIndex, true);
     // Write the changes to the database immediately so we don't lose them
     // if the application crashes.
     // XXX If we're removing a bunch of handlers at once, will flushing
     // after every removal cause a significant performance hit?
     if (this._ds instanceof Ci.nsIRDFRemoteDataSource)
       this._ds.Flush();
   },
   getTypeFromExtension: function HS_getTypeFromExtension(aFileExtension) {
     var fileExtension = aFileExtension.toLowerCase();
     var typeID;
     if (this._existsLiteralTarget(NC_FILE_EXTENSIONS, fileExtension))
       typeID = this._getSourceForLiteral(NC_FILE_EXTENSIONS, fileExtension);
     if (typeID && this._hasValue(typeID, NC_VALUE)) {
       let type = this._getValue(typeID, NC_VALUE);
       if (type == "")
         throw Cr.NS_ERROR_FAILURE;
       return type;
     }
     return "";
   },
   //**************************************************************************//
   // Retrieval Methods
   /**
    * Retrieve the preferred action for the info record with the given ID.
    *
    * @param aInfoID  {string}  the info record ID
    *
    * @returns  {integer}  the preferred action enumeration value
    */
   _retrievePreferredAction: function HS__retrievePreferredAction(aInfoID) {
     if (this._getValue(aInfoID, NC_SAVE_TO_DISK) == "true")
       return Ci.nsIHandlerInfo.saveToDisk;
     if (this._getValue(aInfoID, NC_USE_SYSTEM_DEFAULT) == "true")
       return Ci.nsIHandlerInfo.useSystemDefault;
     if (this._getValue(aInfoID, NC_HANDLE_INTERNALLY) == "true")
       return Ci.nsIHandlerInfo.handleInternally;
     return Ci.nsIHandlerInfo.useHelperApp;
   },
   /**
    * Fill an array of possible handlers with the handlers for the given info ID.
    *
    * @param aInfoID            {string}           the ID of the info record
    * @param aPossibleHandlers  {nsIMutableArray}  the array of possible handlers
    * @param aPreferredHandler  {nsIHandlerApp}    the preferred handler, if any
    */
   _fillPossibleHandlers: function HS__fillPossibleHandlers(aInfoID,
                                                            aPossibleHandlers,
                                                            aPreferredHandler) {
     // The set of possible handlers should include the preferred handler,
     // but legacy datastores (from before we added possible handlers) won't
     // include the preferred handler, so check if it's included as we build
     // the list of handlers, and, if it's not included, add it to the list.
     if (aPreferredHandler)
       aPossibleHandlers.appendElement(aPreferredHandler, false);
     var possibleHandlerTargets = this._getTargets(aInfoID, NC_POSSIBLE_APP);
     while (possibleHandlerTargets.hasMoreElements()) {
       let possibleHandlerTarget = possibleHandlerTargets.getNext();
       if (!(possibleHandlerTarget instanceof Ci.nsIRDFResource))
         continue;
       let possibleHandlerID = possibleHandlerTarget.ValueUTF8;
       let possibleHandler = this._retrieveHandlerApp(possibleHandlerID);
       if (possibleHandler && (!aPreferredHandler ||
                               !possibleHandler.equals(aPreferredHandler)))
         aPossibleHandlers.appendElement(possibleHandler, false);
     }
   },
   /**
    * Retrieve the handler app object with the given ID.
    *
    * @param aHandlerAppID  {string}  the ID of the handler app to retrieve
    *
    * @returns  {nsIHandlerApp}  the handler app, if any; otherwise null
    */
   _retrieveHandlerApp: function HS__retrieveHandlerApp(aHandlerAppID) {
     var handlerApp;
     // If it has a path, it's a local handler; otherwise, it's a web handler.
     if (this._hasValue(aHandlerAppID, NC_PATH)) {
       let executable =
         this._getFileWithPath(this._getValue(aHandlerAppID, NC_PATH));
       if (!executable)
         return null;
       handlerApp = Cc["@mozilla.org/uriloader/local-handler-app;1"].
