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1 office 1 /*
2 * Copyright © 2009, 2010 Codethink Limited
3 * Copyright © 2010 Red Hat, Inc.
4 *
5 * This library is free software; you can redistribute it and/or
6 * modify it under the terms of the GNU Lesser General Public
7 * License as published by the Free Software Foundation; either
8 * version 2 of the licence, or (at your option) any later version.
9 *
10 * This library is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 * Lesser General Public License for more details.
14 *
15 * You should have received a copy of the GNU Lesser General Public
16 * License along with this library; if not, see <http://www.gnu.org/licenses/>.
17 *
18 * Authors: Ryan Lortie <desrt@desrt.ca>
19 * Matthias Clasen <mclasen@redhat.com>
20 */
21  
22 #include "config.h"
23  
24 #include "gsettingsbackendinternal.h"
25 #include "gsimplepermission.h"
26 #include "giomodule-priv.h"
27  
28 #include <string.h>
29 #include <stdlib.h>
30 #include <glib.h>
31 #include <glibintl.h>
32  
33  
34 typedef struct _GSettingsBackendClosure GSettingsBackendClosure;
35 typedef struct _GSettingsBackendWatch GSettingsBackendWatch;
36  
37 struct _GSettingsBackendPrivate
38 {
39 GSettingsBackendWatch *watches;
40 GMutex lock;
41 };
42  
43 G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (GSettingsBackend, g_settings_backend, G_TYPE_OBJECT)
44  
45 /* For g_settings_backend_sync_default(), we only want to actually do
46 * the sync if the backend already exists. This avoids us creating an
47 * entire GSettingsBackend in order to call a do-nothing sync()
48 * operation on it. This variable lets us avoid that.
49 */
50 static gboolean g_settings_has_backend;
51  
52 /**
53 * SECTION:gsettingsbackend
54 * @title: GSettingsBackend
55 * @short_description: Interface for settings backend implementations
56 * @include: gio/gsettingsbackend.h
57 * @see_also: #GSettings, #GIOExtensionPoint
58 *
59 * The #GSettingsBackend interface defines a generic interface for
60 * non-strictly-typed data that is stored in a hierarchy. To implement
61 * an alternative storage backend for #GSettings, you need to implement
62 * the #GSettingsBackend interface and then make it implement the
63 * extension point #G_SETTINGS_BACKEND_EXTENSION_POINT_NAME.
64 *
65 * The interface defines methods for reading and writing values, a
66 * method for determining if writing of certain values will fail
67 * (lockdown) and a change notification mechanism.
68 *
69 * The semantics of the interface are very precisely defined and
70 * implementations must carefully adhere to the expectations of
71 * callers that are documented on each of the interface methods.
72 *
73 * Some of the GSettingsBackend functions accept or return a #GTree.
74 * These trees always have strings as keys and #GVariant as values.
75 * g_settings_backend_create_tree() is a convenience function to create
76 * suitable trees.
77 *
78 * The GSettingsBackend API is exported to allow third-party
79 * implementations, but does not carry the same stability guarantees
80 * as the public GIO API. For this reason, you have to define the
81 * C preprocessor symbol %G_SETTINGS_ENABLE_BACKEND before including
82 * `gio/gsettingsbackend.h`.
