fix: replace the system icon cache whole

The cache was truncated and dumped in place. A concurrent build read
half a file, took it for empty, and an offline run then wrote {} over
every recorded icon. The file is now replaced through a scratch file,
merged with what another run recorded meanwhile, and left alone when
nothing new was learned.

tests/test_site_render.py

@@ -0,0 +1,84 @@
+"""Renderings shared by the site pages, and the icon cache they read.
+
+No network: icon checks are answered by a stub.
+"""
+
+from __future__ import annotations
+
+import json
+import sys
+import tempfile
+import unittest
+import urllib.error
+from pathlib import Path
+from unittest import mock
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+import siterender  # noqa: E402
+
+
+class _Served:
+    status = 200
+
+    def __enter__(self) -> "_Served":
+        return self
+
+    def __exit__(self, *exc: object) -> None:
+        return None
+
+
+class IconCacheIsReplacedWhole(unittest.TestCase):
+    """The cache was truncated then dumped in place: a concurrent reader saw
+    half a file, took it for empty, and an offline run then wrote ``{}``
+    over every recorded icon."""
+
+    def setUp(self) -> None:
+        tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(tmp.cleanup)
+        self.path = Path(tmp.name) / "system_icons.json"
+        patcher = mock.patch.object(siterender, "ICON_CACHE_PATH", self.path)
+        patcher.start()
+        self.addCleanup(patcher.stop)
+        saved = dict(siterender._icon_available)
+        self.addCleanup(lambda: (siterender._icon_available.clear(),
+                                 siterender._icon_available.update(saved)))
+
+    def test_an_offline_run_leaves_an_unreadable_cache_untouched(self):
+        self.path.write_text('{"Sega - Mega Drive": tr')
+        offline = mock.patch.object(
+            siterender.urllib.request, "urlopen",
+            side_effect=urllib.error.URLError("offline"))
+        with offline, mock.patch("sys.stderr"), mock.patch("builtins.print"):
+            siterender.prime_system_icons({"Sega - Mega Drive"})
+        self.assertEqual(self.path.read_text(), '{"Sega - Mega Drive": tr')
+
+    def test_names_another_run_recorded_meanwhile_are_kept(self):
+        self.path.write_text(json.dumps({"A - One": True}))
+
+        def answer(*_args: object, **_kwargs: object) -> _Served:
+            # A second build records its own check while this one runs.
+            self.path.write_text(json.dumps({"A - One": True, "C - Three": False}))
+            return _Served()
+
+        with mock.patch.object(siterender.urllib.request, "urlopen", side_effect=answer), \
+                mock.patch("builtins.print"):
+            siterender.prime_system_icons({"A - One", "B - Two"})
+        self.assertEqual(json.loads(self.path.read_text()),
+                         {"A - One": True, "B - Two": True, "C - Three": False})
+        self.assertEqual(sorted(p.name for p in self.path.parent.iterdir()),
+                         ["system_icons.json"])
+
+    def test_the_write_goes_through_a_scratch_file(self):
+        with mock.patch.object(siterender.urllib.request, "urlopen",
+                               return_value=_Served()), \
+                mock.patch.object(siterender, "write_text_atomic",
+                                  wraps=siterender.write_text_atomic) as atomic, \
+                mock.patch("builtins.print"):
+            siterender.prime_system_icons({"B - Two"})
+        atomic.assert_called_once()
+
+
+if __name__ == "__main__":
+    unittest.main()