                    createInstance(Ci.nsILocalHandlerApp);
       handlerApp.executable = executable;
     }
     else if (this._hasValue(aHandlerAppID, NC_URI_TEMPLATE)) {
       let uriTemplate = this._getValue(aHandlerAppID, NC_URI_TEMPLATE);
       if (!uriTemplate)
         return null;
       handlerApp = Cc["@mozilla.org/uriloader/web-handler-app;1"].
                    createInstance(Ci.nsIWebHandlerApp);
       handlerApp.uriTemplate = uriTemplate;
     }
     else if (this._hasValue(aHandlerAppID, NC_SERVICE)) {
       let service = this._getValue(aHandlerAppID, NC_SERVICE);
       if (!service)
         return null;
       let method = this._getValue(aHandlerAppID, NC_METHOD);
       if (!method)
         return null;
       let objpath = this._getValue(aHandlerAppID, NC_OBJPATH);
       if (!objpath)
         return null;
       let interface = this._getValue(aHandlerAppID, NC_INTERFACE);
       if (!interface)
         return null;
       handlerApp = Cc["@mozilla.org/uriloader/dbus-handler-app;1"].
                    createInstance(Ci.nsIDBusHandlerApp);
       handlerApp.service   = service;
       handlerApp.method    = method;
       handlerApp.objectPath   = objpath;
       handlerApp.dBusInterface = interface;
     }
     else
       return null;
     handlerApp.name = this._getValue(aHandlerAppID, NC_PRETTY_NAME);
     return handlerApp;
   },
   /*
    * Retrieve file extensions, if any, for the MIME type with the given type ID.
    *
    * @param aTypeID  {string}  the type record ID
    */
   _retrieveFileExtensions: function HS__retrieveFileExtensions(aTypeID) {
     var fileExtensions = [];
     var fileExtensionTargets = this._getTargets(aTypeID, NC_FILE_EXTENSIONS);
     while (fileExtensionTargets.hasMoreElements()) {
       let fileExtensionTarget = fileExtensionTargets.getNext();
       if (fileExtensionTarget instanceof Ci.nsIRDFLiteral &&
           fileExtensionTarget.Value != "")
         fileExtensions.push(fileExtensionTarget.Value);
     }
     return fileExtensions;
   },
   /**
    * Get the file with the given path.  This is not as simple as merely
    * initializing a local file object with the path, because the path might be
    * relative to the current process directory, in which case we have to
    * construct a path starting from that directory.
    *
    * @param aPath  {string}  a path to a file
    *
    * @returns {nsILocalFile} the file, or null if the file does not exist
    */
   _getFileWithPath: function HS__getFileWithPath(aPath) {
     var file = Cc["@mozilla.org/file/local;1"].createInstance(Ci.nsILocalFile);
     try {
       file.initWithPath(aPath);
       if (file.exists())
         return file;
     }
     catch(ex) {
       // Note: for historical reasons, we don't actually check to see
       // if the exception is NS_ERROR_FILE_UNRECOGNIZED_PATH, which is what
       // nsILocalFile::initWithPath throws when a path is relative.
       file = this._dirSvc.get("XCurProcD", Ci.nsIFile);
       try {
         file.append(aPath);
         if (file.exists())
           return file;
       }
       catch(ex) {}
     }
     return null;
   },
   //**************************************************************************//
   // Storage Methods
   _storePreferredAction: function HS__storePreferredAction(aHandlerInfo) {
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     switch(aHandlerInfo.preferredAction) {
       case Ci.nsIHandlerInfo.saveToDisk:
         this._setLiteral(infoID, NC_SAVE_TO_DISK, "true");
         this._removeTarget(infoID, NC_HANDLE_INTERNALLY);
         this._removeTarget(infoID, NC_USE_SYSTEM_DEFAULT);
         break;
       case Ci.nsIHandlerInfo.handleInternally:
         this._setLiteral(infoID, NC_HANDLE_INTERNALLY, "true");
         this._removeTarget(infoID, NC_SAVE_TO_DISK);
         this._removeTarget(infoID, NC_USE_SYSTEM_DEFAULT);
         break;
       case Ci.nsIHandlerInfo.useSystemDefault:
         this._setLiteral(infoID, NC_USE_SYSTEM_DEFAULT, "true");
         this._removeTarget(infoID, NC_SAVE_TO_DISK);
         this._removeTarget(infoID, NC_HANDLE_INTERNALLY);
         break;
       // This value is indicated in the datastore either by the absence of
       // the three properties or by setting them all "false".  Of these two
       // options, the former seems preferable, because it reduces the size
       // of the RDF file and thus the amount of stuff we have to parse.