83 **/
84  
85 static gboolean
86 is_key (const gchar *key)
87 {
88 gint length;
89 gint i;
90  
91 g_return_val_if_fail (key != NULL, FALSE);
92 g_return_val_if_fail (key[0] == '/', FALSE);
93  
94 for (i = 1; key[i]; i++)
95 g_return_val_if_fail (key[i] != '/' || key[i + 1] != '/', FALSE);
96  
97 length = i;
98  
99 g_return_val_if_fail (key[length - 1] != '/', FALSE);
100  
101 return TRUE;
102 }
103  
104 static gboolean
105 is_path (const gchar *path)
106 {
107 gint length;
108 gint i;
109  
110 g_return_val_if_fail (path != NULL, FALSE);
111 g_return_val_if_fail (path[0] == '/', FALSE);
112  
113 for (i = 1; path[i]; i++)
114 g_return_val_if_fail (path[i] != '/' || path[i + 1] != '/', FALSE);
115  
116 length = i;
117  
118 g_return_val_if_fail (path[length - 1] == '/', FALSE);
119  
120 return TRUE;
121 }
122  
123 struct _GSettingsBackendWatch
124 {
125 GObject *target;
126 const GSettingsListenerVTable *vtable;
127 GMainContext *context;
128 GSettingsBackendWatch *next;
129 };
130  
131 struct _GSettingsBackendClosure
132 {
133 void (*function) (GObject *target,
134 GSettingsBackend *backend,
135 const gchar *name,
136 gpointer origin_tag,
137 gchar **names);
138  
139 GMainContext *context;
140 GObject *target;
141 GSettingsBackend *backend;
142 gchar *name;
143 gpointer origin_tag;
144 gchar **names;
145 };
146  
147 static void
148 g_settings_backend_watch_weak_notify (gpointer data,
149 GObject *where_the_object_was)
150 {
151 GSettingsBackend *backend = data;
152 GSettingsBackendWatch **ptr;
153  
154 /* search and remove */
155 g_mutex_lock (&backend->priv->lock);
156 for (ptr = &backend->priv->watches; *ptr; ptr = &(*ptr)->next)
157 if ((*ptr)->target == where_the_object_was)
158 {
159 GSettingsBackendWatch *tmp = *ptr;
160  
161 *ptr = tmp->next;
162 g_slice_free (GSettingsBackendWatch, tmp);
163  
164 g_mutex_unlock (&backend->priv->lock);
165 return;
166 }
167  
168 /* we didn't find it. that shouldn't happen. */
169 g_assert_not_reached ();
170 }
171  
172 /*< private >
173 * g_settings_backend_watch:
174 * @backend: a #GSettingsBackend
175 * @target: the GObject (typically GSettings instance) to call back to
176 * @context: (allow-none): a #GMainContext, or %NULL
177 * ...: callbacks...
178 *
179 * Registers a new watch on a #GSettingsBackend.
180 *
181 * note: %NULL @context does not mean "default main context" but rather,
182 * "it is okay to dispatch in any context". If the default main context
183 * is specifically desired then it must be given.
184 *
185 * note also: if you want to get meaningful values for the @origin_tag
186 * that appears as an argument to some of the callbacks, you *must* have
187 * @context as %NULL. Otherwise, you are subject to cross-thread
188 * dispatching and whatever owned @origin_tag at the time that the event
189 * occurred may no longer own it. This is a problem if you consider that
190 * you may now be the new owner of that address and mistakenly think
191 * that the event in question originated from yourself.
192 *
193 * tl;dr: If you give a non-%NULL @context then you must ignore the
194 * value of @origin_tag given to any callbacks.
195 **/
196 void
197 g_settings_backend_watch (GSettingsBackend *backend,
198 const GSettingsListenerVTable *vtable,
199 GObject *target,
200 GMainContext *context)
201 {
202 GSettingsBackendWatch *watch;
203  
204 /* For purposes of discussion, we assume that our target is a
205 * GSettings instance.
206 *
207 * Our strategy to defend against the final reference dropping on the
208 * GSettings object in a thread other than the one that is doing the
209 * dispatching is as follows:
210 *
211 * 1) hold a GObject reference on the GSettings during an outstanding
212 * dispatch. This ensures that the delivery is always possible.
213 *
214 * 2) hold a weak reference on the GSettings at other times. This
215 * allows us to receive early notification of pending destruction
216 * of the object. At this point, it is still safe to obtain a
217 * reference on the GObject to keep it alive, so #1 will work up
218 * to that point. After that point, we'll have been able to drop
219 * the watch from the list.
220 *
221 * Note, in particular, that it's not possible to simply have an
222 * "unwatch" function that gets called from the finalize function of
223 * the GSettings instance because, by that point it is no longer
224 * possible to keep the object alive using g_object_ref() and we would
225 * have no way of knowing this.
226 *
227 * Note also that we do not need to hold a reference on the main
228 * context here since the GSettings instance does that for us and we
229 * will receive the weak notify long before it is dropped. We don't
230 * even need to hold it during dispatches because our reference on the
231 * GSettings will prevent the finalize from running and dropping the
232 * ref on the context.
233 *
234 * All access to the list holds a mutex. We have some strategies to
235 * avoid some of the pain that would be associated with that.
