glib/gio/gsettingsbackend.c

/*
 * Copyright © 2009, 2010 Codethink Limited
 * Copyright © 2010 Red Hat, Inc.
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, see <http://www.gnu.org/licenses/>.
 *
 * Authors: Ryan Lortie <desrt@desrt.ca>
 *          Matthias Clasen <mclasen@redhat.com>
 */

#include "config.h"

#include "gsettingsbackendinternal.h"
#include "gsimplepermission.h"
#include "giomodule-priv.h"

#include <string.h>
#include <stdlib.h>
#include <glib.h>
#include <glibintl.h>


typedef struct _GSettingsBackendClosure GSettingsBackendClosure;
typedef struct _GSettingsBackendWatch   GSettingsBackendWatch;

struct _GSettingsBackendPrivate
{
  GSettingsBackendWatch *watches;
  GMutex lock;
};

G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (GSettingsBackend, g_settings_backend, G_TYPE_OBJECT)

/* For g_settings_backend_sync_default(), we only want to actually do
 * the sync if the backend already exists.  This avoids us creating an
 * entire GSettingsBackend in order to call a do-nothing sync()
 * operation on it.  This variable lets us avoid that.
 */
static gboolean g_settings_has_backend;

/**
 * GSettingsBackend:
 *
 * The `GSettingsBackend` interface defines a generic interface for
 * non-strictly-typed data that is stored in a hierarchy. To implement
 * an alternative storage backend for [class@Gio.Settings], you need to
 * implement the `GSettingsBackend` interface and then make it implement the
 * extension point `G_SETTINGS_BACKEND_EXTENSION_POINT_NAME`.
 *
 * The interface defines methods for reading and writing values, a
 * method for determining if writing of certain values will fail
 * (lockdown) and a change notification mechanism.
 *
 * The semantics of the interface are very precisely defined and
 * implementations must carefully adhere to the expectations of
 * callers that are documented on each of the interface methods.
 *
 * Some of the `GSettingsBackend` functions accept or return a
 * [struct@GLib.Tree]. These trees always have strings as keys and
 * [struct@GLib.Variant] as values.
 *
 * The `GSettingsBackend` API is exported to allow third-party
 * implementations, but does not carry the same stability guarantees
 * as the public GIO API. For this reason, you have to define the
 * C preprocessor symbol `G_SETTINGS_ENABLE_BACKEND` before including
 * `gio/gsettingsbackend.h`.
 **/

static gboolean
is_key (const gchar *key)
{
  gint length;
  gint i;

  g_return_val_if_fail (key != NULL, FALSE);
  g_return_val_if_fail (key[0] == '/', FALSE);

  for (i = 1; key[i]; i++)
    g_return_val_if_fail (key[i] != '/' || key[i + 1] != '/', FALSE);

  length = i;

  g_return_val_if_fail (key[length - 1] != '/', FALSE);

  return TRUE;
}

static gboolean
is_path (const gchar *path)
{
  gint length;
  gint i;

  g_return_val_if_fail (path != NULL, FALSE);
  g_return_val_if_fail (path[0] == '/', FALSE);

  for (i = 1; path[i]; i++)
    g_return_val_if_fail (path[i] != '/' || path[i + 1] != '/', FALSE);

  length = i;

  g_return_val_if_fail (path[length - 1] == '/', FALSE);

  return TRUE;
}

struct _GSettingsBackendWatch
{
  /* Always access the target via the weak reference */
  GWeakRef                       target;
  /* The pointer is only for comparison from the weak notify,
   * at which point the target might already be close to
   * destroyed. It's not safe to use it for anything anymore
   * at that point */
  GObject                       *target_ptr;
  const GSettingsListenerVTable *vtable;
  GMainContext                  *context;
  GSettingsBackendWatch         *next;
};

struct _GSettingsBackendClosure
{
  void (*function) (GObject           *target,
                    GSettingsBackend  *backend,
                    const gchar       *name,
                    gpointer           origin_tag,
                    gchar            **names);