       case Ci.nsIHandlerInfo.useHelperApp:
       default:
         this._removeTarget(infoID, NC_SAVE_TO_DISK);
         this._removeTarget(infoID, NC_HANDLE_INTERNALLY);
         this._removeTarget(infoID, NC_USE_SYSTEM_DEFAULT);
         break;
     }
   },
   _storePreferredHandler: function HS__storePreferredHandler(aHandlerInfo) {
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     var handlerID =
       this._getPreferredHandlerID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     var handler = aHandlerInfo.preferredApplicationHandler;
     if (handler) {
       this._storeHandlerApp(handlerID, handler);
       // Make this app be the preferred app for the handler info.
       //
       // Note: nsExternalHelperAppService::FillContentHandlerProperties ignores
       // this setting and instead identifies the preferred app as the resource
       // whose URI follows the pattern urn:<class>:externalApplication:<type>.
       // But the old downloadactions.js code used to set this property, so just
       // in case there is still some code somewhere that relies on its presence,
       // we set it here.
       this._setResource(infoID, NC_PREFERRED_APP, handlerID);
     }
     else {
       // There isn't a preferred handler.  Remove the existing record for it,
       // if any.
       this._removeTarget(infoID, NC_PREFERRED_APP);
       this._removeAssertions(handlerID);
     }
   },
   /**
    * Store the list of possible handler apps for the content type represented
    * by the given handler info object.
    *
    * @param aHandlerInfo  {nsIHandlerInfo}  the handler info object
    */
   _storePossibleHandlers: function HS__storePossibleHandlers(aHandlerInfo) {
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     // First, retrieve the set of handler apps currently stored for the type,
     // keeping track of their IDs in a hash that we'll use to determine which
     // ones are no longer valid and should be removed.
     var currentHandlerApps = {};
     var currentHandlerTargets = this._getTargets(infoID, NC_POSSIBLE_APP);
     while (currentHandlerTargets.hasMoreElements()) {
       let handlerApp = currentHandlerTargets.getNext();
       if (handlerApp instanceof Ci.nsIRDFResource) {
         let handlerAppID = handlerApp.ValueUTF8;
         currentHandlerApps[handlerAppID] = true;
       }
     }
     // Next, store any new handler apps.
     var newHandlerApps =
       aHandlerInfo.possibleApplicationHandlers.enumerate();
     while (newHandlerApps.hasMoreElements()) {
       let handlerApp =
         newHandlerApps.getNext().QueryInterface(Ci.nsIHandlerApp);
       let handlerAppID = this._getPossibleHandlerAppID(handlerApp);
       if (!this._hasResourceAssertion(infoID, NC_POSSIBLE_APP, handlerAppID)) {
         this._storeHandlerApp(handlerAppID, handlerApp);
         this._addResourceAssertion(infoID, NC_POSSIBLE_APP, handlerAppID);
       }
       delete currentHandlerApps[handlerAppID];
     }
     // Finally, remove any old handler apps that aren't being used anymore,
     // and if those handler apps aren't being used by any other type either,
     // then completely remove their record from the datastore so we don't
     // leave it clogged up with information about handler apps we don't care
     // about anymore.
     for (let handlerAppID in currentHandlerApps) {
       this._removeResourceAssertion(infoID, NC_POSSIBLE_APP, handlerAppID);
       if (!this._existsResourceTarget(NC_POSSIBLE_APP, handlerAppID))
         this._removeAssertions(handlerAppID);
     }
   },
   /**
    * Store the given handler app.
    *
    * Note: the reason this method takes the ID of the handler app in a param
    * is that the ID is different than it usually is when the handler app
    * in question is a preferred handler app, so this method can't just derive
    * the ID of the handler app by calling _getPossibleHandlerAppID, its callers
    * have to do that for it.
