fix: keep the cache when an archive yields nothing

scripts/refresh_data_dirs.py

@@ -210,6 +210,15 @@ def get_remote_sha(source_url: str, version: str) -> str | None:
         return None
 
 
+class NothingExtracted(Exception):
+    """An archive that yields no file for the cache.
+
+    Promoting it replaced the cache with an empty directory, recorded the new
+    version and reported success, so the next run read "up to date" and the
+    packs left without the directory.
+    """
+
+
 def _is_safe_tar_member(member: tarfile.TarInfo, dest: Path) -> bool:
     """Reject path traversal, absolute paths, and symlinks in tar members."""
     if member.issym() or member.islnk():
@@ -293,6 +302,8 @@ def _download_and_extract(
                             shutil.copyfileobj(src, dst)
                     file_count += 1
 
+        if not file_count:
+            raise NothingExtracted(f"no file under {source_path} in the archive")
         _promote(extract_dir, cache_dir, Path(tmpdir))
 
     return file_count
@@ -354,6 +365,8 @@ def _download_and_extract_zip(
                     shutil.copyfileobj(src, dst)
                 file_count += 1
 
+        if not file_count:
+            raise NothingExtracted("the archive holds no file to extract")
         # The old tree is stepped aside rather than deleted: removing it
         # first and then failing to move the new one in left the cache with
         # nothing at all, and the next run reads that as "never fetched".
@@ -450,6 +463,7 @@ def _refresh_entry(
         OSError,
         tarfile.TarError,
         zipfile.BadZipFile,
+        NothingExtracted,
     ) as exc:
         log.warning("[%s] download failed: %s", key, exc)
         return None

tests/test_refresh_failures.py

@@ -8,6 +8,7 @@ failed, the cache left at the old version.
 
 from __future__ import annotations
 
+import shutil
 import subprocess
 import sys
 import tempfile
@@ -63,5 +64,46 @@ class RefreshFailuresAreFailures(unittest.TestCase):
         self.assertIn("sys.exit(1)", source[source.index("failed = update_changed(report)"):])
 
 
+class AnEmptyExtractionKeepsTheCache(unittest.TestCase):
+    """An archive without the cited subtree replaced the cache with an empty
+    directory, recorded the version and reported success."""
+
+    def setUp(self):
+        import refresh_data_dirs
+
+        self.module = refresh_data_dirs
+        scratch = Path(__file__).resolve().parent.parent / "tmp"
+        scratch.mkdir(exist_ok=True)
+        self.root = Path(tempfile.mkdtemp(dir=scratch))
+        self.addCleanup(shutil.rmtree, self.root, True)
+        self.cache = self.root / "cache"
+        self.cache.mkdir()
+        (self.cache / "keep.dat").write_bytes(b"cached")
+
+    def test_a_tarball_without_the_subtree(self):
+        import tarfile
+
+        archive = self.root / "archive.tar.gz"
+        payload = self.root / "a.txt"
+        payload.write_bytes(b"x")
+        with tarfile.open(archive, "w:gz") as tf:
+            tf.add(payload, arcname="repo-main/OtherDir/a.txt")
+        with self.assertRaises(self.module.NothingExtracted):
+            self.module._download_and_extract(
+                archive.as_uri(), "repo-main/Data/Sys", str(self.cache), []
+            )
+        self.assertEqual((self.cache / "keep.dat").read_bytes(), b"cached")
+
+    def test_a_zip_with_no_file(self):
+        import zipfile
+
+        archive = self.root / "archive.zip"
+        with zipfile.ZipFile(archive, "w") as zf:
+            zf.writestr("only/", b"")
+        with self.assertRaises(self.module.NothingExtracted):
+            self.module._download_and_extract_zip(archive.as_uri(), str(self.cache))
+        self.assertEqual((self.cache / "keep.dat").read_bytes(), b"cached")
+
+
 if __name__ == "__main__":
     unittest.main()