236 */
237  
238 watch = g_slice_new (GSettingsBackendWatch);
239 watch->context = context;
240 watch->vtable = vtable;
241 watch->target = target;
242 g_object_weak_ref (target, g_settings_backend_watch_weak_notify, backend);
243  
244 /* linked list prepend */
245 g_mutex_lock (&backend->priv->lock);
246 watch->next = backend->priv->watches;
247 backend->priv->watches = watch;
248 g_mutex_unlock (&backend->priv->lock);
249 }
250  
251 void
252 g_settings_backend_unwatch (GSettingsBackend *backend,
253 GObject *target)
254 {
255 /* Our caller surely owns a reference on 'target', so the order of
256 * these two calls is unimportant.
257 */
258 g_object_weak_unref (target, g_settings_backend_watch_weak_notify, backend);
259 g_settings_backend_watch_weak_notify (backend, target);
260 }
261  
262 static gboolean
263 g_settings_backend_invoke_closure (gpointer user_data)
264 {
265 GSettingsBackendClosure *closure = user_data;
266  
267 closure->function (closure->target, closure->backend, closure->name,
268 closure->origin_tag, closure->names);
269  
270 g_object_unref (closure->backend);
271 g_object_unref (closure->target);
272 g_strfreev (closure->names);
273 g_free (closure->name);
274  
275 g_slice_free (GSettingsBackendClosure, closure);
276  
277 return FALSE;
278 }
279  
280 static void
281 g_settings_backend_dispatch_signal (GSettingsBackend *backend,
282 gsize function_offset,
283 const gchar *name,
284 gpointer origin_tag,
285 const gchar * const *names)
286 {
287 GSettingsBackendWatch *watch;
288 GSList *closures = NULL;
289  
290 /* We're in a little bit of a tricky situation here. We need to hold
291 * a lock while traversing the list, but we don't want to hold the
292 * lock while calling back into user code.
293 *
294 * We work around this by creating a bunch of GSettingsBackendClosure
295 * objects while holding the lock and dispatching them after. We
296 * never touch the list without holding the lock.
297 */
298 g_mutex_lock (&backend->priv->lock);
299 for (watch = backend->priv->watches; watch; watch = watch->next)
300 {
301 GSettingsBackendClosure *closure;
302  
303 closure = g_slice_new (GSettingsBackendClosure);
304 closure->context = watch->context;
305 closure->backend = g_object_ref (backend);
306 closure->target = g_object_ref (watch->target);
307 closure->function = G_STRUCT_MEMBER (void *, watch->vtable,
308 function_offset);
309 closure->name = g_strdup (name);
310 closure->origin_tag = origin_tag;
311 closure->names = g_strdupv ((gchar **) names);
312  
313 closures = g_slist_prepend (closures, closure);
314 }
315 g_mutex_unlock (&backend->priv->lock);
316  
317 while (closures)
318 {
319 GSettingsBackendClosure *closure = closures->data;
320  
321 if (closure->context)
322 g_main_context_invoke (closure->context,
323 g_settings_backend_invoke_closure,
324 closure);
325 else
326 g_settings_backend_invoke_closure (closure);
327  
328 closures = g_slist_delete_link (closures, closures);
329 }
330 }
331  
332 /**
333 * g_settings_backend_changed:
334 * @backend: a #GSettingsBackend implementation
335 * @key: the name of the key
336 * @origin_tag: the origin tag
337 *
338 * Signals that a single key has possibly changed. Backend
339 * implementations should call this if a key has possibly changed its
340 * value.
341 *
342 * @key must be a valid key (ie starting with a slash, not containing
343 * '//', and not ending with a slash).
344 *
345 * The implementation must call this function during any call to
346 * g_settings_backend_write(), before the call returns (except in the
347 * case that no keys are actually changed and it cares to detect this
348 * fact). It may not rely on the existence of a mainloop for
349 * dispatching the signal later.
350 *
351 * The implementation may call this function at any other time it likes
352 * in response to other events (such as changes occurring outside of the
353 * program). These calls may originate from a mainloop or may originate
354 * in response to any other action (including from calls to
355 * g_settings_backend_write()).
356 *
357 * In the case that this call is in response to a call to
358 * g_settings_backend_write() then @origin_tag must be set to the same
359 * value that was passed to that call.