    *
    * @param aHandlerAppID {string}        the ID of the handler app to store
    * @param aHandlerApp   {nsIHandlerApp} the handler app to store
    */
   _storeHandlerApp: function HS__storeHandlerApp(aHandlerAppID, aHandlerApp) {
     aHandlerApp.QueryInterface(Ci.nsIHandlerApp);
     this._setLiteral(aHandlerAppID, NC_PRETTY_NAME, aHandlerApp.name);
     // In the case of the preferred handler, the handler ID could have been
     // used to refer to a different kind of handler in the past (i.e. either
     // a local hander or a web handler), so if the new handler is a local
     // handler, then we remove any web handler properties and vice versa.
     // This is unnecessary but harmless for possible handlers.
     if (aHandlerApp instanceof Ci.nsILocalHandlerApp) {
       this._setLiteral(aHandlerAppID, NC_PATH, aHandlerApp.executable.path);
       this._removeTarget(aHandlerAppID, NC_URI_TEMPLATE);
       this._removeTarget(aHandlerAppID, NC_METHOD);
       this._removeTarget(aHandlerAppID, NC_SERVICE);
       this._removeTarget(aHandlerAppID, NC_OBJPATH);
       this._removeTarget(aHandlerAppID, NC_INTERFACE);
     }
     else if(aHandlerApp instanceof Ci.nsIWebHandlerApp){
       aHandlerApp.QueryInterface(Ci.nsIWebHandlerApp);
       this._setLiteral(aHandlerAppID, NC_URI_TEMPLATE, aHandlerApp.uriTemplate);
       this._removeTarget(aHandlerAppID, NC_PATH);
       this._removeTarget(aHandlerAppID, NC_METHOD);
       this._removeTarget(aHandlerAppID, NC_SERVICE);
       this._removeTarget(aHandlerAppID, NC_OBJPATH);
       this._removeTarget(aHandlerAppID, NC_INTERFACE);
     }
     else if(aHandlerApp instanceof Ci.nsIDBusHandlerApp){
       aHandlerApp.QueryInterface(Ci.nsIDBusHandlerApp);
       this._setLiteral(aHandlerAppID, NC_SERVICE, aHandlerApp.service);
       this._setLiteral(aHandlerAppID, NC_METHOD, aHandlerApp.method);
       this._setLiteral(aHandlerAppID, NC_OBJPATH, aHandlerApp.objectPath);
       this._setLiteral(aHandlerAppID, NC_INTERFACE, aHandlerApp.dBusInterface);
       this._removeTarget(aHandlerAppID, NC_PATH);
       this._removeTarget(aHandlerAppID, NC_URI_TEMPLATE);
     }
     else {
 	throw "unknown handler type";
     }
   },
   _storeAlwaysAsk: function HS__storeAlwaysAsk(aHandlerInfo) {
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     this._setLiteral(infoID,
                      NC_ALWAYS_ASK,
                      aHandlerInfo.alwaysAskBeforeHandling ? "true" : "false");
   },
   //**************************************************************************//
   // Convenience Getters
   // Observer Service
   __observerSvc: null,
   get _observerSvc() {
     if (!this.__observerSvc)
       this.__observerSvc =
         Cc["@mozilla.org/observer-service;1"].
         getService(Ci.nsIObserverService);
     return this.__observerSvc;
   },
   // Directory Service
   __dirSvc: null,
   get _dirSvc() {
     if (!this.__dirSvc)
       this.__dirSvc =
         Cc["@mozilla.org/file/directory_service;1"].
         getService(Ci.nsIProperties);
     return this.__dirSvc;
   },
   // MIME Service
   __mimeSvc: null,
   get _mimeSvc() {
     if (!this.__mimeSvc)
       this.__mimeSvc =
         Cc["@mozilla.org/mime;1"].
         getService(Ci.nsIMIMEService);
     return this.__mimeSvc;
   },
   // Protocol Service
   __protocolSvc: null,
   get _protocolSvc() {
     if (!this.__protocolSvc)
       this.__protocolSvc =
         Cc["@mozilla.org/uriloader/external-protocol-service;1"].
         getService(Ci.nsIExternalProtocolService);
     return this.__protocolSvc;
   },
   // RDF Service
   __rdf: null,
   get _rdf() {
     if (!this.__rdf)
       this.__rdf = Cc["@mozilla.org/rdf/rdf-service;1"].
                    getService(Ci.nsIRDFService);
     return this.__rdf;
   },
   // RDF Container Utils
   __containerUtils: null,
   get _containerUtils() {
     if (!this.__containerUtils)
       this.__containerUtils = Cc["@mozilla.org/rdf/container-utils;1"].
