summaryrefslogtreecommitdiff
path: root/uriloader/exthandler/nsHandlerService.js
blob: 5e6356ac2eda868967a01b1a3c8c9faab3b886ec (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
/* 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 (var schemePrefName of 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 (let handlerNumber in schemes[scheme]) {
        let handlerPrefs = schemes[scheme][handlerNumber];
        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.");
      // If neither of the prefs exists, be paranoid and prompt.
      alwaysAsk =
        prefBranch.getBoolPref("warn-external." + type,
                               prefBranch.getBoolPref("warn-external-default",
                                                      true));
    }
    aHandlerInfo.alwaysAskBeforeHandling = alwaysAsk;

    // If the object represents a MIME type handler, then also retrieve
    // any file extensions.
    if (aHandlerInfo instanceof Ci.nsIMIMEInfo)
      for (let fileExtension of 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);
    this._storeExtensions(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 (let possibleHandlerID of 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;

    // See bug 1100069 for why we want to fail gracefully and silently here.
    try {
      this._ds;
    } catch (ex) {
      Components.returnCode = Cr.NS_ERROR_NOT_AVAILABLE;
      return;
    }

    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 iface = this._getValue(aHandlerAppID, NC_INTERFACE);
      if (!iface)
        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 = iface;
      
    }
    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");
  },

  _storeExtensions: function HS__storeExtensions(aHandlerInfo) {
    if (aHandlerInfo instanceof Ci.nsIMIMEInfo) {
      var typeID = this._getTypeID(this._getClass(aHandlerInfo), aHandlerInfo.type);
      var extEnum = aHandlerInfo.getFileExtensions();
      while (extEnum.hasMore()) {
        let ext = extEnum.getNext().toLowerCase();
        if (!this._hasLiteralAssertion(typeID, NC_FILE_EXTENSIONS, ext)) {
          this._setLiteral(typeID, NC_FILE_EXTENSIONS, ext);
        }
      }
    }
  },


  //**************************************************************************//
  // 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]);