360 *
361 * Since: 2.26
362 **/
363 void
364 g_settings_backend_changed (GSettingsBackend *backend,
365 const gchar *key,
366 gpointer origin_tag)
367 {
368 g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
369 g_return_if_fail (is_key (key));
370  
371 g_settings_backend_dispatch_signal (backend,
372 G_STRUCT_OFFSET (GSettingsListenerVTable,
373 changed),
374 key, origin_tag, NULL);
375 }
376  
377 /**
378 * g_settings_backend_keys_changed:
379 * @backend: a #GSettingsBackend implementation
380 * @path: the path containing the changes
381 * @items: (array zero-terminated=1): the %NULL-terminated list of changed keys
382 * @origin_tag: the origin tag
383 *
384 * Signals that a list of keys have possibly changed. Backend
385 * implementations should call this if keys have possibly changed their
386 * values.
387 *
388 * @path must be a valid path (ie starting and ending with a slash and
389 * not containing '//'). Each string in @items must form a valid key
390 * name when @path is prefixed to it (ie: each item must not start or
391 * end with '/' and must not contain '//').
392 *
393 * The meaning of this signal is that any of the key names resulting
394 * from the contatenation of @path with each item in @items may have
395 * changed.
396 *
397 * The same rules for when notifications must occur apply as per
398 * g_settings_backend_changed(). These two calls can be used
399 * interchangeably if exactly one item has changed (although in that
400 * case g_settings_backend_changed() is definitely preferred).
401 *
402 * For efficiency reasons, the implementation should strive for @path to
403 * be as long as possible (ie: the longest common prefix of all of the
404 * keys that were changed) but this is not strictly required.
405 *
406 * Since: 2.26
407 */
408 void
409 g_settings_backend_keys_changed (GSettingsBackend *backend,
410 const gchar *path,
411 gchar const * const *items,
412 gpointer origin_tag)
413 {
414 g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
415 g_return_if_fail (is_path (path));
416  
417 /* XXX: should do stricter checking (ie: inspect each item) */
418 g_return_if_fail (items != NULL);
419  
420 g_settings_backend_dispatch_signal (backend,
421 G_STRUCT_OFFSET (GSettingsListenerVTable,
422 keys_changed),
423 path, origin_tag, items);
424 }
425  
426 /**
427 * g_settings_backend_path_changed:
428 * @backend: a #GSettingsBackend implementation
429 * @path: the path containing the changes
430 * @origin_tag: the origin tag
431 *
432 * Signals that all keys below a given path may have possibly changed.
433 * Backend implementations should call this if an entire path of keys
434 * have possibly changed their values.
435 *
436 * @path must be a valid path (ie starting and ending with a slash and
437 * not containing '//').
438 *
439 * The meaning of this signal is that any of the key which has a name
440 * starting with @path may have changed.
441 *
442 * The same rules for when notifications must occur apply as per
443 * g_settings_backend_changed(). This call might be an appropriate
444 * reasponse to a 'reset' call but implementations are also free to
445 * explicitly list the keys that were affected by that call if they can
446 * easily do so.
447 *
448 * For efficiency reasons, the implementation should strive for @path to
449 * be as long as possible (ie: the longest common prefix of all of the
450 * keys that were changed) but this is not strictly required. As an
451 * example, if this function is called with the path of "/" then every
452 * single key in the application will be notified of a possible change.
453 *
454 * Since: 2.26
455 */
456 void
457 g_settings_backend_path_changed (GSettingsBackend *backend,
458 const gchar *path,
459 gpointer origin_tag)
460 {
461 g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
462 g_return_if_fail (is_path (path));
463  
464 g_settings_backend_dispatch_signal (backend,
465 G_STRUCT_OFFSET (GSettingsListenerVTable,
466 path_changed),
467 path, origin_tag, NULL);
468 }
469  
470 /**
471 * g_settings_backend_writable_changed:
472 * @backend: a #GSettingsBackend implementation
473 * @key: the name of the key
474 *
475 * Signals that the writability of a single key has possibly changed.
476 *
477 * Since GSettings performs no locking operations for itself, this call
478 * will always be made in response to external events.
479 *
480 * Since: 2.26
481 **/
482 void
483 g_settings_backend_writable_changed (GSettingsBackend *backend,
484 const gchar *key)
485 {
486 g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
487 g_return_if_fail (is_key (key));
488  
489 g_settings_backend_dispatch_signal (backend,
490 G_STRUCT_OFFSET (GSettingsListenerVTable,
491 writable_changed),
492 key, NULL, NULL);
493 }
494  
495 /**
496 * g_settings_backend_path_writable_changed:
497 * @backend: a #GSettingsBackend implementation
498 * @path: the name of the path
499 *
500 * Signals that the writability of all keys below a given path may have
501 * changed.
