Gnome-tweaks does not look at GSETTINGS_SCHEMA_PATH or XDG_DATA_DIRS, it
assumes that schemas are installed in one global directory
(GSETTINGS_SCHEMA_DIR/gsettingsschemadir).

Guix/GuixSD uses a different directory for every gir package and has
packages pick-up files using XDG_DATA_DIRS.

Upstream ticket: https://bugzilla.gnome.org/show_bug.cgi?id=764537
janneke@gnu.org

--- gnome-tweak-3.18.1.orig/gtweak/gsettings.py	2015-04-08 15:21:32.000000000 +0200
+++ gnome-tweak-tool-3.18.1/gtweak/gsettings.py	2016-04-03 11:26:38.658482704 +0200
@@ -16,7 +16,8 @@
 # along with gnome-tweak-tool.  If not, see <http://www.gnu.org/licenses/>.
 
 import logging
-import os.path
+import os
+import sys
 import xml.dom.minidom
 import gettext
 
@@ -31,6 +32,13 @@
 class GSettingsMissingError(Exception):
     pass
 
+def file_from_path(path, file_name):
+    for dir in path:
+        f = os.path.join(dir, file_name)
+        if os.path.exists(f):
+            return f
+    return None
+
 class _GSettingsSchema:
     def __init__(self, schema_name, schema_dir=None, schema_filename=None, **options):
         if not schema_dir:
@@ -38,9 +46,14 @@
         if not schema_filename:
             schema_filename = schema_name + ".gschema.xml"
 
+        schema_prefix = os.path.join('glib-2.0', 'schemas')
         schema_path = os.path.join(schema_dir, schema_filename)
         if not os.path.exists(schema_path):
-            logging.critical("Could not find schema %s" % schema_path)
+            schema_path = file_from_path(os.environ.get ('GSETTINGS_SCHEMA_PATH', '').split(os.path.pathsep), schema_filename)
+        if not (schema_path and os.path.exists(schema_path)):
+            schema_path = file_from_path(os.environ.get ('XDG_DATA_DIRS', '').split(os.path.pathsep), os.path.join(schema_prefix, schema_filename))
+        if not (schema_path and os.path.exists(schema_path)):
+            logging.critical("Could not find schema %s" % schema_filename)
             assert(False)
 
         self._schema_name = schema_name
v class='path'>path: <a href='/guix/log/?id=3f17ee680e433300322e687dd9c0d6e1a3700727'>root</a>/<a href='/guix/log/gnu?id=3f17ee680e433300322e687dd9c0d6e1a3700727'>gnu</a>/<a href='/guix/log/gnu/installer?id=3f17ee680e433300322e687dd9c0d6e1a3700727'>installer</a>/<a href='/guix/log/gnu/installer/connman.scm?id=3f17ee680e433300322e687dd9c0d6e1a3700727'>connman.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/installer/connman.scm?id=3f17ee680e433300322e687dd9c0d6e1a3700727&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-09-21 00:31:15 +0200'>2020-09-21</span></td><td><a href='/guix/commit/gnu/installer/connman.scm?id=e1f3788944ee154ba9b33c891c4ccf039a1ff39a'>installer: Fix docstring typoes.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/connman.scm (connman-state, run-locale-page)
(start-swapping, stop-swapping, run-installer-steps): Fix typo in
docstring.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-04-19 11:46:21 +0200'>2020-04-19</span></td><td><a href='/guix/commit/gnu/installer/connman.scm?id=ea6594e08e2724c64bc07724a07479fc1633dede'>installer: connman: Fix Wifi connection.</a><span class='msg-avail'>...<span class='msg-tooltip'>Issue reported by SergioBG here:
https://lists.gnu.org/archive/html/bug-guix/2020-04/msg00432.html.

Due to default buffering mode, the installer was reading from the pipe without
having actually written a command before.

* gnu/installer/connman.scm (connman-connect-with-auth): Set pipe buffering
mode to 'line.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-05-29 11:44:49 +0200'>2019-05-29</span></td><td><a href='/guix/commit/gnu/installer/connman.scm?id=694cb8dc62f59fb67799ad4908a6b12cf7cb49a3'>installer: Fix wifi menu crash with hidden SSIDs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This fixes https://issues.guix.gnu.org/issue/35622.

* gnu/installer/connman.scm (&lt;service&gt;): Mention that name may be false.
* gnu/installer/newt/wifi.scm (wifi-services): Filter out wifi services
without name.

Co-authored by: Pierre Neidhardt &lt;mail@ambrevar.xyz&gt;
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2019-02-18 01:52:47 +0100'>2019-02-18</span></td><td><a href='/guix/commit/gnu/installer/connman.scm?id=b83e4a93a2118513cdd2186d796729f0813264c0'>gnu: Fix some typos in the installer.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/installer/connman.scm (connman-connect-with-auth): Fix typo in
documentation.
* gnu/installer/newt/keymap.scm (sort-variants): Likewise.
* gnu/installer/newt/page.scm (run-listbox-selection-page): Likewise.
* gnu/installer/parted.scm (mkpart): Likewise.
* gnu/installer/newt/utils.scm (destroy-form-and-pop): Likewise.
* gnu/installer/newt/wifi.scm (run-unknown-error-page): Fix typo.
(wifi-listbox-heigth): Rename to…
(wifi-listbox-height): …this, and adjust caller.
* gnu/installer/timezone.scm (locate-childrens): Rename to…
(locate-children): …this.  Adjust all callers.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-01-17 14:04:20 +0100'>2019-01-17</span></td><td><a href='/guix/commit/gnu/installer/connman.scm?id=d0f3a672dcbdfefd3556b6a21985ff0e35eed3be'>gnu: Add graphical installer support.</a><span class='msg-avail'>...<span class='msg-tooltip'>* configure.ac: Require that guile-newt is available.
* gnu/installer.scm: New file.
* gnu/installer/aux-files/logo.txt: New file.
* gnu/installer/build-installer.scm: New file.
* gnu/installer/connman.scm: New file.
* gnu/installer/keymap.scm: New file.
* gnu/installer/locale.scm: New file.
* gnu/installer/newt.scm: New file.
* gnu/installer/newt/ethernet.scm: New file.
* gnu/installer/newt/hostname.scm: New file.
* gnu/installer/newt/keymap.scm: New file.
* gnu/installer/newt/locale.scm: New file.
* gnu/installer/newt/menu.scm: New file.
* gnu/installer/newt/network.scm: New file.
* gnu/installer/newt/page.scm: New file.
* gnu/installer/newt/timezone.scm: New file.
* gnu/installer/newt/user.scm: New file.
* gnu/installer/newt/utils.scm: New file.
* gnu/installer/newt/welcome.scm: New file.
* gnu/installer/newt/wifi.scm: New file.
* gnu/installer/steps.scm: New file.
* gnu/installer/timezone.scm: New file.
* gnu/installer/utils.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add previous files.
* gnu/system.scm: Export %root-account.
* gnu/system/install.scm (%installation-services): Use kmscon instead of linux
VT for all tty.
(installation-os)[users]: Add the graphical installer as shell of the root
account.
[packages]: Add font related packages.
* po/guix/POTFILES.in: Add installer files.
</span></span></td><td>Mathieu Othacehe</td></tr>