  GMainContext      *context;
  GObject           *target;
  GSettingsBackend  *backend;
  gchar             *name;
  gpointer           origin_tag;
  gchar            **names;
};

static void
g_settings_backend_watch_weak_notify (gpointer  data,
                                      GObject  *where_the_object_was)
{
  GSettingsBackend *backend = data;
  GSettingsBackendWatch **ptr;

  /* search and remove */
  g_mutex_lock (&backend->priv->lock);
  for (ptr = &backend->priv->watches; *ptr; ptr = &(*ptr)->next)
    if ((*ptr)->target_ptr == where_the_object_was)
      {
        GSettingsBackendWatch *tmp = *ptr;

        *ptr = tmp->next;
        g_weak_ref_clear (&tmp->target);
        g_slice_free (GSettingsBackendWatch, tmp);

        g_mutex_unlock (&backend->priv->lock);
        return;
      }

  /* we didn't find it.  that shouldn't happen. */
  g_assert_not_reached ();
}

/*< private >
 * g_settings_backend_watch:
 * @backend: a #GSettingsBackend
 * @target: the GObject (typically GSettings instance) to call back to
 * @context: (nullable): a #GMainContext, or %NULL
 * ...: callbacks...
 *
 * Registers a new watch on a #GSettingsBackend.
 *
 * note: %NULL @context does not mean "default main context" but rather,
 * "it is okay to dispatch in any context".  If the default main context
 * is specifically desired then it must be given.
 *
 * note also: if you want to get meaningful values for the @origin_tag
 * that appears as an argument to some of the callbacks, you *must* have
 * @context as %NULL.  Otherwise, you are subject to cross-thread
 * dispatching and whatever owned @origin_tag at the time that the event
 * occurred may no longer own it.  This is a problem if you consider that
 * you may now be the new owner of that address and mistakenly think
 * that the event in question originated from yourself.
 *
 * tl;dr: If you give a non-%NULL @context then you must ignore the
 * value of @origin_tag given to any callbacks.
 **/
void
g_settings_backend_watch (GSettingsBackend              *backend,
                          const GSettingsListenerVTable *vtable,
                          GObject                       *target,
                          GMainContext                  *context)
{
  GSettingsBackendWatch *watch;

  /* For purposes of discussion, we assume that our target is a
   * GSettings instance.
   *
   * Our strategy to defend against the final reference dropping on the
   * GSettings object in a thread other than the one that is doing the
   * dispatching is as follows:
   *
   *  1) hold a strong reference on the GSettings during an outstanding
   *     dispatch.  This ensures that the delivery is always possible while
   *     the GSettings object is alive, and if this was the last reference
   *     then it will be dropped from the dispatch thread.
   *
   *  2) hold a weak reference on the GSettings at other times.  This
   *     allows us to receive early notification of pending destruction
   *     of the object.  At this point, it is still safe to obtain a
   *     reference on the GObject to keep it alive, so #1 will work up
   *     to that point.  After that point, we'll have been able to drop
   *     the watch from the list.
   *
   * Note, in particular, that it's not possible to simply have an
   * "unwatch" function that gets called from the finalize function of
   * the GSettings instance because, by that point it is no longer
   * possible to keep the object alive using g_object_ref() and we would
   * have no way of knowing this.
   *
   * Note also that we need to hold a reference on the main context here
   * since the GSettings instance may be finalized before the closure runs.
   *
   * All access to the list holds a mutex.  We have some strategies to
   * avoid some of the pain that would be associated with that.
   */

  watch = g_slice_new (GSettingsBackendWatch);
  watch->context = context;
  watch->vtable = vtable;
  g_weak_ref_init (&watch->target, target);
  watch->target_ptr = target;
  g_object_weak_ref (target, g_settings_backend_watch_weak_notify, backend);