502 *
503 * Since GSettings performs no locking operations for itself, this call
504 * will always be made in response to external events.
505 *
506 * Since: 2.26
507 **/
508 void
509 g_settings_backend_path_writable_changed (GSettingsBackend *backend,
510 const gchar *path)
511 {
512 g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
513 g_return_if_fail (is_path (path));
514  
515 g_settings_backend_dispatch_signal (backend,
516 G_STRUCT_OFFSET (GSettingsListenerVTable,
517 path_writable_changed),
518 path, NULL, NULL);
519 }
520  
521 typedef struct
522 {
523 const gchar **keys;
524 GVariant **values;
525 gint prefix_len;
526 gchar *prefix;
527 } FlattenState;
528  
529 static gboolean
530 g_settings_backend_flatten_one (gpointer key,
531 gpointer value,
532 gpointer user_data)
533 {
534 FlattenState *state = user_data;
535 const gchar *skey = key;
536 gint i;
537  
538 g_return_val_if_fail (is_key (key), TRUE);
539  
540 /* calculate longest common prefix */
541 if (state->prefix == NULL)
542 {
543 gchar *last_byte;
544  
545 /* first key? just take the prefix up to the last '/' */
546 state->prefix = g_strdup (skey);
547 last_byte = strrchr (state->prefix, '/') + 1;
548 state->prefix_len = last_byte - state->prefix;
549 *last_byte = '\0';
550 }
551 else
552 {
553 /* find the first character that does not match. we will
554 * definitely find one because the prefix ends in '/' and the key
555 * does not. also: no two keys in the tree are the same.
556 */
557 for (i = 0; state->prefix[i] == skey[i]; i++);
558  
559 /* check if we need to shorten the prefix */
560 if (state->prefix[i] != '\0')
561 {
562 /* find the nearest '/', terminate after it */
563 while (state->prefix[i - 1] != '/')
564 i--;
565  
566 state->prefix[i] = '\0';
567 state->prefix_len = i;
568 }
569 }
570  
571  
572 /* save the entire item into the array.
573 * the prefixes will be removed later.
574 */
575 *state->keys++ = key;
576  
577 if (state->values)
578 *state->values++ = value;
579  
580 return FALSE;
581 }
582  
583 /**
584 * g_settings_backend_flatten_tree:
585 * @tree: a #GTree containing the changes
586 * @path: (out): the location to save the path
587 * @keys: (out) (transfer container) (array zero-terminated=1): the
588 * location to save the relative keys
589 * @values: (out) (allow-none) (transfer container) (array zero-terminated=1):
590 * the location to save the values, or %NULL
591 *
592 * Calculate the longest common prefix of all keys in a tree and write
593 * out an array of the key names relative to that prefix and,
594 * optionally, the value to store at each of those keys.
595 *
596 * You must free the value returned in @path, @keys and @values using
597 * g_free(). You should not attempt to free or unref the contents of
598 * @keys or @values.
599 *
600 * Since: 2.26
601 **/
602 void
603 g_settings_backend_flatten_tree (GTree *tree,
604 gchar **path,
605 const gchar ***keys,
606 GVariant ***values)
607 {
608 FlattenState state = { 0, };
609 gsize nnodes;
610  
611 nnodes = g_tree_nnodes (tree);
612  
613 *keys = state.keys = g_new (const gchar *, nnodes + 1);
614 state.keys[nnodes] = NULL;
615  
616 if (values != NULL)
617 {
618 *values = state.values = g_new (GVariant *, nnodes + 1);
619 state.values[nnodes] = NULL;
620 }
621  
622 g_tree_foreach (tree, g_settings_backend_flatten_one, &state);
623 g_return_if_fail (*keys + nnodes == state.keys);
624  
625 *path = state.prefix;
626 while (nnodes--)
627 *--state.keys += state.prefix_len;
628 }
629  
630 /**
631 * g_settings_backend_changed_tree:
632 * @backend: a #GSettingsBackend implementation
633 * @tree: a #GTree containing the changes
634 * @origin_tag: the origin tag
635 *
636 * This call is a convenience wrapper. It gets the list of changes from
637 * @tree, computes the longest common prefix and calls
638 * g_settings_backend_changed().