                               getService(Ci.nsIRDFContainerUtils);
     return this.__containerUtils;
   },
   // RDF datasource containing content handling config (i.e. mimeTypes.rdf)
   __ds: null,
   get _ds() {
     if (!this.__ds) {
       var file = this._dirSvc.get("UMimTyp", Ci.nsIFile);
       // FIXME: make this a memoizing getter if we use it anywhere else.
       var ioService = Cc["@mozilla.org/network/io-service;1"].
                       getService(Ci.nsIIOService);
       var fileHandler = ioService.getProtocolHandler("file").
                         QueryInterface(Ci.nsIFileProtocolHandler);
       this.__ds =
         this._rdf.GetDataSourceBlocking(fileHandler.getURLSpecFromFile(file));
     }
     return this.__ds;
   },
   //**************************************************************************//
   // Datastore Utils
   /**
    * Get the string identifying whether this is a MIME or a protocol handler.
    * This string is used in the URI IDs of various RDF properties.
    *
    * @param aHandlerInfo {nsIHandlerInfo} the handler for which to get the class
    *
    * @returns {string} the class
    */
   _getClass: function HS__getClass(aHandlerInfo) {
     if (aHandlerInfo instanceof Ci.nsIMIMEInfo)
       return CLASS_MIMEINFO;
     else
       return CLASS_PROTOCOLINFO;
   },
   /**
    * Return the unique identifier for a content type record, which stores
    * the value field plus a reference to the content type's handler info record.
    *
    * |urn:<class>:<type>|
    *
    * XXX: should this be a property of nsIHandlerInfo?
    *
    * @param aClass {string} the class (CLASS_MIMEINFO or CLASS_PROTOCOLINFO)
    * @param aType  {string} the type (a MIME type or protocol scheme)
    *
    * @returns {string} the ID
    */
   _getTypeID: function HS__getTypeID(aClass, aType) {
     return "urn:" + aClass + ":" + aType;
   },
   /**
    * Return the unique identifier for a handler info record, which stores
    * the preferredAction and alwaysAsk fields plus a reference to the preferred
    * handler app.  Roughly equivalent to the nsIHandlerInfo interface.
    *
    * |urn:<class>:handler:<type>|
    *
    * FIXME: the type info record should be merged into the type record,
    * since there's a one to one relationship between them, and this record
    * merely stores additional attributes of a content type.
    *
    * @param aClass {string} the class (CLASS_MIMEINFO or CLASS_PROTOCOLINFO)
    * @param aType  {string} the type (a MIME type or protocol scheme)
    *
    * @returns {string} the ID
    */
   _getInfoID: function HS__getInfoID(aClass, aType) {
     return "urn:" + aClass + ":handler:" + aType;
   },
   /**
    * Return the unique identifier for a preferred handler record, which stores
    * information about the preferred handler for a given content type, including
    * its human-readable name and the path to its executable (for a local app)
    * or its URI template (for a web app).
    *
    * |urn:<class>:externalApplication:<type>|
    *
    * XXX: should this be a property of nsIHandlerApp?
    *
    * FIXME: this should be an arbitrary ID, and we should retrieve it from
    * the datastore for a given content type via the NC:ExternalApplication
    * property rather than looking for a specific ID, so a handler doesn't
    * have to change IDs when it goes from being a possible handler to being
    * the preferred one (once we support possible handlers).
    *
    * @param aClass {string} the class (CLASS_MIMEINFO or CLASS_PROTOCOLINFO)
    * @param aType  {string} the type (a MIME type or protocol scheme)
    *
    * @returns {string} the ID
    */
   _getPreferredHandlerID: function HS__getPreferredHandlerID(aClass, aType) {
     return "urn:" + aClass + ":externalApplication:" + aType;
   },
   /**
    * Return the unique identifier for a handler app record, which stores
    * information about a possible handler for one or more content types,
    * including its human-readable name and the path to its executable (for a
    * local app) or its URI template (for a web app).