  /* linked list prepend */
  g_mutex_lock (&backend->priv->lock);
  watch->next = backend->priv->watches;
  backend->priv->watches = watch;
  g_mutex_unlock (&backend->priv->lock);
}

void
g_settings_backend_unwatch (GSettingsBackend *backend,
                            GObject          *target)
{
  /* Our caller surely owns a reference on 'target', so the order of
   * these two calls is unimportant.
   */
  g_object_weak_unref (target, g_settings_backend_watch_weak_notify, backend);
  g_settings_backend_watch_weak_notify (backend, target);
}

static gboolean
g_settings_backend_invoke_closure (gpointer user_data)
{
  GSettingsBackendClosure *closure = user_data;

  closure->function (closure->target, closure->backend, closure->name,
                     closure->origin_tag, closure->names);

  if (closure->context)
    g_main_context_unref (closure->context);
  g_object_unref (closure->backend);
  g_object_unref (closure->target);
  g_strfreev (closure->names);
  g_free (closure->name);

  g_slice_free (GSettingsBackendClosure, closure);

  return FALSE;
}

static void
g_settings_backend_dispatch_signal (GSettingsBackend    *backend,
                                    gsize                function_offset,
                                    const gchar         *name,
                                    gpointer             origin_tag,
                                    const gchar * const *names)
{
  GSettingsBackendWatch *watch;
  GSList *closures = NULL;

  /* We're in a little bit of a tricky situation here.  We need to hold
   * a lock while traversing the list, but we don't want to hold the
   * lock while calling back into user code.
   *
   * We work around this by creating a bunch of GSettingsBackendClosure
   * objects while holding the lock and dispatching them after.  We
   * never touch the list without holding the lock.
   */
  g_mutex_lock (&backend->priv->lock);
  for (watch = backend->priv->watches; watch; watch = watch->next)
    {
      GSettingsBackendClosure *closure;
      GObject *target = g_weak_ref_get (&watch->target);

      /* If the target was destroyed in the meantime, just skip it here */
      if (!target)
        continue;

      closure = g_slice_new (GSettingsBackendClosure);
      closure->context = watch->context;
      if (closure->context)
        g_main_context_ref (closure->context);
      closure->backend = g_object_ref (backend);
      closure->target = g_steal_pointer (&target);
      closure->function = G_STRUCT_MEMBER (void *, watch->vtable,
                                           function_offset);
      closure->name = g_strdup (name);
      closure->origin_tag = origin_tag;
      closure->names = g_strdupv ((gchar **) names);

      closures = g_slist_prepend (closures, closure);
    }
  g_mutex_unlock (&backend->priv->lock);

  while (closures)
    {
      GSettingsBackendClosure *closure = closures->data;

      if (closure->context)
        g_main_context_invoke (closure->context,
                               g_settings_backend_invoke_closure,
                               closure);
      else
        g_settings_backend_invoke_closure (closure);

      closures = g_slist_delete_link (closures, closures);
    }
}

/**
 * g_settings_backend_changed:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of the key
 * @origin_tag: the origin tag
 *
 * Signals that a single key has possibly changed.  Backend
 * implementations should call this if a key has possibly changed its
 * value.
 *
 * @key must be a valid key (ie starting with a slash, not containing
 * '//', and not ending with a slash).
 *
 * The implementation must call this function during any call to
 * g_settings_backend_write(), before the call returns (except in the
 * case that no keys are actually changed and it cares to detect this
 * fact).  It may not rely on the existence of a mainloop for
 * dispatching the signal later.
 *
 * The implementation may call this function at any other time it likes
 * in response to other events (such as changes occurring outside of the
 * program).  These calls may originate from a mainloop or may originate
 * in response to any other action (including from calls to
 * g_settings_backend_write()).
 *
 * In the case that this call is in response to a call to
 * g_settings_backend_write() then @origin_tag must be set to the same
 * value that was passed to that call.
 *
 * Since: 2.26
 **/
void
g_settings_backend_changed (GSettingsBackend *backend,
                            const gchar      *key,
                            gpointer          origin_tag)
{
  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_key (key));

  g_settings_backend_dispatch_signal (backend,
                                      G_STRUCT_OFFSET (GSettingsListenerVTable,
                                                       changed),
                                      key, origin_tag, NULL);
}