639 *
640 * Since: 2.26
641 **/
642 void
643 g_settings_backend_changed_tree (GSettingsBackend *backend,
644 GTree *tree,
645 gpointer origin_tag)
646 {
647 const gchar **keys;
648 gchar *path;
649  
650 g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
651  
652 g_settings_backend_flatten_tree (tree, &path, &keys, NULL);
653  
654 #ifdef DEBUG_CHANGES
655 {
656 gint i;
657  
658 g_print ("----\n");
659 g_print ("changed_tree(): prefix %s\n", path);
660 for (i = 0; keys[i]; i++)
661 g_print (" %s\n", keys[i]);
662 g_print ("----\n");
663 }
664 #endif
665  
666 g_settings_backend_keys_changed (backend, path, keys, origin_tag);
667 g_free (path);
668 g_free (keys);
669 }
670  
671 /*< private >
672 * g_settings_backend_read:
673 * @backend: a #GSettingsBackend implementation
674 * @key: the key to read
675 * @expected_type: a #GVariantType
676 * @default_value: if the default value should be returned
677 *
678 * Reads a key. This call will never block.
679 *
680 * If the key exists, the value associated with it will be returned.
681 * If the key does not exist, %NULL will be returned.
682 *
683 * The returned value will be of the type given in @expected_type. If
684 * the backend stored a value of a different type then %NULL will be
685 * returned.
686 *
687 * If @default_value is %TRUE then this gets the default value from the
688 * backend (ie: the one that the backend would contain if
689 * g_settings_reset() were called).
690 *
691 * Returns: the value that was read, or %NULL
692 */
693 GVariant *
694 g_settings_backend_read (GSettingsBackend *backend,
695 const gchar *key,
696 const GVariantType *expected_type,
697 gboolean default_value)
698 {
699 GVariant *value;
700  
701 value = G_SETTINGS_BACKEND_GET_CLASS (backend)
702 ->read (backend, key, expected_type, default_value);
703  
704 if (value != NULL)
705 value = g_variant_take_ref (value);
706  
707 if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))
708 {
709 g_variant_unref (value);
710 value = NULL;
711 }
712  
713 return value;
714 }
715  
716 /*< private >
717 * g_settings_backend_read_user_value:
718 * @backend: a #GSettingsBackend implementation
719 * @key: the key to read
720 * @expected_type: a #GVariantType
721 *
722 * Reads the 'user value' of a key.
723 *
724 * This is the value of the key that the user has control over and has
725 * set for themselves. Put another way: if the user did not set the
726 * value for themselves, then this will return %NULL (even if the
727 * sysadmin has provided a default value).
728 *
729 * Returns: the value that was read, or %NULL
730 */
731 GVariant *
732 g_settings_backend_read_user_value (GSettingsBackend *backend,
733 const gchar *key,
734 const GVariantType *expected_type)
735 {
736 GVariant *value;
737  
738 value = G_SETTINGS_BACKEND_GET_CLASS (backend)
739 ->read_user_value (backend, key, expected_type);
740  
741 if (value != NULL)
742 value = g_variant_take_ref (value);
743  
744 if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))
745 {
746 g_variant_unref (value);
747 value = NULL;
748 }
749  
750 return value;
751 }
752  
753 /*< private >
754 * g_settings_backend_write:
755 * @backend: a #GSettingsBackend implementation
756 * @key: the name of the key
757 * @value: a #GVariant value to write to this key
758 * @origin_tag: the origin tag
759 *
760 * Writes exactly one key.
761 *
762 * This call does not fail. During this call a
763 * #GSettingsBackend::changed signal will be emitted if the value of the
764 * key has changed. The updated key value will be visible to any signal
765 * callbacks.
766 *
767 * One possible method that an implementation might deal with failures is
768 * to emit a second "changed" signal (either during this call, or later)
769 * to indicate that the affected keys have suddenly "changed back" to their
770 * old values.
771 *
772 * Returns: %TRUE if the write succeeded, %FALSE if the key was not writable
773 */
774 gboolean
775 g_settings_backend_write (GSettingsBackend *backend,
776 const gchar *key,
777 GVariant *value,
778 gpointer origin_tag)
779 {
780 gboolean success;
781  
782 g_variant_ref_sink (value);
783 success = G_SETTINGS_BACKEND_GET_CLASS (backend)
784 ->write (backend, key, value, origin_tag);
785 g_variant_unref (value);
786  
787 return success;
788 }
789  
790 /*< private >
791 * g_settings_backend_write_tree:
792 * @backend: a #GSettingsBackend implementation
793 * @tree: a #GTree containing key-value pairs to write
794 * @origin_tag: the origin tag
795 *
796 * Writes one or more keys. This call will never block.
