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
'>root</a>/<a href='/guix/log/gnu?id=b3175c709d7fb2e8519902c3dd66608436d0f0ff'>gnu</a>/<a href='/guix/log/gnu/tests.scm?id=b3175c709d7fb2e8519902c3dd66608436d0f0ff'>tests.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/tests.scm?id=b3175c709d7fb2e8519902c3dd66608436d0f0ff&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-08-29 23:32:01 +0200'>2018-08-29</span></td><td><a href='/guix/commit/gnu/tests.scm?id=d258c791441b46705f4360cf141343363d1751f2'>tests: Warn about test module load failures.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-05-28 13:24:19 +0200'>2018-05-28</span></td><td><a href='/guix/commit/gnu/tests.scm?id=9ceeca0880248eee8c2796f5e6ebc172b95277f7'>system: Remove uses of the 'title' field of &lt;file-system&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-02-19 22:04:11 +0100'>2018-02-19</span></td><td><a href='/guix/commit/gnu/tests.scm?id=27a2c9c3e071fdb380c2f4f389b7cf4008dc75f7'>marionette: Use QEMU's "VM channel" mechanism.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-01-09 20:02:26 +0100'>2018-01-09</span></td><td><a href='/guix/commit/gnu/tests.scm?id=16d78a8f7207271e662d9152150b0cf790492373'>tests: marionette-operating-system: Add initrd parameter and kernel-arguments.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>
<tr><td><span title='2017-12-06 15:02:24 +0100'>2017-12-06</span></td><td><a href='/guix/commit/gnu/tests.scm?id=fdfdecdb7ebfd05b4ae66cc5140078dbfc10fc35'>gnu, doc, tests: Use ‘bootloader-configuration’ everywhere.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-08-23 22:23:19 +0200'>2017-08-23</span></td><td><a href='/guix/commit/gnu/tests.scm?id=045ebb3e58d74c75f39ce47380045d3cd00359c3'>gnu: bootloader: Deprecate "device" field in favor of "target".</a><span class='msg-avail'>...</span></td><td>Andy Wingo</td></tr>
<tr><td><span title='2017-05-16 14:41:01 +0200'>2017-05-16</span></td><td><a href='/guix/commit/gnu/tests.scm?id=b09a8da4a2e50845a297e041762f3ff9e649c047'>bootloader: Add extlinux support.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2017-05-04 18:05:05 +0200'>2017-05-04</span></td><td><a href='/guix/commit/gnu/tests.scm?id=67d84d6335ae84ac7c217061570e6b968060073c'>tests: Use 'fold-module-public-variables' for discovery.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-05-03 23:50:15 +0200'>2017-05-03</span></td><td><a href='/guix/commit/gnu/tests.scm?id=cd903ef7871170d3c4eced45418459d293ef48a7'>Add (guix discovery).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-04-01 00:45:18 +0200'>2017-04-01</span></td><td><a href='/guix/commit/gnu/tests.scm?id=892d9089a88abaa2ef1127f16308d03f4f08a4ce'>tests: Introduce 'simple-operating-system' and use it.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>