/**
 * g_settings_backend_keys_changed:
 * @backend: a #GSettingsBackend implementation
 * @path: the path containing the changes
 * @items: (array zero-terminated=1): the %NULL-terminated list of changed keys
 * @origin_tag: the origin tag
 *
 * Signals that a list of keys have possibly changed.  Backend
 * implementations should call this if keys have possibly changed their
 * values.
 *
 * @path must be a valid path (ie starting and ending with a slash and
 * not containing '//').  Each string in @items must form a valid key
 * name when @path is prefixed to it (ie: each item must not start or
 * end with '/' and must not contain '//').
 *
 * The meaning of this signal is that any of the key names resulting
 * from the contatenation of @path with each item in @items may have
 * changed.
 *
 * The same rules for when notifications must occur apply as per
 * g_settings_backend_changed().  These two calls can be used
 * interchangeably if exactly one item has changed (although in that
 * case g_settings_backend_changed() is definitely preferred).
 *
 * For efficiency reasons, the implementation should strive for @path to
 * be as long as possible (ie: the longest common prefix of all of the
 * keys that were changed) but this is not strictly required.
 *
 * Since: 2.26
 */
void
g_settings_backend_keys_changed (GSettingsBackend    *backend,
                                 const gchar         *path,
                                 gchar const * const *items,
                                 gpointer             origin_tag)
{
  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_path (path));

  /* XXX: should do stricter checking (ie: inspect each item) */
  g_return_if_fail (items != NULL);

  g_settings_backend_dispatch_signal (backend,
                                      G_STRUCT_OFFSET (GSettingsListenerVTable,
                                                       keys_changed),
                                      path, origin_tag, items);
}

/**
 * g_settings_backend_path_changed:
 * @backend: a #GSettingsBackend implementation
 * @path: the path containing the changes
 * @origin_tag: the origin tag
 *
 * Signals that all keys below a given path may have possibly changed.
 * Backend implementations should call this if an entire path of keys
 * have possibly changed their values.
 *
 * @path must be a valid path (ie starting and ending with a slash and
 * not containing '//').
 *
 * The meaning of this signal is that any of the key which has a name
 * starting with @path may have changed.
 *
 * The same rules for when notifications must occur apply as per
 * g_settings_backend_changed().  This call might be an appropriate
 * reasponse to a 'reset' call but implementations are also free to
 * explicitly list the keys that were affected by that call if they can
 * easily do so.
 *
 * For efficiency reasons, the implementation should strive for @path to
 * be as long as possible (ie: the longest common prefix of all of the
 * keys that were changed) but this is not strictly required.  As an
 * example, if this function is called with the path of "/" then every
 * single key in the application will be notified of a possible change.
 *
 * Since: 2.26
 */
void
g_settings_backend_path_changed (GSettingsBackend *backend,
                                 const gchar      *path,
                                 gpointer          origin_tag)
{
  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_path (path));

  g_settings_backend_dispatch_signal (backend,
                                      G_STRUCT_OFFSET (GSettingsListenerVTable,
                                                       path_changed),
                                      path, origin_tag, NULL);
}

/**
 * g_settings_backend_writable_changed:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of the key
 *
 * Signals that the writability of a single key has possibly changed.
 *
 * Since GSettings performs no locking operations for itself, this call
 * will always be made in response to external events.
 *
 * Since: 2.26
 **/
void
g_settings_backend_writable_changed (GSettingsBackend *backend,
                                     const gchar      *key)
{
  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_key (key));

  g_settings_backend_dispatch_signal (backend,
                                      G_STRUCT_OFFSET (GSettingsListenerVTable,
                                                       writable_changed),
                                      key, NULL, NULL);
}

