test(nix): run font compatibility regressions in CI

.github/workflows/source-check.yml

@@ -21,6 +21,8 @@ jobs:
       - run: python tools/riot-launcher-offline-check.py
       - run: python tools/riot-runtime-cleanup-offline-check.py
       - run: python tools/riot-cdn-dns-offline-check.py
+      - run: python -m unittest discover -s tests -p 'test_nix_fonts.py'
+      - run: python -m unittest discover -s tests -p 'test_nix_repairs.py'
       - run: python -m unittest discover -s tests -p 'test_hidden_riot_recovery.py'
       - run: python -m unittest discover -s tests -p 'test_desktop_run.py'
       - run: python -m unittest discover -s tests -p 'test_urlsession_runner.py'

docs/NIX.md

@@ -215,9 +215,10 @@ the host's `/usr/share/fonts`, which the FHS environment replaces. Fonts in your
 user font directories remain available. This also prevents an icon-only user
 font inventory from becoming the only fallback for ordinary text.
 
-Inherited `FONTCONFIG_PATH` and `FONTCONFIG_SYSROOT` overrides are cleared inside
-the FHS environment so foreign include paths or an invalid sysroot cannot hide
-the packaged fonts. Custom XDG font/configuration directories and legacy
+The FHS environment selects the packaged fontconfig configuration and clears
+inherited `FONTCONFIG_PATH` and `FONTCONFIG_SYSROOT` overrides. Clearing the
+sysroot prevents it from redirecting the packaged configuration and font paths.
+Custom XDG font/configuration directories and legacy
 `~/.fonts` remain supported, including paths with spaces. The host configuration
 and user font files are not modified.
 

flake.nix

@@ -19,6 +19,8 @@
         fontDirectories = darlingFonts;
         includes = [
           "${pkgs.fontconfig.out}/etc/fonts/conf.d"
+          # makeFontsConf adds XDG font directories, but neither it nor
+          # conf.d/50-user.conf adds the legacy ~/.fonts directory.
           (pkgs.writeText "darling-user-fonts.conf" ''
             <fontconfig>
               <dir>~/.fonts</dir>

tests/test_nix_fonts.py

@@ -88,9 +88,9 @@ class NixFontsConfigTest(unittest.TestCase):
 
                 self.assertNotIn("FONTCONFIG_SYSROOT", resulting_env)
                 self.assertNotIn("FONTCONFIG_PATH", resulting_env)
-                self.assertTrue(
-                    resulting_env.get("FONTCONFIG_FILE", "").startswith("/nix/store/"),
-                    f"Unexpected FONTCONFIG_FILE: {resulting_env.get('FONTCONFIG_FILE')}",
+                self.assertEqual(
+                    resulting_env.get("FONTCONFIG_FILE"),
+                    "/nix/store/mock-darlingFontsConf",
                 )
                 self.assertEqual(resulting_env.get("XDG_DATA_HOME"), "/tmp/mock-xdg data with spaces")
                 self.assertEqual(resulting_env.get("XDG_CONFIG_HOME"), "/tmp/mock-xdg config with spaces")

tests/test_nix_repairs.py

@@ -454,7 +454,8 @@ class SessionTests(unittest.TestCase):
             self.assertIn('terminal', result.stderr.lower())
             self.assertFalse((root / 'build').exists())
         self.assertIn('Terminal=true', (ROOT / 'flake.nix').read_text())
-        self.assertIn('checks.${system}', (ROOT / 'flake.nix').read_text())
+        self.assertIn('packaging = pkgs.runCommand "darling-lol-packaging-tests"',
+                      (ROOT / 'flake.nix').read_text())
 
     def test_rootless_update_refuses_live_marker_without_build_or_stop(self):
         with tempfile.TemporaryDirectory(prefix='nix-update-idle-') as directory: