fix: write artifacts whole or not at all

tests/test_atomic_artifacts.py

@@ -0,0 +1,69 @@
+"""Generated artifacts are written whole or not at all.
+
+write_if_changed got the scratch-and-rename treatment, but the install
+manifests, the target manifests and release.json were still truncated at
+their final name, and restore_large_files copied an asset straight to its
+path in bios/: a scan running beside the copy hashed the truncated file.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+from unittest import mock
+
+REPO_ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(REPO_ROOT / "scripts"))
+
+import artifacts  # noqa: E402
+
+
+class WholeOrNothing(unittest.TestCase):
+    def setUp(self):
+        self._tmp = tempfile.TemporaryDirectory()
+        self.dir = Path(self._tmp.name)
+
+    def tearDown(self):
+        self._tmp.cleanup()
+
+    def test_an_interrupted_write_keeps_the_previous_file(self):
+        target = self.dir / "manifest.json"
+        target.write_text("{}")
+        with mock.patch("os.replace", side_effect=OSError("cut")), self.assertRaises(OSError):
+            artifacts.write_text_atomic(str(target), "{\"files\": []}")
+        self.assertEqual(target.read_text(), "{}")
+        self.assertEqual(os.listdir(self.dir), ["manifest.json"])
+
+    def test_an_interrupted_copy_leaves_no_file(self):
+        source = self.dir / "asset.bin"
+        source.write_bytes(b"x" * 1000)
+        target = self.dir / "bios" / "asset.bin"
+        target.parent.mkdir()
+        with mock.patch("shutil.copy2", side_effect=OSError("cut")), self.assertRaises(OSError):
+            artifacts.copy_file_atomic(str(source), str(target))
+        self.assertEqual(os.listdir(target.parent), [])
+
+
+class EveryWriterGoesThroughIt(unittest.TestCase):
+    def test_manifests_record_and_restore_use_the_atomic_helpers(self):
+        scripts = REPO_ROOT / "scripts"
+        for name, helper in (
+            ("generate_pack.py", "write_text_atomic("),
+            ("release_record.py", "write_text_atomic("),
+            ("restore_large_files.py", "copy_file_atomic("),
+        ):
+            source = (scripts / name).read_text(encoding="utf-8")
+            with self.subTest(script=name):
+                self.assertIn(helper, source)
+        pack = (scripts / "generate_pack.py").read_text(encoding="utf-8")
+        self.assertNotIn('with open(path, "w") as f:\n        f.write(new_json)', pack)
+        self.assertNotIn("json.dump(result, f", pack)
+        self.assertNotIn("args.output.write_text(", (scripts / "release_record.py").read_text())
+        self.assertNotIn("shutil.copy2(source, path)", (scripts / "restore_large_files.py").read_text())
+
+
+if __name__ == "__main__":
+    unittest.main()