/**
 * g_settings_backend_path_writable_changed:
 * @backend: a #GSettingsBackend implementation
 * @path: the name of the path
 *
 * Signals that the writability of all keys below a given path may have
 * changed.
 *
 * Since GSettings performs no locking operations for itself, this call
 * will always be made in response to external events.
 *
 * Since: 2.26
 **/
void
g_settings_backend_path_writable_changed (GSettingsBackend *backend,
                                          const gchar      *path)
{
  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_path (path));

  g_settings_backend_dispatch_signal (backend,
                                      G_STRUCT_OFFSET (GSettingsListenerVTable,
                                                       path_writable_changed),
                                      path, NULL, NULL);
}

typedef struct
{
  const gchar **keys;
  GVariant **values;
  gint prefix_len;
  gchar *prefix;
} FlattenState;

static gboolean
g_settings_backend_flatten_one (gpointer key,
                                gpointer value,
                                gpointer user_data)
{
  FlattenState *state = user_data;
  const gchar *skey = key;
  gint i;

  g_return_val_if_fail (is_key (key), TRUE);

  /* calculate longest common prefix */
  if (state->prefix == NULL)
    {
      gchar *last_byte;

      /* first key?  just take the prefix up to the last '/' */
      state->prefix = g_strdup (skey);
      last_byte = strrchr (state->prefix, '/') + 1;
      state->prefix_len = last_byte - state->prefix;
      *last_byte = '\0';
    }
  else
    {
      /* find the first character that does not match.  we will
       * definitely find one because the prefix ends in '/' and the key
       * does not.  also: no two keys in the tree are the same.
       */
      for (i = 0; state->prefix[i] == skey[i]; i++);

      /* check if we need to shorten the prefix */
      if (state->prefix[i] != '\0')
        {
          /* find the nearest '/', terminate after it */
          while (state->prefix[i - 1] != '/')
            i--;

          state->prefix[i] = '\0';
          state->prefix_len = i;
        }
    }


  /* save the entire item into the array.
   * the prefixes will be removed later.
   */
  *state->keys++ = key;

  if (state->values)
    *state->values++ = value;

  return FALSE;
}

/**
 * g_settings_backend_flatten_tree:
 * @tree: a #GTree containing the changes
 * @path: (out): the location to save the path
 * @keys: (out) (transfer container) (array zero-terminated=1): the
 *        location to save the relative keys
 * @values: (out) (optional) (transfer container) (array zero-terminated=1):
 *          the location to save the values, or %NULL
 *
 * Calculate the longest common prefix of all keys in a tree and write
 * out an array of the key names relative to that prefix and,
 * optionally, the value to store at each of those keys.
 *
 * You must free the value returned in @path, @keys and @values using
 * g_free().  You should not attempt to free or unref the contents of
 * @keys or @values.
 *
 * Since: 2.26
 **/
void
g_settings_backend_flatten_tree (GTree         *tree,
                                 gchar        **path,
                                 const gchar ***keys,
                                 GVariant    ***values)
{
  FlattenState state = { 0, };
  gsize nnodes;

  nnodes = g_tree_nnodes (tree);

  *keys = state.keys = g_new (const gchar *, nnodes + 1);
  state.keys[nnodes] = NULL;

  if (values != NULL)
    {
      *values = state.values = g_new (GVariant *, nnodes + 1);
      state.values[nnodes] = NULL;
    }

  g_tree_foreach (tree, g_settings_backend_flatten_one, &state);
  g_return_if_fail (*keys + nnodes == state.keys);

  *path = state.prefix;
  while (nnodes--)
    *--state.keys += state.prefix_len;
}

