The Tor Browser / file revision

 /* 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]);