fix: key the manifest hash cache by its check

The cache of manifest hashes was keyed by the database's id without
holding the database, so a freed database replaced at the same
address got another scan's verified hashes back. The key now carries
the sha1 that database expects for the path.

scripts/generate_pack.py

@@ -3236,13 +3236,16 @@ def _stable_hashes(local_path: str, db: dict) -> dict:
     refused rather than published.
     """
     stat = os.stat(local_path)
-    key = (os.path.normpath(local_path), stat.st_size, stat.st_mtime_ns, id(db))
+    expected = _db_sha1_by_path(db).get(os.path.normpath(local_path))
+    # Keyed by what was checked, the sha1 this database expects there, never
+    # by id(db): a database freed and replaced at the same address answered
+    # with hashes verified against the first one.
+    key = (os.path.normpath(local_path), stat.st_size, stat.st_mtime_ns, expected)
     known = _STABLE_HASHES.get(key)
     if known is not None:
         # Verified against the scan once: twelve manifests name a file up
         # to a dozen times, and rehashing it each time spent 163 s of 218.
         return known
-    expected = _db_sha1_by_path(db).get(os.path.normpath(local_path))
     hashes = compute_hashes(local_path, _MANIFEST_HASHES)
     for _attempt in range(2):
         if expected is None or hashes["sha1"] == expected:

tests/test_install_experience.py

@@ -212,6 +212,16 @@ class ManifestHashesMatchTheScan(unittest.TestCase):
                 generate_pack._stable_hashes(str(path), bad)
             self.assertEqual(generate_pack._stable_hashes(str(path), {"files": {}})["sha1"], real)
 
+    def test_the_cache_is_keyed_by_what_was_checked(self):
+        """Keyed by id(db), a database freed and replaced at the same address
+        got hashes verified against the first one back, unchecked."""
+        source = (REPO_ROOT / "scripts" / "generate_pack.py").read_text(encoding="utf-8")
+        body = source[source.index("def _stable_hashes("):]
+        body = body[: body.index("\ndef ")]
+        key_line = next(line for line in body.splitlines() if line.strip().startswith("key ="))
+        self.assertNotIn("id(", key_line)
+        self.assertIn("expected", key_line)
+
 
 class RetriesWait(unittest.TestCase):
     def test_a_server_error_is_retried_after_the_delay_it_asks(self):