/**
 * g_settings_backend_changed_tree:
 * @backend: a #GSettingsBackend implementation
 * @tree: a #GTree containing the changes
 * @origin_tag: the origin tag
 *
 * This call is a convenience wrapper.  It gets the list of changes from
 * @tree, computes the longest common prefix and calls
 * g_settings_backend_changed().
 *
 * Since: 2.26
 **/
void
g_settings_backend_changed_tree (GSettingsBackend *backend,
                                 GTree            *tree,
                                 gpointer          origin_tag)
{
  const gchar **keys;
  gchar *path;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_settings_backend_flatten_tree (tree, &path, &keys, NULL);

#ifdef DEBUG_CHANGES
  {
    gint i;

    g_print ("----\n");
    g_print ("changed_tree(): prefix %s\n", path);
    for (i = 0; keys[i]; i++)
      g_print ("  %s\n", keys[i]);
    g_print ("----\n");
  }
#endif

  g_settings_backend_keys_changed (backend, path, keys, origin_tag);
  g_free (path);
  g_free (keys);
}

/*< private >
 * g_settings_backend_read:
 * @backend: a #GSettingsBackend implementation
 * @key: the key to read
 * @expected_type: a #GVariantType
 * @default_value: if the default value should be returned
 *
 * Reads a key. This call will never block.
 *
 * If the key exists, the value associated with it will be returned.
 * If the key does not exist, %NULL will be returned.
 *
 * The returned value will be of the type given in @expected_type.  If
 * the backend stored a value of a different type then %NULL will be
 * returned.
 *
 * If @default_value is %TRUE then this gets the default value from the
 * backend (ie: the one that the backend would contain if
 * g_settings_reset() were called).
 *
 * Returns: (nullable) (transfer full): the value that was read, or %NULL
 */
GVariant *
g_settings_backend_read (GSettingsBackend   *backend,
                         const gchar        *key,
                         const GVariantType *expected_type,
                         gboolean            default_value)
{
  GVariant *value;

  value = G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->read (backend, key, expected_type, default_value);

  if (value != NULL)
    value = g_variant_take_ref (value);

  if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))
    {
      g_variant_unref (value);
      value = NULL;
    }

  return value;
}

/*< private >
 * g_settings_backend_read_user_value:
 * @backend: a #GSettingsBackend implementation
 * @key: the key to read
 * @expected_type: a #GVariantType
 *
 * Reads the 'user value' of a key.
 *
 * This is the value of the key that the user has control over and has
 * set for themselves.  Put another way: if the user did not set the
 * value for themselves, then this will return %NULL (even if the
 * sysadmin has provided a default value).
 *
 * Returns: (nullable) (transfer full): the value that was read, or %NULL
 */
GVariant *
g_settings_backend_read_user_value (GSettingsBackend   *backend,
                                    const gchar        *key,
                                    const GVariantType *expected_type)
{
  GVariant *value;

  value = G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->read_user_value (backend, key, expected_type);

  if (value != NULL)
    value = g_variant_take_ref (value);

  if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))
    {
      g_variant_unref (value);
      value = NULL;
    }

  return value;
}

/*< private >
 * g_settings_backend_write:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of the key
 * @value: a #GVariant value to write to this key
 * @origin_tag: the origin tag
 *
 * Writes exactly one key.
 *
 * This call does not fail.  During this call a
 * #GSettingsBackend::changed signal will be emitted if the value of the
 * key has changed.  The updated key value will be visible to any signal
 * callbacks.
 *
 * One possible method that an implementation might deal with failures is
 * to emit a second "changed" signal (either during this call, or later)
 * to indicate that the affected keys have suddenly "changed back" to their
 * old values.
 *
 * If @value has a floating reference, it will be sunk.
 *
 * Returns: %TRUE if the write succeeded, %FALSE if the key was not writable
 */
gboolean
g_settings_backend_write (GSettingsBackend *backend,
                          const gchar      *key,
                          GVariant         *value,
                          gpointer          origin_tag)
{
  gboolean success;