    *
    * Note: handler app IDs for preferred handlers are different.  For those,
    * see the _getPreferredHandlerID method.
    *
    * @param aHandlerApp  {nsIHandlerApp}   the handler app object
    */
   _getPossibleHandlerAppID: function HS__getPossibleHandlerAppID(aHandlerApp) {
     var handlerAppID = "urn:handler:";
     if (aHandlerApp instanceof Ci.nsILocalHandlerApp)
       handlerAppID += "local:" + aHandlerApp.executable.path;
     else if(aHandlerApp instanceof Ci.nsIWebHandlerApp){
       aHandlerApp.QueryInterface(Ci.nsIWebHandlerApp);
       handlerAppID += "web:" + aHandlerApp.uriTemplate;
     }
     else if(aHandlerApp instanceof Ci.nsIDBusHandlerApp){
       aHandlerApp.QueryInterface(Ci.nsIDBusHandlerApp);
       handlerAppID += "dbus:" + aHandlerApp.service + " " + aHandlerApp.method + " " + aHandlerApp.uriTemplate;
     }else{
 	throw "unknown handler type";
     }
     return handlerAppID;
   },
   /**
    * Get the list of types for the given class, creating the list if it doesn't
    * already exist. The class can be either CLASS_MIMEINFO or CLASS_PROTOCOLINFO
    * (i.e. the result of a call to _getClass).
    *
    * |urn:<class>s|
    * |urn:<class>s:root|
    *
    * @param aClass {string} the class for which to retrieve a list of types
    *
    * @returns {nsIRDFContainer} the list of types
    */
   _ensureAndGetTypeList: function HS__ensureAndGetTypeList(aClass) {
     var source = this._rdf.GetResource("urn:" + aClass + "s");
     var property =
       this._rdf.GetResource(aClass == CLASS_MIMEINFO ? NC_MIME_TYPES
                                                      : NC_PROTOCOL_SCHEMES);
     var target = this._rdf.GetResource("urn:" + aClass + "s:root");
     // Make sure we have an arc from the source to the target.
     if (!this._ds.HasAssertion(source, property, target, true))
       this._ds.Assert(source, property, target, true);
     // Make sure the target is a container.
     if (!this._containerUtils.IsContainer(this._ds, target))
       this._containerUtils.MakeSeq(this._ds, target);
     // Get the type list as an RDF container.
     var typeList = Cc["@mozilla.org/rdf/container;1"].
                    createInstance(Ci.nsIRDFContainer);
     typeList.Init(this._ds, target);
     return typeList;
   },
   /**
    * Make sure there are records in the datasource for the given content type
    * by creating them if they don't already exist.  We have to do this before
    * storing any specific data, because we can't assume the presence
    * of the records (the nsIHandlerInfo object might have been created
    * from the OS), and the records have to all be there in order for the helper
    * app service to properly construct an nsIHandlerInfo object for the type.
    *
    * Based on old downloadactions.js::_ensureMIMERegistryEntry.
    *
    * @param aHandlerInfo {nsIHandlerInfo} the type to make sure has a record
    */
   _ensureRecordsForType: function HS__ensureRecordsForType(aHandlerInfo) {
     // Get the list of types.
     var typeList = this._ensureAndGetTypeList(this._getClass(aHandlerInfo));
     // If there's already a record in the datastore for this type, then we
     // don't need to do anything more.
     var typeID = this._getTypeID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     var type = this._rdf.GetResource(typeID);
     if (typeList.IndexOf(type) != -1)
       return;
     // Create a basic type record for this type.
     typeList.AppendElement(type);
     this._setLiteral(typeID, NC_VALUE, aHandlerInfo.type);
     // Create a basic info record for this type.
     var infoID = this._getInfoID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     this._setLiteral(infoID, NC_ALWAYS_ASK, "false");
     this._setResource(typeID, NC_HANDLER_INFO, infoID);
     // XXX Shouldn't we set preferredAction to useSystemDefault?
     // That's what it is if there's no record in the datastore; why should it
     // change to useHelperApp just because we add a record to the datastore?
     // Create a basic preferred handler record for this type.
