Gnome-tweak-tool 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-tool-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
></tr></table>
<div class='path'>path: <a href='/guix/log/?id=42da990859d41a4da9f74e4d20fe5c5b972ada18'>root</a>/<a href='/guix/log/gnu?id=42da990859d41a4da9f74e4d20fe5c5b972ada18'>gnu</a>/<a href='/guix/log/gnu/packages?id=42da990859d41a4da9f74e4d20fe5c5b972ada18'>packages</a>/<a href='/guix/log/gnu/packages/cybersecurity.scm?id=42da990859d41a4da9f74e4d20fe5c5b972ada18'>cybersecurity.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/cybersecurity.scm?id=42da990859d41a4da9f74e4d20fe5c5b972ada18&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-02-15 18:09:39 +0100'>2021-02-15</span></td><td><a href='/guix/commit/gnu/packages/cybersecurity.scm?id=dbbc6c0059e7f9543e1cfc8a07140237c3e79401'>gnu: ropgadget: Update to 6.5.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-12-06 23:50:43 +0100'>2020-12-06</span></td><td><a href='/guix/commit/gnu/packages/cybersecurity.scm?id=71efb620cb2927f3028e4c2259ad5c3ff3fabd3b'>gnu: ropgadget: Update to 6.4.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-06-02 00:28:52 +0200'>2020-06-02</span></td><td><a href='/guix/commit/gnu/packages/cybersecurity.scm?id=d038100ef08a03b9294e03e19f76d187fe5f9abe'>gnu: ropgadget: Update to 6.3.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-03-26 22:53:02 +0100'>2020-03-26</span></td><td><a href='/guix/commit/gnu/packages/cybersecurity.scm?id=60eee3e6de2a11b425496bbdf90bfe1f807c8f49'>gnu: Add ROPgadget.</a><span class='msg-avail'>...</span></td><td>Jakub Kądziołka</td></tr>