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
d=6e6769dde0a3e613784834aeded53b3aaa2e4cec'>root</a>/<a href='/guix/log/tests?id=6e6769dde0a3e613784834aeded53b3aaa2e4cec'>tests</a>/<a href='/guix/log/tests/guix-build.sh?id=6e6769dde0a3e613784834aeded53b3aaa2e4cec'>guix-build.sh</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/tests/guix-build.sh?id=6e6769dde0a3e613784834aeded53b3aaa2e4cec&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-03-23 23:19:57 -0700'>2021-03-23</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=a16eb6c5f97f136b678540ba61f12b2c08e43e13'>Add powerpc64le-linux as a supported Guix architecture.</a><span class='msg-avail'>...</span></td><td>Chris Marusich</td></tr>
<tr><td><span title='2020-10-20 16:30:15 +0200'>2020-10-20</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=8db4ebb0cd9bfdcf1aea63eb8d20eb6af0c87c93'>packages: Better preserve object identity when rewriting.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-10-15 18:50:22 +0200'>2020-10-15</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=5ef1508942ee083ed22b844f5291e59320016b79'>ui: Only suggest modules that export the unbound variable identifier.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-09-28 21:06:36 -0500'>2020-09-28</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=d8934360d2453a403b5433e71d09188e4ed23b57'>tests: Simplify shell exit status negation;</a><span class='msg-avail'>...</span></td><td>Eric Bavier</td></tr>
<tr><td><span title='2020-09-27 21:41:08 +0200'>2020-09-27</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=2bf6f962b91123b0474c0f7123cd17efe7f09a66'>packages: 'package-input-rewriting/spec' can rewrite implicit dependencies.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-04-11 00:32:41 +0200'>2020-04-11</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=e85d4cecbe253e59a8a2a42b6ce427d96ff10534'>gnu: commencement: Memoize packages as a function of the system.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-05 16:33:37 +0100'>2020-03-05</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=5a675b2c67825e581cd8193f643f66c4cb1ea1e4'>guix build: Allow non-package objects in manifest.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-02-14 17:49:26 +0100'>2020-02-14</span></td><td><a href='/guix/commit/tests/guix-build.sh?id=11415d35064cdba5cec1139aede18099cfa14547'>guix build: Add '--manifest' option.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>