  g_variant_ref_sink (value);
  success = G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->write (backend, key, value, origin_tag);
  g_variant_unref (value);

  return success;
}

/*< private >
 * g_settings_backend_write_tree:
 * @backend: a #GSettingsBackend implementation
 * @tree: a #GTree containing key-value pairs to write
 * @origin_tag: the origin tag
 *
 * Writes one or more keys.  This call will never block.
 *
 * The key of each item in the tree is the key name to write to and the
 * value is a #GVariant to write.  The proper type of #GTree for this
 * call can be created with g_settings_backend_create_tree().  This call
 * might take a reference to the tree; you must not modified the #GTree
 * after passing it to this call.
 *
 * This call does not fail.  During this call a #GSettingsBackend::changed
 * signal will be emitted if any keys have been changed.  The new values of
 * all updated keys will be visible to any signal callbacks.
 *
 * One possible method that an implementation might deal with failures is
 * to emit a second "changed" signal (either during this call, or later)
 * to indicate that the affected keys have suddenly "changed back" to their
 * old values.
 */
gboolean
g_settings_backend_write_tree (GSettingsBackend *backend,
                               GTree            *tree,
                               gpointer          origin_tag)
{
  return G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->write_tree (backend, tree, origin_tag);
}

/*< private >
 * g_settings_backend_reset:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of a key
 * @origin_tag: the origin tag
 *
 * "Resets" the named key to its "default" value (ie: after system-wide
 * defaults, mandatory keys, etc. have been taken into account) or possibly
 * unsets it.
 */
void
g_settings_backend_reset (GSettingsBackend *backend,
                          const gchar      *key,
                          gpointer          origin_tag)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->reset (backend, key, origin_tag);
}

/*< private >
 * g_settings_backend_get_writable:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of a key
 *
 * Finds out if a key is available for writing to.  This is the
 * interface through which 'lockdown' is implemented.  Locked down
 * keys will have %FALSE returned by this call.
 *
 * You should not write to locked-down keys, but if you do, the
 * implementation will deal with it.
 *
 * Returns: %TRUE if the key is writable
 */
gboolean
g_settings_backend_get_writable (GSettingsBackend *backend,
                                 const gchar      *key)
{
  return G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->get_writable (backend, key);
}

/*< private >
 * g_settings_backend_unsubscribe:
 * @backend: a #GSettingsBackend
 * @name: a key or path to subscribe to
 *
 * Reverses the effect of a previous call to
 * g_settings_backend_subscribe().
 */
void
g_settings_backend_unsubscribe (GSettingsBackend *backend,
                                const char       *name)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->unsubscribe (backend, name);
}

/*< private >
 * g_settings_backend_subscribe:
 * @backend: a #GSettingsBackend
 * @name: a key or path to subscribe to
 *
 * Requests that change signals be emitted for events on @name.
 */
void
g_settings_backend_subscribe (GSettingsBackend *backend,
                              const gchar      *name)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->subscribe (backend, name);
}

static void
g_settings_backend_finalize (GObject *object)
{
  GSettingsBackend *backend = G_SETTINGS_BACKEND (object);

  g_mutex_clear (&backend->priv->lock);

  G_OBJECT_CLASS (g_settings_backend_parent_class)
    ->finalize (object);
}

static void
ignore_subscription (GSettingsBackend *backend,
                     const gchar      *key)
{
}

static GVariant *
g_settings_backend_real_read_user_value (GSettingsBackend   *backend,
                                         const gchar        *key,
                                         const GVariantType *expected_type)
{
  return g_settings_backend_read (backend, key, expected_type, FALSE);
}

static void
g_settings_backend_init (GSettingsBackend *backend)
{
  backend->priv = g_settings_backend_get_instance_private (backend);
  g_mutex_init (&backend->priv->lock);
}

static void
g_settings_backend_class_init (GSettingsBackendClass *class)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (class);

  class->subscribe = ignore_subscription;
  class->unsubscribe = ignore_subscription;

  class->read_user_value = g_settings_backend_real_read_user_value;

  gobject_class->finalize = g_settings_backend_finalize;
}

static void
g_settings_backend_variant_unref0 (gpointer data)
{
  if (data != NULL)
    g_variant_unref (data);
}