797 *
798 * The key of each item in the tree is the key name to write to and the
799 * value is a #GVariant to write. The proper type of #GTree for this
800 * call can be created with g_settings_backend_create_tree(). This call
801 * might take a reference to the tree; you must not modified the #GTree
802 * after passing it to this call.
803 *
804 * This call does not fail. During this call a #GSettingsBackend::changed
805 * signal will be emitted if any keys have been changed. The new values of
806 * all updated keys will be visible to any signal callbacks.
807 *
808 * One possible method that an implementation might deal with failures is
809 * to emit a second "changed" signal (either during this call, or later)
810 * to indicate that the affected keys have suddenly "changed back" to their
811 * old values.
812 */
813 gboolean
814 g_settings_backend_write_tree (GSettingsBackend *backend,
815 GTree *tree,
816 gpointer origin_tag)
817 {
818 return G_SETTINGS_BACKEND_GET_CLASS (backend)
819 ->write_tree (backend, tree, origin_tag);
820 }
821  
822 /*< private >
823 * g_settings_backend_reset:
824 * @backend: a #GSettingsBackend implementation
825 * @key: the name of a key
826 * @origin_tag: the origin tag
827 *
828 * "Resets" the named key to its "default" value (ie: after system-wide
829 * defaults, mandatory keys, etc. have been taken into account) or possibly
830 * unsets it.
831 */
832 void
833 g_settings_backend_reset (GSettingsBackend *backend,
834 const gchar *key,
835 gpointer origin_tag)
836 {
837 G_SETTINGS_BACKEND_GET_CLASS (backend)
838 ->reset (backend, key, origin_tag);
839 }
840  
841 /*< private >
842 * g_settings_backend_get_writable:
843 * @backend: a #GSettingsBackend implementation
844 * @key: the name of a key
845 *
846 * Finds out if a key is available for writing to. This is the
847 * interface through which 'lockdown' is implemented. Locked down
848 * keys will have %FALSE returned by this call.
849 *
850 * You should not write to locked-down keys, but if you do, the
851 * implementation will deal with it.
852 *
853 * Returns: %TRUE if the key is writable
854 */
855 gboolean
856 g_settings_backend_get_writable (GSettingsBackend *backend,
857 const gchar *key)
858 {
859 return G_SETTINGS_BACKEND_GET_CLASS (backend)
860 ->get_writable (backend, key);
861 }
862  
863 /*< private >
864 * g_settings_backend_unsubscribe:
865 * @backend: a #GSettingsBackend
866 * @name: a key or path to subscribe to
867 *
868 * Reverses the effect of a previous call to
869 * g_settings_backend_subscribe().
870 */
871 void
872 g_settings_backend_unsubscribe (GSettingsBackend *backend,
873 const char *name)
874 {
875 G_SETTINGS_BACKEND_GET_CLASS (backend)
876 ->unsubscribe (backend, name);
877 }
878  
879 /*< private >
880 * g_settings_backend_subscribe:
881 * @backend: a #GSettingsBackend
882 * @name: a key or path to subscribe to
883 *
884 * Requests that change signals be emitted for events on @name.