     // XXX Not sure this is necessary, since preferred handlers are optional,
     // and nsExternalHelperAppService::FillHandlerInfoForTypeFromDS doesn't seem
     // to require the record , but downloadactions.js::_ensureMIMERegistryEntry
     // used to create it, so we'll do the same.
     var preferredHandlerID =
       this._getPreferredHandlerID(this._getClass(aHandlerInfo), aHandlerInfo.type);
     this._setLiteral(preferredHandlerID, NC_PATH, "");
     this._setResource(infoID, NC_PREFERRED_APP, preferredHandlerID);
   },
   /**
    * Append known handlers of the given class to the given array.  The class
    * can be either CLASS_MIMEINFO or CLASS_PROTOCOLINFO.
    *
    * @param aHandlers   {array} the array of handlers to append to
    * @param aClass      {string} the class for which to append handlers
    */
   _appendHandlers: function HS__appendHandlers(aHandlers, aClass) {
     var typeList = this._ensureAndGetTypeList(aClass);
     var enumerator = typeList.GetElements();
     while (enumerator.hasMoreElements()) {
       var element = enumerator.getNext();
       // This should never happen.  If it does, that means our datasource
       // is corrupted with type list entries that point to literal values
       // instead of resources.  If it does happen, let's just do our best
       // to recover by ignoring this entry and moving on to the next one.
       if (!(element instanceof Ci.nsIRDFResource))
         continue;
       // Get the value of the element's NC:value property, which contains
       // the MIME type or scheme for which we're retrieving a handler info.
       var type = this._getValue(element.ValueUTF8, NC_VALUE);
       if (!type)
         continue;
       var handler;
       if (typeList.Resource.ValueUTF8 == "urn:mimetypes:root")
         handler = this._mimeSvc.getFromTypeAndExtension(type, null);
       else
         handler = this._protocolSvc.getProtocolHandlerInfo(type);
       aHandlers.appendElement(handler, false);
     }
   },
   /**
    * Whether or not a property of an RDF source has a value.
    *
    * @param sourceURI   {string}  the URI of the source
    * @param propertyURI {string}  the URI of the property
    * @returns           {boolean} whether or not the property has a value
    */
   _hasValue: function HS__hasValue(sourceURI, propertyURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     return this._ds.hasArcOut(source, property);
   },
   /**
    * Get the value of a property of an RDF source.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @returns           {string} the value of the property
    */
   _getValue: function HS__getValue(sourceURI, propertyURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._ds.GetTarget(source, property, true);
     if (!target)
       return null;
     if (target instanceof Ci.nsIRDFResource)
       return target.ValueUTF8;
     if (target instanceof Ci.nsIRDFLiteral)
       return target.Value;
     return null;
   },
   /**
    * Get all targets for the property of an RDF source.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    *
    * @returns {nsISimpleEnumerator} an enumerator of targets
    */
   _getTargets: function HS__getTargets(sourceURI, propertyURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     return this._ds.GetTargets(source, property, true);
   },
   /**
    * Set a property of an RDF source to a literal value.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @param value       {string} the literal value
    */
   _setLiteral: function HS__setLiteral(sourceURI, propertyURI, value) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetLiteral(value);
     this._setTarget(source, property, target);
   },
   /**
    * Set a property of an RDF source to a resource target.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @param targetURI   {string} the URI of the target
    */
   _setResource: function HS__setResource(sourceURI, propertyURI, targetURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetResource(targetURI);
     this._setTarget(source, property, target);
   },
   /**
    * Assert an arc into the RDF datasource if there is no arc with the given
    * source and property; otherwise, if there is already an existing arc,
    * change it to point to the given target. _setLiteral and _setResource
    * call this after converting their string arguments into resources
    * and literals, and most callers should call one of those two methods
    * instead of this one.
    *
    * @param source    {nsIRDFResource}  the source
    * @param property  {nsIRDFResource}  the property
    * @param target    {nsIRDFNode}      the target
    */
   _setTarget: function HS__setTarget(source, property, target) {
     if (this._ds.hasArcOut(source, property)) {
       var oldTarget = this._ds.GetTarget(source, property, true);
       this._ds.Change(source, property, oldTarget, target);
     }
     else
       this._ds.Assert(source, property, target, true);
   },
   /**
    * Assert that a property of an RDF source has a resource target.