/*< private >
 * g_settings_backend_create_tree:
 *
 * This is a convenience function for creating a tree that is compatible
 * with g_settings_backend_write().  It merely calls g_tree_new_full()
 * with strcmp(), g_free() and g_variant_unref().
 *
 * Returns: a new #GTree
 */
GTree *
g_settings_backend_create_tree (void)
{
  return g_tree_new_full ((GCompareDataFunc) strcmp, NULL,
                          g_free, g_settings_backend_variant_unref0);
}

static gboolean
g_settings_backend_verify (gpointer impl)
{
  GSettingsBackend *backend = impl;

  if (strcmp (G_OBJECT_TYPE_NAME (backend), "GMemorySettingsBackend") == 0 &&
      g_strcmp0 (g_getenv ("GSETTINGS_BACKEND"), "memory") != 0)
    {
      g_message ("Using the 'memory' GSettings backend.  Your settings "
		 "will not be saved or shared with other applications.");
    }

  g_settings_has_backend = TRUE;
  return TRUE;
}

/* We need to cache the default #GSettingsBackend for the entire process
 * lifetime, especially if the backend is #GMemorySettingsBackend: it needs to
 * keep the in-memory settings around even while there are no #GSettings
 * instances alive. */
static GSettingsBackend *settings_backend_default_singleton = NULL;  /* (owned) (atomic) */

/**
 * g_settings_backend_get_default:
 *
 * Returns the default #GSettingsBackend. It is possible to override
 * the default by setting the `GSETTINGS_BACKEND` environment variable
 * to the name of a settings backend.
 *
 * The user gets a reference to the backend.
 *
 * Returns: (not nullable) (transfer full): the default #GSettingsBackend,
 *     which will be a dummy (memory) settings backend if no other settings
 *     backend is available.
 *
 * Since: 2.28
 */
GSettingsBackend *
g_settings_backend_get_default (void)
{
  if (g_once_init_enter_pointer (&settings_backend_default_singleton))
    {
      GSettingsBackend *singleton;

      singleton = _g_io_module_get_default (G_SETTINGS_BACKEND_EXTENSION_POINT_NAME,
                                            "GSETTINGS_BACKEND",
                                            g_settings_backend_verify);

      g_once_init_leave_pointer (&settings_backend_default_singleton, singleton);
    }

  return g_object_ref (settings_backend_default_singleton);
}

/*< private >
 * g_settings_backend_get_permission:
 * @backend: a #GSettingsBackend
 * @path: a path
 *
 * Gets the permission object associated with writing to keys below
 * @path on @backend.
 *
 * If this is not implemented in the backend, then a %TRUE
 * #GSimplePermission is returned.
 *
 * Returns: (not nullable) (transfer full): a non-%NULL #GPermission.
 *     Free with g_object_unref()
 */
GPermission *
g_settings_backend_get_permission (GSettingsBackend *backend,
                                   const gchar      *path)
{
  GSettingsBackendClass *class = G_SETTINGS_BACKEND_GET_CLASS (backend);

  if (class->get_permission)
    return class->get_permission (backend, path);

  return g_simple_permission_new (TRUE);
}

/*< private >
 * g_settings_backend_sync_default:
 *
 * Syncs the default backend.
 */
void
g_settings_backend_sync_default (void)
{
  if (g_settings_has_backend)
    {
      GSettingsBackendClass *class;
      GSettingsBackend *backend;

      backend = g_settings_backend_get_default ();
      class = G_SETTINGS_BACKEND_GET_CLASS (backend);

      if (class->sync)
        class->sync (backend);

      g_object_unref (backend);
    }
}