885 */
886 void
887 g_settings_backend_subscribe (GSettingsBackend *backend,
888 const gchar *name)
889 {
890 G_SETTINGS_BACKEND_GET_CLASS (backend)
891 ->subscribe (backend, name);
892 }
893  
894 static void
895 g_settings_backend_finalize (GObject *object)
896 {
897 GSettingsBackend *backend = G_SETTINGS_BACKEND (object);
898  
899 g_mutex_clear (&backend->priv->lock);
900  
901 G_OBJECT_CLASS (g_settings_backend_parent_class)
902 ->finalize (object);
903 }
904  
905 static void
906 ignore_subscription (GSettingsBackend *backend,
907 const gchar *key)
908 {
909 }
910  
911 static GVariant *
912 g_settings_backend_real_read_user_value (GSettingsBackend *backend,
913 const gchar *key,
914 const GVariantType *expected_type)
915 {
916 return g_settings_backend_read (backend, key, expected_type, FALSE);
917 }
918  
919 static void
920 g_settings_backend_init (GSettingsBackend *backend)
921 {
922 backend->priv = g_settings_backend_get_instance_private (backend);
923 g_mutex_init (&backend->priv->lock);
924 }
925  
926 static void
927 g_settings_backend_class_init (GSettingsBackendClass *class)
928 {
929 GObjectClass *gobject_class = G_OBJECT_CLASS (class);
930  
931 class->subscribe = ignore_subscription;
932 class->unsubscribe = ignore_subscription;
933  
934 class->read_user_value = g_settings_backend_real_read_user_value;
935  
936 gobject_class->finalize = g_settings_backend_finalize;
937 }
938  
939 static void
940 g_settings_backend_variant_unref0 (gpointer data)
941 {
942 if (data != NULL)
943 g_variant_unref (data);
944 }
945  
946 /*< private >
947 * g_settings_backend_create_tree:
948 *
949 * This is a convenience function for creating a tree that is compatible
950 * with g_settings_backend_write(). It merely calls g_tree_new_full()
951 * with strcmp(), g_free() and g_variant_unref().
952 *
953 * Returns: a new #GTree
954 */
955 GTree *
956 g_settings_backend_create_tree (void)
957 {
958 return g_tree_new_full ((GCompareDataFunc) strcmp, NULL,
959 g_free, g_settings_backend_variant_unref0);
960 }
961  
962 static gboolean
963 g_settings_backend_verify (gpointer impl)
964 {
965 GSettingsBackend *backend = impl;
966  
967 if (strcmp (G_OBJECT_TYPE_NAME (backend), "GMemorySettingsBackend") == 0 &&
968 g_strcmp0 (g_getenv ("GSETTINGS_BACKEND"), "memory") != 0)
969 {
970 g_message ("Using the 'memory' GSettings backend. Your settings "
971 "will not be saved or shared with other applications.");
972 }
973  
974 g_settings_has_backend = TRUE;
975 return TRUE;
976 }
977  
978 /**
979 * g_settings_backend_get_default:
980 *
981 * Returns the default #GSettingsBackend. It is possible to override
982 * the default by setting the `GSETTINGS_BACKEND` environment variable
983 * to the name of a settings backend.
984 *
985 * The user gets a reference to the backend.
986 *
987 * Returns: (transfer full): the default #GSettingsBackend
988 *
989 * Since: 2.28
990 */
991 GSettingsBackend *
992 g_settings_backend_get_default (void)
993 {
994 GSettingsBackend *backend;
995  
996 backend = _g_io_module_get_default (G_SETTINGS_BACKEND_EXTENSION_POINT_NAME,
997 "GSETTINGS_BACKEND",
998 g_settings_backend_verify);
999 return g_object_ref (backend);
1000 }
1001  
1002 /*< private >
1003 * g_settings_backend_get_permission:
1004 * @backend: a #GSettingsBackend
1005 * @path: a path
1006 *
1007 * Gets the permission object associated with writing to keys below
1008 * @path on @backend.
1009 *
1010 * If this is not implemented in the backend, then a %TRUE
1011 * #GSimplePermission is returned.
1012 *
1013 * Returns: a non-%NULL #GPermission. Free with g_object_unref()
1014 */
1015 GPermission *
1016 g_settings_backend_get_permission (GSettingsBackend *backend,
1017 const gchar *path)
1018 {
1019 GSettingsBackendClass *class = G_SETTINGS_BACKEND_GET_CLASS (backend);
1020  
1021 if (class->get_permission)
1022 return class->get_permission (backend, path);
1023  
1024 return g_simple_permission_new (TRUE);
1025 }
1026  
1027 /*< private >
1028 * g_settings_backend_sync_default:
1029 *
1030 * Syncs the default backend.
1031 */
1032 void
1033 g_settings_backend_sync_default (void)
1034 {
1035 if (g_settings_has_backend)
1036 {
1037 GSettingsBackendClass *class;
1038 GSettingsBackend *backend;
1039  
1040 backend = g_settings_backend_get_default ();
1041 class = G_SETTINGS_BACKEND_GET_CLASS (backend);
1042  
1043 if (class->sync)
1044 class->sync (backend);
1045 }
1046 }