fix: swap a pack in only once it is complete

Packs were opened on their final name, so a rebuild truncated the
previous one and a build that failed half way left a readable ZIP
of part of the files. Each pack is now written beside its name and
replaces it only when the write finished.

scripts/generate_pack.py

@@ -182,6 +182,29 @@ def _narrowings(
     return applied
 
 
+@contextlib.contextmanager
+def _writing_pack(zip_path: str) -> Iterator[zipfile.ZipFile]:
+    """A pack written beside its final name, swapped in once complete.
+
+    Opened on the final name, a rebuild truncated the previous pack at once,
+    and a build that raised half way still wrote a central directory: a
+    readable ZIP holding part of the files, which the run then checked and
+    summed as the pack.
+    """
+    directory = os.path.dirname(zip_path) or "."
+    fd, scratch = tempfile.mkstemp(
+        prefix=f".{os.path.basename(zip_path)}.", suffix=".partial", dir=directory
+    )
+    os.close(fd)
+    try:
+        with zipfile.ZipFile(scratch, "w", zipfile.ZIP_DEFLATED) as zf:
+            yield zf
+        os.replace(scratch, zip_path)
+    except BaseException:
+        Path(scratch).unlink(missing_ok=True)
+        raise
+
+
 def _target_tag(target_name: str) -> str:
     """Filename tag for a hardware target.
 
@@ -735,7 +758,7 @@ def generate_pack(
         target_cores, source, regions, one_per_slot, required_only,
     )
 
-    with zipfile.ZipFile(zip_path, "w", zipfile.ZIP_DEFLATED) as zf:
+    with _writing_pack(zip_path) as zf:
       if source != "truth":
         for sys_id, system in sorted(pack_systems.items()):
             for file_entry in system.get("files", []):
@@ -1385,7 +1408,7 @@ def generate_emulator_pack(
         platforms_dir or os.path.join(os.path.dirname(__file__), "..", "platforms")
     )
 
-    with zipfile.ZipFile(zip_path, "w", zipfile.ZIP_DEFLATED) as zf:
+    with _writing_pack(zip_path) as zf:
         for emu_name, profile in sorted(selected):
             pack_structure = profile.get("pack_structure")
             files = expand_directory_entries(
@@ -2014,7 +2037,7 @@ def generate_md5_pack(
     not_in_db: list[str] = []
     seen: set[str] = set()
 
-    with zipfile.ZipFile(zip_path, "w", zipfile.ZIP_DEFLATED) as zf:
+    with _writing_pack(zip_path) as zf:
         for hash_type, hash_val in hashes:
             sha1 = None
             if hash_type == "sha1" and hash_val in files_db:

tests/test_audit_regressions.py

@@ -407,6 +407,56 @@ class OnePlatformPerFlag(unittest.TestCase):
         self.assertIn("batocera", str(caught.exception))
 
 
+class APackIsSwappedInWhole(unittest.TestCase):
+    """Packs were opened on their final name: a rebuild truncated the good
+    pack at once, and a build that raised half way left a readable ZIP of
+    part of the files, which the run then verified and summed."""
+
+    def _dir(self) -> Path:
+        import shutil
+        import tempfile
+
+        (ROOT / "tmp").mkdir(exist_ok=True)
+        directory = Path(tempfile.mkdtemp(dir=ROOT / "tmp"))
+        self.addCleanup(shutil.rmtree, directory, True)
+        return directory
+
+    def test_a_failed_build_keeps_the_previous_pack(self):
+        import zipfile
+
+        from generate_pack import _writing_pack
+
+        directory = self._dir()
+        pack = directory / "Pack.zip"
+        with zipfile.ZipFile(pack, "w") as zf:
+            zf.writestr("old.bin", b"complete")
+        with self.assertRaises(OSError):
+            with _writing_pack(str(pack)) as zf:
+                zf.writestr("new.bin", b"half")
+                raise OSError("disk full")
+        with zipfile.ZipFile(pack) as zf:
+            self.assertEqual(zf.namelist(), ["old.bin"])
+        self.assertEqual(sorted(p.name for p in directory.iterdir()), ["Pack.zip"])
+
+    def test_a_finished_build_replaces_it(self):
+        import zipfile
+
+        from generate_pack import _writing_pack
+
+        directory = self._dir()
+        pack = directory / "Pack.zip"
+        pack.write_bytes(b"stale")
+        with _writing_pack(str(pack)) as zf:
+            zf.writestr("new.bin", b"whole")
+        with zipfile.ZipFile(pack) as zf:
+            self.assertEqual(zf.namelist(), ["new.bin"])
+        self.assertEqual(sorted(p.name for p in directory.iterdir()), ["Pack.zip"])
+
+    def test_no_pack_is_opened_on_its_final_name(self):
+        source = (ROOT / "scripts" / "generate_pack.py").read_text()
+        self.assertNotIn('ZipFile(zip_path, "w"', source)
+
+
 class FaqRegressions(unittest.TestCase):
     """The FAQ states facts the code owns; these tie it back to the source.