    *
    * The difference between this method and _setResource is that this one adds
    * an assertion even if one already exists, which allows its callers to make
    * sets of assertions (i.e. to set a property to multiple targets).
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @param targetURI   {string} the URI of the target
    */
   _addResourceAssertion: function HS__addResourceAssertion(sourceURI,
                                                            propertyURI,
                                                            targetURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetResource(targetURI);
     this._ds.Assert(source, property, target, true);
   },
   /**
    * Remove an assertion with a resource target.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @param targetURI   {string} the URI of the target
    */
   _removeResourceAssertion: function HS__removeResourceAssertion(sourceURI,
                                                                  propertyURI,
                                                                  targetURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetResource(targetURI);
     this._ds.Unassert(source, property, target);
   },
   /**
    * Whether or not a property of an RDF source has a given resource target.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @param targetURI   {string} the URI of the target
    *
    * @returns {boolean} whether or not there is such an assertion
    */
   _hasResourceAssertion: function HS__hasResourceAssertion(sourceURI,
                                                            propertyURI,
                                                            targetURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetResource(targetURI);
     return this._ds.HasAssertion(source, property, target, true);
   },
   /**
    * Whether or not a property of an RDF source has a given literal value.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    * @param value       {string} the literal value
    *
    * @returns {boolean} whether or not there is such an assertion
    */
   _hasLiteralAssertion: function HS__hasLiteralAssertion(sourceURI,
                                                          propertyURI,
                                                          value) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetLiteral(value);
     return this._ds.HasAssertion(source, property, target, true);
   },
   /**
    * Whether or not there is an RDF source that has the given property set to
    * the given literal value.
    *
    * @param propertyURI {string} the URI of the property
    * @param value       {string} the literal value
    *
    * @returns {boolean} whether or not there is a source
    */
   _existsLiteralTarget: function HS__existsLiteralTarget(propertyURI, value) {
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetLiteral(value);
     return this._ds.hasArcIn(target, property);
   },
   /**
    * Get the source for a property set to a given literal value.
    *
    * @param propertyURI {string} the URI of the property
    * @param value       {string} the literal value
    */
   _getSourceForLiteral: function HS__getSourceForLiteral(propertyURI, value) {
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetLiteral(value);
     var source = this._ds.GetSource(property, target, true);
     if (source)
       return source.ValueUTF8;
     return null;
   },
   /**
    * Whether or not there is an RDF source that has the given property set to
    * the given resource target.
    *
    * @param propertyURI {string} the URI of the property
    * @param targetURI   {string} the URI of the target
    *
    * @returns {boolean} whether or not there is a source
    */
   _existsResourceTarget: function HS__existsResourceTarget(propertyURI,
                                                            targetURI) {
     var property = this._rdf.GetResource(propertyURI);
     var target = this._rdf.GetResource(targetURI);
     return this._ds.hasArcIn(target, property);
   },
   /**
    * Remove a property of an RDF source.
    *
    * @param sourceURI   {string} the URI of the source
    * @param propertyURI {string} the URI of the property
    */
   _removeTarget: function HS__removeTarget(sourceURI, propertyURI) {
     var source = this._rdf.GetResource(sourceURI);
     var property = this._rdf.GetResource(propertyURI);
     if (this._ds.hasArcOut(source, property)) {
       var target = this._ds.GetTarget(source, property, true);
       this._ds.Unassert(source, property, target);
     }
   },
  /**
   * Remove all assertions about a given RDF source.
   *
   * Note: not recursive.  If some assertions point to other resources,
   * and you want to remove assertions about those resources too, you need
   * to do so manually.
   *
   * @param sourceURI {string} the URI of the source
   */
   _removeAssertions: function HS__removeAssertions(sourceURI) {
     var source = this._rdf.GetResource(sourceURI);
     var properties = this._ds.ArcLabelsOut(source);
     while (properties.hasMoreElements()) {
       let property = properties.getNext();
       let targets = this._ds.GetTargets(source, property, true);
       while (targets.hasMoreElements()) {
         let target = targets.getNext();
         this._ds.Unassert(source, property, target);
       }
     }
   }
 };
 //****************************************************************************//
 // More XPCOM Plumbing
 this.NSGetFactory = XPCOMUtils.generateNSGetFactory([HandlerService]);