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
r></table>
<div class='path'>path: <a href='/guix/log/?id=d8ac82c209a786c39de20102cd90810b4ddb0f44'>root</a>/<a href='/guix/log/gnu?id=d8ac82c209a786c39de20102cd90810b4ddb0f44'>gnu</a>/<a href='/guix/log/gnu/packages?id=d8ac82c209a786c39de20102cd90810b4ddb0f44'>packages</a>/<a href='/guix/log/gnu/packages/cyrus-sasl.scm?id=d8ac82c209a786c39de20102cd90810b4ddb0f44'>cyrus-sasl.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/packages/cyrus-sasl.scm?id=d8ac82c209a786c39de20102cd90810b4ddb0f44&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-11-11 05:17:58 +0100'>2017-11-11</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=9629c6b4b2b75884fdac79877dbd8d7a451b1fd1'>gnu: cyrus-sasl: Use new home page.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2017-01-23 22:33:10 +0100'>2017-01-23</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=58ea4d407c2e4adbe51b2d7b71dc8bef095677c7'>Merge branch 'master' into core-updates</a></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-19 00:11:15 +0100'>2017-01-19</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=89e34644d94860fafc053b759961ba8b55a6c77c'>gnu: Move Kerberos implemetations to (gnu packages kerberos).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-11-26 16:23:42 -0500'>2016-11-26</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=e30abbbb02583021eaf7f0533f234ea92f291d55'>gnu: cyrus-sasl: Incorporate grafted patch.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2016-11-26 16:21:02 -0500'>2016-11-26</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=cd65d600ac6e8701ef9c54f5d09a45cd6c149949'>gnu: cyrus-sasl: Fix CVE-2013-4122.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2016-05-30 12:53:23 -0400'>2016-05-30</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=3d55d04ab8f11cf8e9612c0ef92b10347a867309'>gnu: cyrus-sasl: Update URLs.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2015-10-29 19:07:58 +0100'>2015-10-29</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=b416aadf7455470b702a557f4f475b04198ce698'>gnu: Move gdbm to (gnu packages databases).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2015-07-05 18:27:24 +0200'>2015-07-05</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=cc2b77dfc715c2133bd0c9e318bd7dc90bfedece'>gnu: Move content of openssl module into tls module.</a><span class='msg-avail'>...</span></td><td>Andreas Enge</td></tr>
<tr><td><span title='2015-03-14 19:27:14 +0100'>2015-03-14</span></td><td><a href='/guix/commit/gnu/packages/cyrus-sasl.scm?id=166191b377e2e6117ab2a620e759724f7e446678'>gnu: Update packages to use 'non-copyleft' instead of 'bsd-style'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>